(Circulation. 1995;91:838-844.)
© 1995 American Heart Association, Inc.
Articles |
Metabolic Determinants of Defibrillation
Role of Adenosine
From the Department of Medicine and the Division of Cardiology, The New York Hospital–Cornell Medical Center, New York, NY.
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Abstract |
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Background The single most important determinant of cardiac arrest outcome is the duration of ventricular fibrillation (VF) preceding delivery of a high-energy shock, because of the adverse effect of VF duration on defibrillation threshold (DFT). Although a metabolic mechanism has been proposed, hypoxia, metabolic acidosis, or alkalosis do not adversely affect DFT. However, since (1) catecholamines and adenosine levels are markedly increased during hypoxia, (2) exogenous catecholamines decrease DFT, and (3) adenosine is a potent antagonist of the electrophysiological effects of catecholamines on ventricular myocardium, we hypothesized that release of adenosine during prolonged VF adversely affects DFT and that this effect occurs through an antiadrenergic mechanism.
Methods and Results DFT was determined in dogs during infusion of adenosine (300 µg · kg-1 · min-1) and dipyridamole (0.25 mg/kg), an adenosine uptake blocker, a regimen that resulted in adenosine levels in the myocardial effluent equivalent to those achieved after 5 minutes of VF. Adenosine increased transthoracic DFT in each dog by 49±14% (n=21) (mean±SEM) and transmyocardial DFT in a separate group of 10 dogs by 103±16%, P=.0003. Pretreatment with the specific A1 adenosine receptor antagonist 8-cyclopentyltheophylline (CPT) 5 mg/kg completely abolished the effects of adenosine on DFT. The effects of adenosine on DFT were also examined in the denervated state (propranolol 0.2 mg/kg plus bilateral vagotomy). In contrast to its effect in the innervated condition, adenosine had no effect on DFT in the same dogs when denervated, 49±11 versus 53±10 J (P=NS).
Conclusions Adenosine significantly increases transthoracic and transmyocardial DFT, effects that are mediated by the A1 adenosine myocardial receptor through an antiadrenergic mechanism. These results suggest that enhanced release of adenosine during VF may have a deleterious effect on defibrillation and that intramyocardial delivery of a specific A1 adenosine antagonist during VF may facilitate defibrillation and significantly reduce defibrillation threshold.
Key Words: electrophysiology • dipyridamole • fibrillation
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Introduction |
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Sudden death due to ventricular fibrillation (VF) is the major cause of death in the United States. The single most important factor determining the outcome of cardiac arrest is the duration of VF before a high-energy shock is delivered, because of the adverse effect of VF duration on defibrillation threshold (DFT).1 2 3 4 The mechanism governing this phenomenon is unknown. Although a metabolic cause has been hypothesized, it has not been proved, since metabolic acidosis or alkalosis does not elevate defibrillation requirements.5 6 The potential role for the endogenous metabolite adenosine in mediating elevation of DFT has not been systematically evaluated. We considered a possible role for adenosine in mediating this phenomenon on the basis of several lines of evidence: First, myocardial production and interstitial release of adenosine is triggered by an imbalance in O2 supply-demand,7 a condition that exists during VF. Second, the effects of adenosine in ventricular myocardium are antiadrenergic (inhibiting production of cAMP) and are mediated by the inhibitory GTP-binding protein Gi.8 Third, endogenous catecholamine levels are markedly increased during VF, and catecholamines are known to decrease DFT, whereas ß-adrenergic blockade elevates DFT.9 10 11 In view of these data, we hypothesized that enhanced release of adenosine from the myocardium during VF may mediate an increase in energy requirements for defibrillation and that this occurs through an antiadrenergic action.
