(Circulation. 1995;91:201-214.)
© 1995 American Heart Association, Inc.
Articles |
Premature Beats Elicit a Phase Reversal of Mechanoelectrical Alternans in Cat Ventricular Myocytes
A Possible Mechanism for Reentrant Arrhythmias
From the Department of Cardiology (D.S.R.) and the Department of Physiology (S.L.L.), Loyola University of Chicago, Maywood, Ill.
Correspondence to Donald S. Rubenstein, MD, PhD, Department of Cardiology, Loyola University of Chicago, 2160 S First Ave, Maywood, IL 60153.
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Abstract |
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 Top Abstract IntroductionMethods Results Discussion References |
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Background Alternans of the ST segment of the ECG is an important risk factor for sudden cardiac death. Premature beats during alternans and the development of discordant alternans are associated with the onset of ventricular tachycardia and ventricular fibrillation. Moreover, premature beats can switch the pattern of alternans from discordant to concordant alternans. The mechanisms of how a premature beat can elicit a pattern shift in alternans and develop malignant ventricular arrhythmias are not clear. The purpose of this cellular study was to determine the electrical and mechanical restitution properties during cycle length–induced alternans and to determine how premature and delayed beats affect the resultant phase of alternans.
Methods and Results A perforated patch recording method and video-based edge detector were used to record action potentials and contractions, respectively, from single ventricular myocytes enzymatically isolated from the cat heart. Electrical and mechanical restitution curves were determined by programmed test beats delivered at different cycle lengths during mechanoelectrical alternans. At 35°C, 97.8% of cells exhibited concordant cellular alternans (action potentials with the larger action potential duration [APD] were associated with the larger contraction, and action potentials with the smaller APD exhibited the smaller contraction). The sequence or phase of concordant cellular alternans could be systematically reversed by (1) early premature beats that followed only action potentials with the shorter APD and smaller contraction (type 1 phase reversal; n=34) or (2) late delayed beats that followed only action potentials with the longer duration and the larger contraction (type 2 phase reversal; n=14). A phase reversal point was defined as a threshold time interval that resulted in switching the sequence of the alternating beats. A test stimulus at the phase reversal point caused temporary suppression of mechanoelectrical alternans. Lower temperatures (32°C) or decreases in the basic cycle length induced larger beat-to-beat changes in the magnitude of alternans (APD or contraction) and significantly shifted the phase reversal point to earlier premature intervals for type 1 phase reversal. The interval of the phase reversal point was a function of the contractile ratio (the magnitude of the larger contraction/smaller contraction for two consecutive beats, r=.93) and not the APD ratio (longer APD/shorter APD; r=.501). In cells stimulated at cycle lengths longer than the threshold of alternans, a single premature beat could elicit a damped form of concordant mechanoelectrical alternans. A critically timed second premature beat reversed the phase of the damped alternans.
Conclusions Properly timed premature or delayed beats during cycle length–induced alternans consistently reversed the phase of cellular mechanoelectrical alternans. Reversal of the phase of alternans was dependent on recovery of mechanical activity, not electrical activity. The premature stimulus interval at the phase reversal point can be predicted by the magnitude of mechanical alternans. Thus, during cycle length–induced alternans, mechanical alternans governs the phase of electrical alternans. From the present results, a multicellular model is proposed that may explain how critically timed premature beats cause a regional change in the phase of mechanical alternans and thereby result in discordant electrical alternans or dispersion of refractoriness. Premature beats that induce phase reversal in mechanoelectrical alternans may contribute to the development of reentrant arrhythmias.
Key Words: alternans • potentials • electrophysiology • reentry • arrhythmia
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Introduction |
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TopAbstractIntroductionMethods Results Discussion References |
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Alternation of the ST segment in the ECG is generated from alternation of the action potential duration (APD) at the cellular level,1 which is believed to be responsible for the alternating effective refractory period of electrical activity.1 During alternans, it has been hypothesized that adjacent regions of cardiac tissue that exhibit differences in refractory period (dispersion of refractoriness) may be an important factor that initiates malignant ventricular arrhythmias.2 In addition, programmed stimulation protocols in patients exhibiting alternans were found to be more easily induced into reentrant arrhythmias such as ventricular tachycardia than in patients without alternans.3 Moreover, in patients with coronary artery disease, electrical alternans of the ST segment has recently been determined to be an important risk factor for ventricular arrhythmias and sudden cardiac death.4
Several animal models of alternans have shown that mechanical alternans coexists with electrical alternans and that the heart can exhibit two patterns, either concordant or discordant alternans. Concordant and discordant alternans at the whole-heart level refer to how one tissue region exhibiting alternans correlates with a second region of tissue also exhibiting alternans.5 6 7 For example, discordant electrical alternans occurs when simultaneous recordings from adjacent tissue sites show long APDs in one region and short APDs in an adjacent region and vice versa with the alternating beat. Consequently, the APDs of one region are alternating out of phase with the second region.6 Concordance at the whole-heart level is simply the simultaneous demonstration of similar action potential morphologies or contractile states from two adjacent tissue sites.
Importantly, the terms concordance and discordance at the whole-heart level are defined differently at the cellular level.8 Concordant or discordant alternans within a single cell refers to how the electrical alternation is associated with the mechanical alternation. In some studies, mechanical alternans is concordant with the electrical alternans.6 9 10 In other words, beats exhibiting the larger contraction of mechanical alternans are associated with action potentials that have the longer APD. These beats then alternate with beats exhibiting smaller contractions and shorter APDs. However, other studies have shown that the electrical and mechanical alternans also may be discordant.11 12 That is, beats exhibiting larger contractions are associated with action potentials that have shorter APDs, and these alternate with beats exhibiting smaller contractions and longer APDs. The type of cellular alternans may depend on temperature.11 These definitions imply that concordant alternans at the cellular level can exist at two different sites in the heart, yet be out of phase with one another and therefore be viewed at the whole-heart level as a discordant pattern of alternans.10 13
In whole-animal models of mechanoelectrical alternans, a single premature ventricular beat can switch the pattern between concordance and discordance.10 13 Premature ventricular beats during alternans and the development of discordant alternans have been associated with the onset of ventricular fibrillation.5 14 Pattern shifts between concordance and discordance at the level of the whole heart suggest that the phase of alternans at the cellular level reversed in only one of the two tissue regions.10 13 Thus, switching the sequence of action potentials with the longer APD for the shorter APD and vice versa at one tissue site could cause the pattern shift. Although it is currently unknown how one region of cardiac tissue can change its phase of alternans, at least two possible mechanisms may account for this phenomenon. First, alternating paths of electrical conduction may be a cause of the electrical alternans.15 The premature beat may locally alter the wave of excitation and thus change the phase of alternans in one tissue area and not another. Another possible explanation is that the premature beat might affect specific mechanisms of alternans at the cellular level. Because the stimulated cycle length influences the magnitude of alternans (beat-to-beat difference in APD or contraction),16 17 we hypothesized that changes in the phase of alternans following a premature beat might depend on either the recovery of electrical or mechanical activity (restitution properties) of the cell. The purpose of the present study, therefore, is to investigate the effect of premature and delayed stimuli on the phase of cycle length–induced alternans at the single cell level.