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Methods |
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General
All studies were performed in accordance with the guidelines of the American Physiological Society in a protocol approved by the Institutional Animal Care and Use Committee at the Cornell University Medical College. Experiments were performed in adult mongrel dogs weighing 15 to 28 kg. All animals were premedicated with acepromethazine (2 mg/kg IM), anesthetized with sodium pentobarbital (30 mg/kg IV), and ventilated with a Harvard respirator adjusted to maintain arterial pH within the physiological range. Additional doses of pentobarbital were administered as necessary. ECG surface lead I and arterial blood pressure were continuously monitored on a physiological recorder.
Surgical Preparation
The right or left internal jugular vein
was cannulated
percutaneously, and a 6F quadripolar catheter (Bard Inc) was advanced
to the right ventricle. The position of the catheter was verified by
the intracardiac recording and by pacing with an external pacemaker. In
open-chest dogs, the catheter was sutured to the surface of the right
ventricle. For protocols requiring a bilateral vagotomy, a paramedian
incision was made on both sides of the trachea, and the vagus nerve was
identified, isolated adjacent to the carotid artery, and then
ligated.
Sampling of Coronary Sinus Effluent
A median sternotomy was
performed, the pericardium was
reflected, and the heart was suspended in a pericardial cradle. A 6F
right coronary Judkins catheter was introduced via an internal jugular
vein and advanced under manual guidance into the coronary sinus (CS).
The position of the catheter was verified by manual palpation before
each sampling of CS blood.
Measurement of Plasma Adenosine, Catecholamines, and Lactate
For adenosine measurements, blood samples from the CS and the
femoral artery were withdrawn simultaneously and placed immediately
into iced tubes containing a stop solution that inhibits adenosine
metabolism. The solution contained 0.2 mmol/L dipyridamole, 4.2 mmol/L
Na2EDTA, 5 µmol/L erythro-9(2-hydroxy-3-nonyl)adenine, 79
µmol/L 
,ß-methylene adenosine 5'-diphosphate, and 25 U/mL
heparin in 0.9% NaCl.12 Samples were obtained immediately
before induction of VF (0 minutes) and after 5 minutes of VF. The
transit time of blood through the catheters was approximately 5
seconds. The samples were cold-centrifuged (4°C) for 20 minutes at
11 000g to separate blood cells from the diluted plasma.
The supernatant fraction was then ultrafiltered during centrifugation
at 900g to 950g for 60 minutes. Adenosine content
was determined from aliquots of reconstituted ultrafiltrate by
high-performance liquid chromatography.
Epinephrine, norepinephrine, and lactate serum levels were determined from blood samples withdrawn from the CS and femoral artery simultaneously. As for adenosine, samples were obtained just before induction of VF and 5 minutes after continuous VF. Serum epinephrine and norepinephrine levels were measured by radioenzymatic assay,13 and serum lactate was measured by an enzymatic method previously described.14
Determination of Transthoracic Defibrillation Threshold
Defibrillation shocks were delivered by a modified commercially
available defibrillator (Lifepak 6, Physio-Control) designed to deliver
a pulse calibrated in units of current through a constant-load current
divider circuit.15 Shocks were calibrated to deliver 3000
V at 400 J across a 50-
load. Variable parallel and series resistors
were adjusted to deliver the desired current and simultaneously
maintain a 50- load to the defibrillator. Electrode area (60
cm2, circular stainless steel) and force (50 N) were
held constant with a precision control system as previously
described.15 The electrodes were covered with a fresh,
thin film of electrode paste (Redux Paste, Hewlett-Packard) and were
positioned over the shaven right and left lateral chest walls at the
transverse level of the heart.
At the beginning of the protocol,
transthoracic resistance was
determined in each dog by a 20-A shock delivered during sinus rhythm.
Voltage delivered across the thorax was measured with a 1000:1 voltage
divider in parallel with the defibrillator output, and delivered
current was measured with a 0.10- resistor in series with the
defibrillator output. Voltage and current wave forms were displayed on
a triggered-sweep storage oscilloscope (model 5113, Tektronix) with a
frequency response from DC to 1 MHz.