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Methods |
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TopAbstractIntroductionMethods Results Discussion References |
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Adult cats of either sex were anesthetized with sodium pentobarbital (70 mg/kg IP). Hearts were rapidly excised and perfused via a Langendorff apparatus. The initial perfusion was with a bicarbonate-buffered Tyrode's solution for 5 minutes, followed by perfusion with a nominally Ca2+-free Tyrode's solution. After 5 minutes, the solution was switched to a low-Ca2+ (36 µmol/L) Tyrode's solution containing 0.05% collagenase (Worthington Biochemical, type II) for approximately 25 to 30 minutes. After collagenase perfusion of the whole heart, small pieces of ventricular muscle (1 to 2 mm thick) were sliced from the free wall of the left ventricular endocardial surface and agitated in fresh collagenase (0.015%) for 5 minutes.
Cells used for study were transferred to a small tissue bath on the
stage of an inverted microscope (Nikon Diaphot) and superfused with a
modified Tyrode's solution containing (in mmol/L): NaCl 137, KCl 5.4,
MgCl2 1.0, CaCl2 2.0, HEPES 5, and glucose 11,
titrated with NaOH to a pH of 7.4. Temperature in the tissue bath was
maintained at 35±0.2°C (unless otherwise stated) by a feedback
temperature controller (NBD) at the fluid inlet and bath base. A single
suction pipette (tip resistance of 1 to 5 M
) was used to record
voltage in a whole-cell configuration18 by use of a
nystatin perforated patch recording method.19 Nystatin was
dissolved in dimethyl sulfoxide at a concentration of 25 mg/mL and then
added to the internal pipette solution to yield a concentration of 75
µg/mL. The internal pipette solution contained (in mmol/L): potassium
glutamate 100, KCl 40, Na2-ATP 5, EGTA 5, and HEPES 5,
titrated with KOH to a pH of 7.2. The nystatin-containing pipette stock
solution was ultrasonicated for 5 minutes. Pipettes were filled by
placing pipette tips in the pipette solution for 2 to 5 minutes,
followed by backfilling the pipette with the same
solution.19
Suction pipettes were used to record voltage (bridge mode) with an Axoclamp-2A amplifier (Axon Instruments, Inc). Computer software (LabVIEW, National Instruments) was used to create a pulse generator and acquire data on-line. The pulse generator delivered programmed stimulation protocols (see "Results" section for specific patterns used). Action potential and cell length measurements were acquired and displayed graphically on-line. Stimuli were delivered at 1.5 times the threshold stimulus. All analog data were digitally converted (AT-MIO-16X, National Instruments) at a rate of 1 kHz. Cell length measurements were made by a video edge detector (Crescent Electronics). A raster line was placed on one edge of the tissue, and all cell length data are expressed in micrometers of detected motion. The custom-designed software program measured APD at 90% of complete repolarization and the change in cell length.
All data were stored on a computer hard drive (Dell 486DX, 66 MHz) and
displayed graphically on-line. In addition, analog data were stored on
a four-channel FM tape recorder (4DS, Racal-Dana Instruments). The
cycle length threshold for alternans was defined as a 
1% alternation
in the APD or cell length for at least 150 consecutive beats. The
threshold cycle length that elicited concordant alternans was
determined by initially stimulating a cell at a basic cycle length of
300 ms. After a 100-beat warm-up period, cell length and APD
measurements were made for the next 150 consecutive beats. After a
10-second rest, the cycle length was decreased by 10 ms, and the
stimulation protocol was repeated. Five to 10 different cycle lengths
were used to establish the threshold cycle length for stable
alternans.
To construct curves describing the recovery of electrical activity (electrical restitution) and the recovery of mechanical activity (mechanical restitution) during alternans, cells were stimulated according to a protocol designed to simulate premature ventricular contractions followed by a compensatory pause. Premature stimuli were delivered after a beat exhibiting either a shorter or longer APD to determine whether there was an alternation in the rates of recovery. This was accomplished by delivering test stimuli on every 21st beat of a stimulated train with a test interval that progressively decreased by a preprogrammed interval (see "Results"). We observed that approximately 15 beats were required between test stimuli to achieve the control magnitude of alternation. Because the test stimuli were delivered on the odd-numbered beat (21st), each subsequent test stimulus followed an alternating APD, ie, short then long APD. This allowed us to determine the restitution curves that followed both the short and long APD beats in a single cell. However, as will be developed in more detail below, a major finding of this study is that properly timed premature beats can change the phase of alternans. After a phase reversal occurred, the subsequent test stimuli delivered on odd-numbered beats no longer followed a beat with the alternate APD. Therefore, the test stimuli with very short test intervals would generate data of the restitution curve only for the beat that followed an action potential with a shorter duration. To complete the restitution curve for the beat that followed the action potential with the longer duration, it was also necessary to deliver early test stimuli on an even-numbered beat (every 20th beat). In addition, this protocol was necessary in evaluation of the restitution properties generated by delayed test stimuli.
Data are presented as mean±SEM. The difference between the means was tested by Student's t distribution. Results were considered significant when P<.05. Restitution and postextrasystolic potentiation (PESP) curves were fitted by monoexponential functions17 with commercial software (SigmaPlot 4.0).