DFT was determined by a method previously described.16 In brief, VF was induced by AC delivered through the right ventricular catheter. After induction of VF, the endotracheal tube was clamped at peak inspiration. Twenty seconds after induction of VF, a subthreshold shock, usually starting at 15 A, was delivered, and the current increased in 2- to 5-A steps every 10 seconds until defibrillation occurred. This sequence was repeated with an initial shock strength 2 A lower than the successful shock from the preceding episode. Data were used from second-shock conversions only (a successful shock that followed an initial failed shock), since by definition threshold cannot be determined from a first-shock conversion. Thresholds determined during two consecutive trials were averaged in each phase of the protocol. Thresholds for consecutive trials varied by approximately ±2 A. A symmetrical protocol was implemented, with control DFTs determined before and after an intervention.16
Determination of Transmyocardial Defibrillation Threshold
A
median sternotomy was performed, and the heart was suspended
in a pericardial cradle. A small and a large ventricular patch
electrode (models 0040 and 0041, Cardiac Pacemakers Inc) were sutured
onto the anterior and posterior surfaces of the heart, respectively, so
as to maximize the amount of interventricular septum between the leads.
The electrode surface areas including insulation were 30 and 53
cm2, respectively. The leads were connected to an
external cardioverter defibrillator (Ventak ECD, model 2805, Cardiac
Pacemakers Inc) that can be set to deliver energy-based shocks of 1, 2,
3, 4, 5, 8, 10, 15, 20, 25, 30, 35, or 40 J. DFT was determined by a
subthreshold shock delivered 20 seconds after induction of VF, and two
subsequent shocks of incremental energy levels were applied until
defibrillation occurred. A rescue shock of 40 J was delivered if
defibrillation did not occur with the first three shocks. As described
above for transthoracic shocks, data from second-shock conversions from
two consecutive successful trials were averaged to determine DFT during
each phase of the protocol.
Drugs and Chemicals
The following drugs and preparations were
used for the
experiments. Adenosine (Sigma Chemical Co) was dissolved in normal
saline and diluted to a concentration of 1 mg/mL. Adenosine was
administered as a constant infusion at a rate of 300
µg · kg-1 · min-1 with an
infusion
pump (LifeCare Pump, model 4P, Abbott Laboratories). Atropine
(Elkins-Sinn Inc) was administered intravenously at a dose of 0.04
mg/kg. Dipyridamole (Sigma) was dissolved in normal saline at a pH of 2
to 3 (after addition of HCl) and was diluted to a concentration of 1
mg/mL. It was administered as an infusion over 30 to 60 seconds at a
dose of 0.25 mg/kg. 8-Cyclopentyltheophylline (CPT) (Research
Biochemical Inc) was dissolved in a solution of normal saline with 1N
NaOH in a 3:1 ratio. It was administered as an infusion over 1 minute
at a dose of 5 mg/kg. Isoproterenol (Elkins-Sinn Inc) was diluted in
normal saline or 5% dextrose to a concentration of 4 µg/mL and
infused at a continuous rate of 6 to 12 µg/min.9
Propranolol (Inderal 1 mg/mL, Ayerst Laboratories Inc) was administered
intravenously at a dose of 0.2 mg/kg and at a rate of 1 mg/min.
Experimental Protocols
Coronary Sinus Levels of
Adenosine, Catecholamines, and Lactate
During Ventricular Fibrillation (Protocol 1)
After surgical
instrumentation, the animals were allowed to rest
for 15 to 30 minutes until a stable baseline blood pressure and heart
rate were observed. Blood samples for control measurements of
adenosine, catecholamines (epinephrine and norepinephrine), and lactate
were obtained simultaneously from the femoral artery and CS. VF was
then induced, and samples for adenosine, catecholamines, and lactate
were obtained from the femoral artery and CS 5 minutes after induction
of VF.