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Results |
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TopAbstractIntroductionMethods Results Discussion References |
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Threshold Cycle Length of Alternans
In this series of experiments we sought to determine whether electrical alternans could be elicited without evidence of mechanical alternans. Fig 1
shows typical recordings of concordant
mechanoelectrical alternans at different cycle lengths in a ventricular
muscle cell. Action potentials exhibiting longer APDs were associated
with larger changes in cell length, and action potentials with shorter
APDs were associated with smaller changes in cell length. At a
stimulated cycle length of 290 ms (Fig 1A), the APD varied by
about 7
ms between alternating beats. In this cell, changes in contractile
amplitude were more apparent than changes in APD. When the stimulated
cycle length was decreased to 250 ms, changes in APD and mechanical
activity became more apparent (Fig 1B). APD varied by about 27
ms
between beats, and the shorter action potentials elicited little if any
cell shortening. In addition, as shown in the superimposed action
potentials, the longer APDs exhibited more negative plateau voltages
than the shorter APDs, resulting in a crossover in final repolarization
voltage. Similar results were obtained in 90 of 92 cells studied
(97.8%). In 8 of 8 cells tested, exposure to 10-6 mol/L
ryanodine abolished all cellular contractions, and there was no longer
any evidence of electrical alternans at cycle lengths ranging from 200
to 300 ms. Furthermore, in 4 cells, we continued to shorten the cycle
length until block occurred (mean cycle length at block was 112±10
ms). Shorter cycle lengths elicited even shorter APDs, and electrical
alternation of the APD was not seen in any cell tested (not shown).
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The
three-dimensional histograms in Fig 2A and 2B
show
the development of cycle length–induced electrical and mechanical
alternans, respectively. At a relatively long cycle length that
exhibited no evidence of alternans (290 ms; right insets), the
histograms show that the APDs and cell length changes are distributed
within a single narrow peak at approximately 130 ms (Fig 2A)
and 4.25
µm (Fig 2B), respectively. As stimulated cycle length was
decreased,
the distribution of the APDs and cell length changes widened. At a
cycle length of approximately 260 ms, two peaks emerged in both the
electrical and mechanical measurements (Fig 3A and
3B).
At the shortest cycle length tested (200 ms, left insets), the
histograms showed the widest separation of the two APDs and two
contractile peaks, reflective of stable mechanoelectrical alternans.
The bifurcation at 260 ms identified the threshold cycle length for
electrical and mechanical alternans for this cell. In 64 cells tested,
the threshold cycle length that elicited stable electrical and
mechanical alternans was the same. The mean threshold cycle length was
256±6 ms. Electrical alternans was never found without a concomitant
mechanical alternans.
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Effect of Premature and Delayed Stimuli on the Phase of Concordant
Mechanoelectrical Alternans
The traces displayed in Fig
3
are taken from an experiment that
tested progressively shorter intervals (4 ms) of a premature beat that
followed the shorter APD and smaller contraction of stable concordant
alternans. These specific traces were chosen to show the gradual
changes of cell length shortening and APD caused by earlier premature
beats while also eliciting an abrupt change in the phase of alternans.
The action potentials shown in E1 (top) and E5 (bottom) of the figure
were recorded simultaneously with the contractions shown in M1 and M5,
respectively. The remaining contractile traces (M2 through M4) were
recorded from the same cell, but the action potentials are not shown
for clarity. Traces E1 and M1 show the control action potential and
contractile activities at a stimulated cycle length of 300 ms. As the
premature beats were delivered progressively earlier, from 264 to 236
ms (M2 through M5), the contraction, as expected, became progressively
smaller in amplitude (dashed line b). In addition, as expected, the
contraction of the beat following the premature beat, the PESP beat,
became progressively larger in amplitude (dashed line c). Moreover, as
the premature beat was delivered progressively earlier, the premature
APD became progressively shorter (E5) and the APD of the PESP beat
became progressively longer. Of particular importance, at certain test
intervals the premature and postextrasystolic contractions were similar
in amplitude. In this experiment, equal amplitudes of the premature
test beat and PESP beat occurred at a test interval of about 260 ms
(M3). This can be viewed as a temporary suppression of alternans.
Moreover, premature stimuli that had intervals shorter than the
interval that caused temporary suppression of alternans (<260 ms)
caused mechanoelectrical alternans to reverse its phase (type 1 phase
reversal). Thus, at a test interval of 236 ms (traces E5 and M5), the
beats following the test beat are reversed in sequence: beats that had
longer APDs and larger contractions now exhibit the shorter APDs and
smaller contractions and vice versa (see dashed line d). This new phase
persisted until the next premature stimulus that elicited another phase
reversal. Every cell tested (n=34) exhibited phase reversal of
alternans only when the longer APD beat was activated earlier than a
critical interval. The phase reversal was always observed as a change
in the sequence of the alternating beats. Electrical alternans always
remained concordant with mechanical alternans after a phase reversal.
Phase reversal was never observed when the shorter APD (and smaller
contraction) beat was activated prematurely at any test interval (not
shown). If these results were due to a balance of less calcium
available for the premature beat and more available for the PESP beat,
then a similar phase reversal should be achieved by delaying the
activation of the beat with the smaller contraction of alternans.
The
traces in Fig 4 are displayed similarly to those in
Fig 3. In this experiment, the basic stimulated cycle length
was 360
ms, and activation of the shorter APD beat (*) was progressively
delayed in the cycle. As the test beat was delayed from 360 to 432 ms
(traces M1 through M5), the contraction amplitude of the delayed beat
became progressively larger (dashed line b) and the postdelay beat (the
beat that directly followed the delayed test beat) became progressively
smaller in amplitude (dashed line c). At a delayed test interval of 408
ms (trace M3), the magnitude of mechanical alternans was suppressed. At
test intervals longer than 408 ms, the phase of mechanoelectrical
alternans following the test stimulus was reversed (dashed line d).
Every cell tested (n=14) exhibited phase reversal of alternans when the
shorter APD beat was delayed past a critical interval (type 2 phase
reversal). In addition, phase reversal was never observed when the
longer APD beat was delayed at any interval tested (not shown). These
results indicate that a premature test stimulus or a delayed test
stimulus delivered at a critical time in the cycle of alternans can
induce a phase reversal or a switch in the sequence of the alternating
beats. These critical intervals, at which the premature or delayed beat
elicits a suppression of alternans, will be defined as the phase
reversal points.
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Restitution Curves
The results obtained with premature and
delayed test stimuli
indicate that the recovery of the electrical and mechanical activities
is an important factor determining the phase of alternans. The previous
experiments showed that the phase reversal was associated with the
relationship between the test stimulus and the subsequent beat, ie, the
PESP beat. Therefore, to examine the effect of the test stimulus
interval on the APD and cell shortening of the test stimulus and to
compare it with the effects caused on the PESP beat, we constructed
simultaneous restitution and PESP curves. The graphs in Fig 5
display the restitution curves (test stimulus beats)
together with the PESP curves in a typical cell. The data presented
in Fig 5A and 5B were obtained from a cell
stimulated at a cycle length
that does not exhibit mechanoelectrical alternans. A shows the
electrical restitution (re) and PESP (pe)
curves, and B shows the mechanical restitution (rm) and
PESP (pm) of a cell stimulated at a basic cycle length of
350 ms with test pulses incremented by 5 ms. The rate of recovery
(restitution rate constant) was slightly greater than the rate constant
of potentiation for electrical activity (11.23 and 8.28
s-1, respectively) and mechanical activity (3.70
and 2.74 s-1, respectively). The restitution curves
of both the electrical and mechanical activities (re and
rm) intersected their respective PESP curves
(pe and pm) when the test stimulus interval
equaled the control cycle length of 350 ms. This result was expected,
because the APD and contraction of the test beat should equal that of
the PESP beat, since there was no evidence of electrical or mechanical
alternans at this cycle length. Similar results were obtained in 14
cells.
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In Fig 5C and 5D, the same cell
was stimulated at a cycle length of 210
ms, which induced stable concordant mechanoelectrical alternans. As a
result, two separate restitution and two PESP curves were determined,
one following the shorter and one following the longer APD beat of
alternans. The graph in Fig 5C shows the electrical restitution
curves
for beats following the longer APD beats (rel) and
following the shorter APD beats (res) and the corresponding
PESP curves (pel and pes, respectively).
The curve depicting electrical recovery after a beat with a long APD
(rel) was shifted to the right of the recovery curve after
a beat with a short APD (res). The two curves had different
intercepts with the time axis, as seen by the different intercepts with
the time axis, indicating that the relative electrical refractory
period of this cell alternated between 137 and 147 ms. Furthermore,
recovery of electrical activity following beats with the longer APD
(rel) was slower than recovery following the beats with the
shorter APD (res) (25.56 versus 139.38
s-1, respectively). The graph in Fig 5D shows
similar curves when the same action potentials for mechanical
restitution are analyzed (rms and rml). Once
again, the different time axis intercepts indicate that the relative
mechanical refractory period alternated between 128 and 179 ms. As in
electrical alternans, the rate of recovery for mechanical activity
following beats with the larger contraction (rml) was
slower than the rate of recovery following beats with the smaller
contraction (rms) (8.73 versus 41.92
s-1, respectively).
The characteristics of each curve varied among different cells, and the variability appeared to depend on the beat-to-beat changes in cell length shortening of mechanical alternans. Therefore, to statistically compare the two restitution curves (recovery after the large versus small contraction), we selected four cells that when stimulated at 210 ms each had similar magnitudes of mechanical alternation. The rate constants of mechanical restitution following the larger contraction beats were significantly slower than the restitution following the smaller contraction beats (12.2±1.5 versus 27.1±5.2 s-1; P<.05). Furthermore, the mechanical relative refractory period after the larger contraction was significantly longer than the relative refractory period after the smaller contraction (157±9 versus 125±2 ms; P<.02). The mean rate constant of electrical restitution after the longer APD was not significantly slower than electrical restitution following the shorter APD (89.5±24.2 versus 55.6±18.3 s-1). However, as expected, the relative electrical refractory period following the larger APD was significantly longer than the refractory period following the shorter APD (148±4 versus 122±6 ms; P<.02). The recovery of mechanical activity was significantly slower than electrical activity after either beat of alternans (P<.05).
Inspection of the curves in Fig
5C showed that at a test interval of
177 ms, the APD of the premature stimulus equaled the APD of the PESP
beat (intersection of curves res and pes; solid
lines). As shown in Fig 5D, this same test stimulus interval
also
corresponded to the interval in which the cell length shortening of the
premature stimulus equaled that of the PESP beat (intersection of
curves rms and pms; solid lines). In other
words, this premature stimulus interval caused temporary suppression of
mechanoelectrical alternans, similar to that shown in Fig 3
(trace M3).
Furthermore, the intersection of the restitution and potentiation
curves identified the phase reversal point, since an interval 
177 ms
caused a switch in the sequence of the alternating beats. Similarly,
for test stimuli following the longer APD beats, the restitution curve
(rel; dashed line) and the PESP curve (pel;
dashed line) intersected at a test interval of 237 ms, which is the
same value as the intersection of the corresponding mechanical
restitution (rml; dashed line) and PESP curves
(pml; dashed line). Therefore, a test interval of 237 ms
caused temporary suppression of mechanoelectrical alternans similar to
that shown in Fig 4 (trace M3). This interval identified the
phase
reversal point caused by a delayed test stimulus. In all cells tested
(n=14), two phase reversal points were identified during alternans: a
phase reversal point that occurred at an early interval following the
shorter APD beats of alternans (type 1 phase reversal) and a phase
reversal point that occurred at a late interval following the longer
APD beats of alternans (type 2 phase reversal). All cells tested
exhibited mechanical and electrical phase reversal points at identical
test intervals.
Relationship of the Phase Reversal Point With APD and Contractile
Ratios
In the restitution experiments described above, we observed
that
different magnitudes of alternans (beat-to-beat difference in APD or
cell length shortening) were associated with shifts in their
restitution curves (different rates of recovery and relative refractory
periods). These curve shifts suggested that the magnitude of alternans
might predict the interval of the phase reversal point (the
intersection of the restitution and the potentiation curves). The
magnitude of the beat-to-beat changes in APD and contraction during
alternans could be altered by changes in the stimulated cycle length
(Figs 1 and 3). The ratio of longer APD to
shorter APD (APD ratio) or
larger contraction to smaller contraction (contractile ratio) of the
alternating beats was used as an index of the magnitude of alternans.
Stimulating cells (n=34) at various cycle lengths to generate different
magnitudes of alternans, we therefore compared the APD or contractile
ratios with their respective phase reversal points. The phase reversal
points were determined for the type 1 phase reversal (by premature
stimulation), and this interval was normalized to the stimulated basic
cycle length. Fig 6A shows that there was a poor
correlation of APD ratio with the phase reversal points
(r=.501). In contrast, Fig 6B shows that the
contractile
ratio of alternans was highly predictive of the phase reversal points
(r=.93). In addition, the graph in Fig 6B
indicates that
narrow forms of mechanical alternans (similar to Fig 1A) had a
phase
reversal point close to the control cycle length, whereas larger
contractile ratios during alternans (similar to Fig 1B)
exhibited a
phase reversal point earlier in the alternans cycle. Thus, larger
magnitudes of mechanical alternans required earlier premature stimuli
to switch the sequence of the alternating beats. Clearly, the
contractile ratios were not correlated with APD ratios. This
corresponds with the observation that large alternation of cell length
shortening could be observed in cells with either large or small
alternation of the APD. As the contractile ratio increased, the phase
reversal point approached a maximum of about 0.77 of the normalized
cycle length.