Effects of Adenosine on Transthoracic and
Transmyocardial
Defibrillation Threshold (Protocol 2)
In this protocol, DFT was
determined either transthoracically
(protocol 2A) or transmyocardially (protocol 2B). The effect of
adenosine on DFT was evaluated during concurrent administration of
adenosine and dipyridamole, a nucleoside transport blocker, and
isoproterenol. Steady-state effects of isoproterenol were achieved by
its infusion being started 15 minutes before administration of
adenosine. Two minutes after the adenosine infusion was begun (300
µg · kg-1 · min-1),
dipyridamole
0.25 mg/kg was given. One minute after the dipyridamole bolus, DFT was
determined. A control DFT was determined before adenosine and
dipyridamole, and a recontrol DFT was determined 1 hour after the
adenosine and isoproterenol infusions were discontinued (Fig
1
). The two control DFTs were averaged and compared with
DFT determined during adenosine and dipyridamole.
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Pilot experiments in
3 dogs showed that the doses of adenosine
and dipyridamole given in protocol 2 (during sinus rhythm) were
sufficient for achieving CS levels of adenosine comparable to those
obtained after 5 minutes of VF, 1098±476 (sinus rhythm) versus
817±257 nmol/L (VF, protocol 1), P=NS. This was an
important objective because, although DFT was determined after
approximately 30 seconds of VF in the protocol, myocardial effluent
levels of adenosine were intended to correlate with those achieved
after 5 minutes of VF. In the pilot experiments as well as in protocol
2, adenosine infusion was maintained for 15 minutes after the bolus
dose of dipyridamole was given. A reduction in heart rate and blood
pressure (30% to 50%) was maintained during this time frame; heart
rate and blood pressure returned toward baseline within 5 minutes of
discontinuation of the adenosine infusion (Fig 2).
Therefore, the effects of a single dose of dipyridamole, in combination
with adenosine infusion, lasted approximately 15 minutes, which was the
maximum time required to determine DFT. CPT, a selective A1
adenosine receptor antagonist (5 mg/kg), was shown to rapidly reverse
the effects of adenosine and dipyridamole on heart rate (Fig
2B).
Although pretreatment with CPT alone had no effect on heart rate or
blood pressure, it also markedly attenuated the effects of adenosine
and dipyridamole on these parameters (Fig 2C).
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Effect of A1 Adenosine Antagonist on
Transthoracic
Defibrillation Threshold (Protocol 3)
In a subset of dogs from
protocol 2A in which adenosine elevated
DFT, the effects of CPT (5 mg/kg) were evaluated. The effects of
adenosine on DFT were initially determined as described in protocol 2.
The sequence was then repeated after pretreatment with CPT, which was
infused 10 minutes before adenosine and dipyridamole were infused.
Recontrol DFT was determined after a 90-minute washout period of the
antagonist (Fig 1
). Preliminary experiments in 6 dogs showed
that CPT
alone (5 mg/kg) had no effect on DFT (<1.0%, P=NS).
Effects of Adenosine on Transthoracic Defibrillation
Threshold in
Denervated Dogs (Protocol 4)
To determine whether the effects of
adenosine were
catecholamine-independent or catecholamine-dependent, the effects of
adenosine on DFT were determined after surgical (bilateral vagotomy in
1 dog) or pharmacological (atropine 0.04 mg/kg in 4 dogs) vagal
denervation and intravenous propranolol, 0.2 mg/kg, in a subset of dogs
from protocol 2A in which adenosine elevated DFT. DFT was first
determined during control and then during dipyridamole and adenosine
infusion as outlined in protocol 2. After autonomic denervation and
steady-state heart rate and blood pressure were achieved, control DFT
in the denervated dog was determined. Adenosine (300
µg · kg-1 · min-1) and
dipyridamole
(0.25 mg/kg) were then administered as described in protocol 2, and DFT
was determined. Adenosine infusion was then discontinued, and a
recontrol DFT was determined after a 30-minute washout period (Fig
1).