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Another method that changes the magnitude of cycle
length–induced
alternans is to lower the temperature. In the experiment shown in Fig
7A through 7C, alternans was elicited by pacing at cycle
lengths that induced a mechanical alternans in which the large
contraction was approximately twice the amplitude of the small
contraction. While the basic cycle length was kept constant in any
individual cell, the temperature was reduced from 35°C to 32°C.
Normalized phase reversal points were determined by premature test
stimuli that were delivered only after shorter APDs and small
contractions. All mechanoelectrical alternans were concordant at either
temperature tested. Fig 7A shows that lowering the temperature
elicited
a significant shift in the position of the phase reversal point, from
0.89±0.01 to 0.78±0.2 of the normalized cycle length
(P<.001; n=5). Because the phase reversal point was shown
to be determined primarily by the contractile and not electrical
activity (Fig 7), the temperature-induced changes in the phase
reversal
point should be related to changes in contractile, not APD, ratios. As
shown in Fig 7B, there was no significant difference between
the APD
ratio at 35°C (1.30±0.12) compared with 32°C (1.40±0.14)
(P>.4). In contrast, the contractile ratio was
significantly increased when the temperature was lowered from 35°C
(2.39±0.47) to 32°C (15.37±2.03) (P<.001). These
results provide further support for the idea that the phase reversal
point is dependent on the contractile ratio, not the APD ratio.
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Effect of Premature and Delayed Stimuli on the Phase of Discordant
Mechanoelectrical Alternans
An important question arises as to how
type 1 and type 2 phase
reversals apply to discordant cellular alternans. In concordant
alternans, a premature beat elicited a phase reversal when the
premature stimulus was applied early enough after a beat with a short
APD and a small contraction (Fig 3). In discordant alternans,
however,
the shorter APD is associated with the larger contraction. Therefore,
if electrical restitution determines the phase reversal point, then an
early premature stimulus after a beat with the smaller APD and a larger
contraction should elicit a phase reversal similar to that found with
concordant alternans. On the other hand, if mechanical restitution is
more important, then the phase reversal should be elicited by an early
premature stimulus that occurred after a beat with the longer APD with
the smaller contraction. Discordant mechanoelectrical alternans was
achieved by cooling cells to room temperature (26°C). The top two
traces in Fig 8A show the control electrical and
mechanical recordings during discordant alternans at a stimulated cycle
length of 500 ms (dashed line a). Note that the beats exhibiting the
longer APDs elicited little if any contraction, whereas the beats with
the shorter APDs elicited relatively large contractions. As shown in
the bottom two traces of Fig 8A, a premature test stimulus
(*)
delivered after a beat with a longer APD and smaller contraction
elicited a delayed and smaller contraction followed by a reversal in
the phase of alternans (dashed line b). Note that after the phase
reversal, the association of the electrical and mechanical activity
remained discordant. Premature stimuli delivered after a beat with a
shorter APD and larger contraction never resulted in phase reversal
(not shown).
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The experiment illustrated in Fig 8B shows
that the ability of a
delayed stimulus to elicit a phase reversal is also dependent primarily
on the mechanical rather than the electrical activity. The top two
traces show the control electrical and mechanical recordings during
discordant mechanoelectrical alternans at a cycle length of 420 ms
(dashed line a). In the lower two traces of Fig 8B, a delayed
test
stimulus (*) was delivered 500 ms after a beat with a shorter APD and
larger contraction. The test beat now elicited a small contraction
where there was no contraction before and the subsequent phase of
alternans was reversed (dashed line b). Delayed stimuli delivered after
a beat with a longer APD and smaller contraction never resulted in a
phase reversal (not shown). Similar results were obtained in all 4
cells tested. These results support the idea that the ability of a
single premature or delayed stimulus to reverse the phase of alternans
is dependent on the mechanical, not the electrical, restitution
properties of the cell.
Effect of Premature Beats During Nonalternating Activity
Stimulating cells at long cycle lengths with no evidence of
mechanoelectrical alternans, we studied a programmed stimulation
protocol identical to that used in clinical electrophysiological
studies that induce ventricular arrhythmias believed to result from
dispersed electrical refractoriness.20 21 We
specifically
studied the effects of changing the intervals of two sequential
premature beats (S2, S3) on the APD and
cell length shortening. Previous experimental studies have shown that
abrupt shortening of the cycle length to a value close to but longer
than the threshold of alternans can elicit a damped alternans of the
APD and contraction
amplitude.22 23 24 25 Based
on the phase
reversal phenomenon described above, we hypothesized that if a single
premature beat could elicit a damped form of alternans, then a properly
timed second consecutive premature beat could reverse the phase of the
damped alternans. The upper two traces in Fig 9A show
the mechanoelectrical activity recorded from a cell stimulated at a
cycle length of 340 ms (S1), which was longer than the
threshold of alternans (300 ms). A single premature beat
(S2; 190 ms) elicited a damped alternans of both APD and
cell length shortening (electrically and mechanically concordant) that
lasted for several beats. If the interval of the PESP beat was now
moved earlier in the compensatory pause, it would simulate the
situation described above (Fig 3), in which a cell exhibiting
alternans
was prematurely activated after the shorter APD and smaller
contraction. As shown in the two lower traces of Fig 9A,
S2
was followed by a second premature beat (S3) delivered 270
ms after S2. This resulted in a damped alternans of both
APD and contraction that was now reversed in phase with the alternans
elicited by a single premature beat (dashed line a). The beat-to-beat
changes in APD and cell length shortening illustrated in Fig 9A
are
plotted in Fig 9B and 9C, respectively. In Fig
9B, the changes in APD
elicited by a single premature beat (S2; solid circles) are
superimposed with the changes in APD elicited by the two consecutive
premature beats (S2 and S3; open circles).
Shown in Fig 9C are similar plots of changes in cell length
shortening
for single (closed circles) and consecutive (open circles) premature
beats. These graphs clearly illustrate that the damped alternans
elicited by a single premature compared with a pair of premature beats
are reversed in phase. This is most apparent when the first beats that
were stimulated at the same cycle length are compared. Thus, in Fig
9A,
the APDs of beat 6 (dashed line a) differed by 24 ms.
|
. The action potentials
corresponding to the bottom electrical trace with two consecutive
premature beats (S2, S3) are plotted as open circles 
. C,
Corresponding changes in cell lengths for each beat in A are plotted
against their respective beat number. The cell length changes
associated with the top mechanical trace with a single premature beat
(S2) are plotted as closed circles |
Discussion |
|---|
|
TopAbstractIntroductionMethods Results Discussion References |
|---|
The new findings of the present study of cycle length–induced mechanoelectrical alternans are that (1) properly timed premature or delayed stimuli can reproducibly reverse the phase of both electrical and mechanical alternans; (2) the relative refractory periods alternated for both electrical and mechanical activity; (3) the threshold interval of the test stimulus that switched the sequence of the alternating beats (phase reversal point) was dependent primarily on the mechanical and not the electrical restitution properties of the cell; (4) the phase reversal point could be predicted by the magnitude of mechanical alternans at the control stimulated cycle length; and (5) programmed stimulation protocols identical to those used clinically to induce ventricular arrhythmias can cause a phase reversal of a damped form of alternans.
To the best of our knowledge, this investigation is the first simultaneous study of the restitution and PESP curves of electrical and mechanical alternans in single ventricular myocytes. In previous separate studies, two distinct curves for electrical restitution16 and mechanical restitution17 could be generated during cycle length–induced alternans in papillary muscle preparations. Potentiation curves were not evaluated in either study. Furthermore, our use of the nystatin perforated patch recording technique provided a new method to study stable cellular alternans for prolonged periods (up to 60 minutes). Our results showed that the threshold cycle length to induce alternans was 256±6 ms. Therefore, this threshold in a single-cell model of alternans is in close agreement with previous multicellular studies of cycle length–induced alternans in the cat.11
As the cycle length was shortened to generate concordant alternans, we
observed the development of two distinct restitution and potentiation
curves for both electrical and mechanical activity (Fig 6C and
6D). The
recovery curves for electrical and mechanical activity following beats
with the longer APD and larger contraction were significantly shifted
to the left of the recovery of electrical and mechanical activity
following beats with the shorter APD and smaller contraction. This was
reflected by the alternation of the electrical and mechanical relative
refractory periods. It was not surprising that beats with the longer
APD and larger contraction of alternans had a significantly longer
electrical and mechanical relative refractory period than beats with
the shorter APD and smaller contraction.
Alternation of the electrical refractory period in our single-cell model of alternans is in accord with alternation of the effective refractory period seen in the whole hearts that exhibit ST alternans.1 However, our finding of the significant alternation of the mechanical refractory period is in contrast with a constant mechanical refractory period in a guinea pig multicellular papillary muscle model of cycle length–induced alternans.17 Three reasons may account for this discrepancy. First, there may be a species difference in alternans mechanisms. Different species have previously been shown to exhibit different characteristics of alternans such as threshold cycle length.11 Second, our investigation used single ventricular myocytes as opposed to their multicellular preparation.17 The cycle length threshold in our experiments had a small standard error, yet the total range of the threshold cycle length spanned almost 200 ms. Thus, the multicellular restitution, being an average restitution among several hundred cells, may cause a convergence of the curves at the time axis. Third, we noted that early premature test stimuli reversed the phase of alternans, requiring us to perform two separate stimulation protocols to obtain data at these early intervals. If the phase reversal was missed in the multicellular experiments, it would have resulted in a leftward shift of the restitution curve for beats following the longer APD beats (rel and rml) at the early premature test intervals. Therefore, without changing the stimulation protocol, an artificial convergence of the restitution curves would be expected and would also result in an exaggeration of the slower rate constant. It is not known whether phase reversal of mechanical alternans was seen in that study, but it would have been expected to occur on the basis of their results of generating two distinct restitution curves.
The mechanical and electrical recovery following beats with the shorter
APD (smaller contraction) of alternans elicited restitution curves that
intersected its corresponding PESP curve at intervals shorter than the
control stimulated cycle length (Fig 5C and 5D;
solid lines). In
addition, test stimuli following beats with the longer APD (larger
contraction) elicited restitution curves that intersected the
corresponding PESP curves at intervals longer than the control
stimulated cycle length (Fig 5C and 5D; dashed
lines). Therefore, these
intersections, or phase reversal points, identified the stimulus
interval that resulted in the amplitude of the APD or contraction of
the test stimulus that was equal to the amplitude of the subsequent
beat. The importance of the critical timing around these intersection
points is illustrated by the fact that small changes in the test
interval stimulus can result in completely opposite phases of
alternans.
To analyze the phase reversal point, we used
methods11 17
known to change the magnitude of alternans and thereby shift the
mechanical and electrical restitution curves. Decreases in the basic
cycle length and decreases in temperature resulted in different
magnitudes of alternans, as seen by the increases in APD and
contractile ratios. We determined that the contractile ratio and not
the APD ratio could reliably predict the interval of the phase reversal
point (Fig 7A and 7B). Therefore, the phase
reversal point for both
electrical and mechanical alternans was dependent primarily on recovery
of mechanical activity, not electrical activity. Three other findings
were observed to strengthen the argument that mechanical alternans
importantly influenced electrical alternans. First, electrical
alternans was never found without the occurrence of mechanical
alternans. Second, suppression of all mechanical activity and
mechanical alternans with ryanodine eliminated alternation of the APD
at all cycle lengths tested (85 to 300 ms), suggesting that
mechanoelectrical alternans is entirely a calcium-mediated event
requiring a functional sarcoplasmic reticulum (SR). Third, when
electrical and mechanical alternans were discordant, control of the
phase was still determined by factors responsible for mechanical
restitution, not electrical restitution. Thus, it appears that
mechanical alternans not only may initiate and maintain the presence of
electrical alternation of the APD but also governs the phase of the
electrical alternans.
In the experiments performed at 35°C, all our cells showed concordance between the mechanical and electrical alternans. The larger amount of calcium released from the SR during the larger contraction beats has been correlated with a greater inward Na/Ca exchange current, leading to an alternation in this inward current.9 Alternation of the Na/Ca exchange current may be responsible for the alternation of the APD. This idea is consistent with our finding that ryanodine abolished electrical alternans. At lower temperatures tested (26°C), the mechanical and electrical activities were found to be discordant. The Na/Ca exchange current is highly temperature sensitive, with a temperature coefficient (Q10) of approximately 3.6,26 which is significantly greater than the Q10 (1.3 to 1.6) found for ionic channel current.27 Therefore, for example, the outward calcium-activated chloride current,28 in addition to other repolarization currents, may predominate over the Na/Ca exchange inward current at lower temperatures. We hypothesize that the larger contraction of alternans at 26°C would elicit a greater calcium-mediated outward current, resulting in APDs shorter than those associated with the smaller contractions of alternans, creating a cellular discordance between mechanical and electrical activity.