Statistics
Statistical analysis was performed with a
two-tailed
Student's t test for paired or unpaired observations.
Multiple comparisons within and between groups were analyzed by
repeated-measures ANOVA. A value of P<.05 was considered
significant. All data are expressed as mean±SEM.
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Results |
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Coronary Sinus and Systemic Plasma Levels of Adenosine, Catecholamines, and Lactate During Ventricular Fibrillation
Protocol 1
VF resulted in marked elevation of myocardial adenosine, as reflected indirectly by CS plasma concentration. During sinus rhythm (control), the CS adenosine concentration measured in 10 dogs was 64±16 nmol/L. After 5 minutes of VF, the adenosine concentration increased to 816±257 nmol/L (P=.01) (Fig 3
).
Plasma concentrations in the systemic circulation also increased in
response to VF, but to a lesser degree. During control, the plasma
concentration measured from the femoral artery was 58±28 nmol/L, and
after 5 minutes of VF, it was 108±46 nmol/L, P=.04.
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Lactate levels also increased in response to VF. The control CS level
was 22±2 mg/dL, which increased to 38±4 mg/dL during VF,
P=.002 (Fig 3). Plasma lactate levels
determined from
femoral artery samples were not altered by VF.
VF resulted in a marked
release of endogenous catecholamines from the
myocardium. The CS plasma level of norepinephrine during control was
322±54 pg/mL, and it increased to 7935±1936 pg/mL during VF,
P=.004 (Fig 3). Similarly, there was a marked
increase in
myocardial release of epinephrine during VF. The control level was
290±63 pg/mL, which increased to 28 451±10 308 pg/mL during VF,
P=.03. In contrast, systemic increments of endogenous
catecholamines were not observed.
Effects of Adenosine on Transthoracic and Transmyocardial
Defibrillation Threshold
Protocol 2A
The effects of
adenosine on transthoracic DFT were determined in
21 dogs. Control DFT for the group of dogs receiving isoproterenol was
49±6 J, which increased to 67±7 J during adenosine and
dipyridamole
infusion, P=.0003. (The control DFT before adenosine and
dipyridamole was 52±6 J, and recontrol was 47±6 J,
P=NS.)
The mean increment in DFT in response to adenosine for each dog was
49±14% (Fig 4A). Nine of 21 dogs had an increment of
50% to 241% in DFT, whereas 7 of 21 dogs had <15% change in DFT.
Since pilot studies in 5 dogs showed that after 30 seconds of VF,
myocardial effluent concentrations of norepinephrine and epinephrine
increased by approximately 4- and 15-fold, respectively, the effects of
adenosine and dipyridamole were examined in 11 other dogs that did not
receive concurrent isoproterenol infusion. In these dogs, control DFT
was 58±11 J, which increased to 76±17 J during adenosine and
dipyridamole, P<.0001. There was no statistical difference
in the effect of adenosine on DFT between the two groups of dogs that
were either infused or not infused with isoproterenol.
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Protocol 2B
The effects of adenosine on
transmyocardial DFT were determined in
10 other dogs. Control DFT was 10±3 J, which increased to 19±4 J
during adenosine infusion, P=.0003. The mean increment for
each dog was 103±16%; range, 24% to 204% (Fig 4B).
Adenosine and
dipyridamole did not alter transmyocardial resistance. The control
transmyocardial resistance during VF was 59±2 , and the
resistance
during VF when adenosine and dipyridamole were administered was 59±2
, P=NS.
Effects of Adenosine Antagonist (CPT) on Defibrillation
Threshold
Protocol 3
As indicated above, CPT alone had
no effect on DFT; however, it
did abolish the adverse effects of adenosine on transthoracic DFT. In a
subset of 12 dogs from protocol 2A, adenosine increased DFT from 62±9
J (control) to 89±16 J (adenosine), P=.0001. When these
dogs were pretreated with CPT, adenosine had no effect on DFT (Fig
5).