Some investigators have proposed that abnormal mechanical restitution underlies the alternans phenomenon.9 10 12 Evidence for involvement of a dysfunctional SR was obtained with the evidence of alternation in the amplitude of calcium transients in both ischemia-induced and cycle length–induced models of alternans.6 12 29 A proposed mechanism for mechanical alternans was that the SR calcium recycling time was slowed.12 Therefore, action potentials with short APD and small contractions would allow greater calcium entry via Na/Ca exchange than after a large contraction. The larger Ca entry could generate greater refilling of the SR after the smaller contraction than after the larger contraction, and the alternating mechanical activity could be maintained.
Yet, in our experiments and in those performed by others,17 30 the development of cycle length–induced alternans was associated with an increase in rate constants of mechanical restitution, which implies that SR recycling time is increased, not decreased. Therefore, if cycle length–induced and ischemia-induced alternans are caused by a common mechanism, then the proposed mechanism of alternans would need modification. One possibility might be that the recovery or restitution of the mechanisms that control contraction becomes slower, not in general, but on alternate beats. The generation of two distinct restitution curves supports this hypothesis and has now been demonstrated in single-cell and multicellular models of cycle length–induced alternans.17 It remains to be determined whether there are also two distinct restitution curves that are generated during ischemia-induced alternans.
We have previously shown that transmembrane voltage changes can shift the phase of periodic SR calcium release.31 This shift in phase resulted in further changes in the membrane voltage. We concluded that phase shifts in the release of calcium from the SR could be an important mechanism for the genesis of arrhythmias. It is clear that a single properly timed premature beat can shift the subsequent phase of electrical and mechanical alternans in a single cell. In addition, a cell stimulated at a basic cycle length longer than the alternans threshold can generate a damped form of alternans by delivering one premature beat. A critically timed second consecutive premature beat can also result in a phase reversal for both APD and contractile events. These stimulation protocols are identical to some of those used to induce ventricular arrhythmias in patients undergoing electrophysiological studies.20 21 The possible importance of a sudden phase reversal of alternans at the single-cell level becomes evident as one applies this new information to a multicellular model of alternans.
Multicellular Model of a Regional Phase Reversal of Alternans and
Development of Unidirectional Block
The stimulation protocol used in
cycle length–induced alternans
was chosen to simulate a premature ventricular beat that was followed
by a compensatory pause. The result that mechanoelectrical alternans
can be reversed in phase within a single cell suggests that a similar
response might be possible at the multicellular level. Thus, a
critically timed premature beat during concordant alternans at the
whole-heart level might elicit a pattern change to discordant
alternans. We used actual voltage recordings obtained from a
ventricular cell during concordant cellular alternans and simulated
similar recordings from 7 adjacent cells in our model of multicellular
concordant alternans (Fig 10A). The phase reversal
point in one cell becomes a phase reversal line at the multicellular
level. The dashed line represents the time in each cell that is
associated with its respective phase reversal point. Note that the
phase reversal line would occur only after the beats with the small
APDs. Realistically, the phase reversal line would probably be
curvilinear, since the magnitude of alternans varies across the surface
of the heart.5 The solid line identifies the time interval
at which a premature stimulus passes across these 8 cells. The time
interval difference of the premature beat occurring between cells 1 and
8 would vary with distance and conduction velocities. In this example,
the premature beat crossed the phase reversal line between cells 4 and
5. Cells 1 through 4 but not cells 5 through 8 would reverse their
phase because of the timing of the premature stimulus. The resultant
form of multicellular alternans would be electrically discordant
between cell group 1 through 4 and cell group 5 through 8 (Fig
10B).
|
The staggered phase reversal lines that occur
only after beats with the
small APD during discordant alternans are illustrated by the dashed
lines in Fig 10B. A second early premature stimulus that
occurred
before the phase reversal line in cell group 1 through 4 would again
result in a phase reversal in these cells. However, in cell 5, the
early premature stimulus might occur when the cell was exhibiting an
action potential with the larger APD and thus have a longer relative
refractory period. Therefore, if the premature beat reached cell 5 not
fully repolarized, the premature stimulus could be blocked from
entering cells 5 through 8 because of the dispersion of refractory
periods. The first premature beat that caused a localized phase
reversal in cells 4 through 8 created a functional area of
unidirectional block in cells 5 through 8. Since reentrant arrhythmias
require a region of unidirectional block,32 properly timed
premature impulses following the small beat of alternans may be an
important trigger of reentrant arrhythmias. Importantly, slightly later
premature impulses during discordant alternans could result in an
entirely different outcome. If the premature beat in Fig 10B
arrived at
cell 5 later in time, not being blocked, then the discordant alternans
could be converted back to concordant alternans. On the basis of our
experimental results, the different outcomes would be dependent on the
state of mechanical activity of the cell immediately before the
premature beat.
Extrapolation of our cellular data to the whole-animal
models of ST
alternans must be done cautiously. Our results of cycle length–induced
alternans at the single-cell level, however, are consistent with
results from other investigators studying ischemic and nonischemic
models of alternans. Monophasic action potential recordings in ischemic
regions of the whole heart have confirmed that ST alternans is due to
alternation of the repolarization phase of the action potential in both
animals14 33 34 and
humans.35 At 35°C, we
showed that the larger contractions of alternans correspond with the
longer APD of mechanical alternans, and shorter stimulation cycle
lengths induce larger magnitudes of alternans. This is in direct accord
with results obtained in the ischemic multicellular studies in the
ferret36 and the whole-animal ischemic pig and dog
models.14 37 Experimental studies in whole hearts
have
repeatedly shown that premature beats during ischemia-induced alternans
can cause localized changes in the phase of alternans while not
affecting the phase of alternans in adjacent
tissue.10 13
Therefore, premature beats that reverse the phase of alternans in one
region can convert a discordant pattern of alternans to concordant
alternans.10 13 Most recently, preliminary
experiments on
nonischemic anesthetized dogs have confirmed results from our work at
the single-cell level that properly timed premature beats can reverse
the phase of cycle length–induced mechanoelectrical alternans
(unpublished observations). Further testing will be needed to determine
whether the phase reversal of alternans can occur locally to create a
discordant pattern of alternans in this nonischemic model. It is
important that our evidence of alternation of the electrical refractory
period agrees with alternation of the effective refractory period
during ischemia-mediated ST alternans in dogs.1 Thus,
during discordant alternans, two adjacent regions of cardiac tissue
have significantly different refractory periods, creating dispersion of
refractoriness. It has already been shown that premature beats and
discordant patterns of alternans are both precursors to the development
of ventricular arrhythmias.5 14 Finally, certain
pacing
protocols used to induce arrhythmias in patients20 21
are
identical to the stimulation protocol we used in single cells (Fig
9).
Such pacing protocols in our experiments showed a phase reversal of the
damped alternans. The phase reversal resulted in differences in APD of
up to 30 ms for comparable beats with the same cycle length. These
differences in APD might be an important contributor to electrical
refractory dispersion.
The mechanical events during cycle length–induced alternans importantly influence the phase of electrical alternans, probably by mechanisms responsible for recovery of SR calcium release. Stimuli that reverse the phase of mechanical alternans can result in a sudden switch in the sequence of the electrical alternation of the APD. Thus, a phase reversal of mechanical alternans may lead to dispersed electrical refractoriness and contribute to the onset of malignant ventricular arrhythmias.
|
Acknowledgments |
|---|
This study was supported by National Research Service Award HL-08817-02 (Dr Rubenstein) and grant HL-27652 (Dr Lipsius) from the National Heart, Lung, and Blood Institute of the National Institutes of Health. We would like to thank Dr Brian Olshansky and Dr David Euler for their critical comments and review of the manuscript and Christine Rechenmacher for technical assistance.
Received May 6, 1994; accepted July 31, 1994.
|
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J. I. Goldhaber, L.-H. Xie, T. Duong, C. Motter, K. Khuu, and J. N. Weiss Action Potential Duration Restitution and Alternans in Rabbit Ventricular Myocytes: The Key Role of Intracellular Calcium Cycling Circ. Res., March 4, 2005; 96(4): 459 - 466. [Abstract] [Full Text] [PDF]
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E. J. Pruvot, R. P. Katra, D. S. Rosenbaum, and K. R. Laurita Role of Calcium Cycling Versus Restitution in the Mechanism of Repolarization Alternans Circ. Res., April 30, 2004; 94(8): 1083 - 1090. [Abstract] [Full Text] [PDF]
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M. L. Walker, X. Wan, G. E. Kirsch, and D. S. Rosenbaum Hysteresis Effect Implicates Calcium Cycling as a Mechanism of Repolarization Alternans Circulation, November 25, 2003; 108(21): 2704 - 2709. [Abstract] [Full Text] [PDF]
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 M. L Walker and D. S Rosenbaum Repolarization alternans: implications for the mechanism and prevention of sudden cardiac death Cardiovasc Res, March 1, 2003; 57(3): 599 - 614. [Abstract] [Full Text] [PDF]
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L. A Blatter, J. Kockskamper, K. A Sheehan, A. V Zima, J. Huser, and S. L Lipsius Local calcium gradients during excitation-contraction coupling and alternans in atrial myocytes J. Physiol., January 1, 2003; 546(1): 19 - 31. [Abstract] [Full Text] [PDF]
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J. Kockskamper and L. A Blatter Subcellular Ca2+ alternans represents a novel mechanism for the generation of arrhythmogenic Ca2+ waves in cat atrial myocytes J. Physiol., November 15, 2002; 545(1): 65 - 79. [Abstract] [Full Text] [PDF]
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B. Pieske and J. Kockskamper Alternans Goes Subcellular: A "Disease" of the Ryanodine Receptor? Circ. Res., October 4, 2002; 91(7): 553 - 555. [Full Text] [PDF]
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K. R. Laurita and A. Singal Mapping action potentials and calcium transients simultaneously from the intact heart Am J Physiol Heart Circ Physiol, May 1, 2001; 280(5): H2053 - H2060. [Abstract] [Full Text] [PDF]
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Y. G. Wang, W. J. Benedict, J. Huser, A. M. Samarel, L. A. Blatter, and S. L. Lipsius Brief rapid pacing depresses contractile function via Ca2+/PKC-dependent signaling in cat ventricular myocytes Am J Physiol Heart Circ Physiol, January 1, 2001; 280(1): H90 - H98. [Abstract] [Full Text] [PDF]
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Z. Qu, A. Garfinkel, P.-S. Chen, and J. N. Weiss Mechanisms of Discordant Alternans and Induction of Reentry in Simulated Cardiac Tissue Circulation, October 3, 2000; 102(14): 1664 - 1670. [Abstract] [Full Text] [PDF]
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S. M. Narayan and J. M. Smith Exploiting rate-related hysteresis in repolarization alternans to improve risk stratification for ventricular tachycardia J. Am. Coll. Cardiol., May 1, 2000; 35(6): 1485 - 1492. [Abstract] [Full Text] [PDF]
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J. Huser, Y. G. Wang, K. A Sheehan, F. Cifuentes, S. L Lipsius, and L. A Blatter Functional coupling between glycolysis and excitation--contraction coupling underlies alternans in cat heart cells J. Physiol., May 1, 2000; 524(3): 795 - 806. [Abstract] [Full Text] [PDF]
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M. Zaniboni, A. E. Pollard, L. Yang, and K. W. Spitzer Beat-to-beat repolarization variability in ventricular myocytes and its suppression by electrical coupling Am J Physiol Heart Circ Physiol, March 1, 2000; 278(3): H677 - H687. [Abstract] [Full Text] [PDF]
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S. M. Narayan, B. D. Lindsay, and J. M. Smith Demonstration of the Proarrhythmic Preconditioning of Single Premature Extrastimuli by Use of the Magnitude, Phase, and Distribution of Repolarization Alternans Circulation, November 2, 1999; 100(18): 1887 - 1893. [Abstract] [Full Text] [PDF]
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D. E Euler Cardiac alternans: mechanisms and pathophysiological significance Cardiovasc Res, June 1, 1999; 42(3): 583 - 590. [Full Text] [PDF]
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W. Shimizu and C. Antzelevitch Cellular and Ionic Basis for T-Wave Alternans Under Long-QT Conditions Circulation, March 23, 1999; 99(11): 1499 - 1507. [Abstract] [Full Text] [PDF]
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J. M. Pastore, S. D. Girouard, K. R. Laurita, F. G. Akar, and D. S. Rosenbaum Mechanism Linking T-Wave Alternans to the Genesis of Cardiac Fibrillation Circulation, March 16, 1999; 99(10): 1385 - 1394. [Abstract] [Full Text] [PDF]
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M. J. Burgess, A. E. Pollard, K. W. Spitzer, and L. Yang Effects of Premature Beats on Repolarization of Postextrasystolic Beats Circulation, October 1, 1995; 92(7): 1969 - 1980. [Abstract] [Full Text]
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