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Effects of Adenosine on DFT in Denervated Dogs
Protocol
4
In contrast to adenosine's effects on DFT in innervated dogs,
adenosine had no effect on DFT when the same dogs were denervated. Five
dogs from protocol 2A were denervated, and the effects of adenosine on
DFT were reassessed. Adenosine increased DFT in the innervated
condition from 38±8 J (control) to 61±15 J (adenosine),
P=.03. However, when these dogs were denervated, adenosine
had no effect on DFT, 49±11 J (control) versus 53±10 J
(adenosine),
P=NS (Fig 6).
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Discussion |
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The major findings in this study are that (1) myocardial effluent levels of adenosine are significantly increased during VF; (2) adenosine markedly elevates transthoracic and transmyocardial DFT; (3) these adverse effects on threshold are mediated by the A1 adenosine myocardial receptor, since they are reversed by the specific A1 receptor antagonist CPT; and (4) the effects of adenosine on defibrillation are mediated through a catecholamine-dependent (antiadrenergic) mechanism. These results suggest that enhanced myocardial production of adenosine during VF has a deleterious effect on defibrillation and in part mediates the increase in threshold related to VF duration.
Although it is recognized that for a given shock strength, the duration of VF before countershock is the major determinant of defibrillation (that is, the longer the VF duration before shock, the more difficult it is to defibrillate), the mechanism governing elevated defibrillation requirements under these conditions is poorly understood. Respiratory acidosis and alkalosis do not elevate threshold requirements, nor does metabolic acidosis or alkalosis.5 On the contrary, neurohumoral substances, such as catecholamines, that are released during hypoxia have been shown to reduce DFT,9 10 11 whereas ß-blockers increase it.9 10
One hypothesis of this study was that the effects of adenosine release during VF are in part responsible for mediating the increase in DFT related to VF duration. Related to this hypothesis, enhanced release of adenosine during VF has been shown to mediate postdefibrillation bradycardia, AV block, hypotension, and myocardial depression,17 18 effects that are reversed by adenosine antagonists.17 18 19 Moreover, aminophylline, a nonselective adenosine antagonist, has been shown to decrease DFT. Although this result was attributed to possible phosphodiesterase inhibition and elevation of cellular cAMP levels,20 this explanation is unlikely, since methylxanthines inhibit phosphodiesterase at levels at least 20-fold greater than those required to antagonize the A1 receptor.21
In this study, we have shown that adenosine has potent adverse effects on DFT, whether evaluated during transmyocardial or transthoracic defibrillation. Preliminary data from a previous study failed to demonstrate any effect of adenosine on DFT. In that study, adenosine (60 µg · kg-1 · min-1) was administered without concurrent infusion of dipyridamole in two dogs.5 Coronary sinus effluent levels of adenosine were not determined, but it is unlikely that adenosine levels were significantly elevated, because DFT was determined 15 seconds after induction of VF, before appreciable endog- enous adenosine is released from the myocardium.
To permit measurement of DFT 30 seconds after induction of VF and yet achieve coronary sinus effluent concentrations of adenosine similar to those observed during cardiac arrest, ie, after 5 minutes of VF, we determined the effluent concentrations of adenosine after 5 minutes of continuous VF. These concentrations were then reproduced during our study by administration of both dipyridamole (0.25 mg/kg IV) and adenosine (300 µg · kg-1 · min-1). These doses produced consistent electrophysiological (sinus slowing) and hemodynamic (decrease in blood pressure) effects in each animal. The CS effluent concentrations of adenosine measured during VF showed a >12-fold increase compared with control measurements during sinus rhythm. In contrast, the other ischemic metabolite measured in CS effluent, lactate, increased by less than twofold. Despite the large increment in effluent concentrations of adenosine achieved in the study, it is likely that our protocol underestimated the true adverse impact of adenosine on DFT. First, adenosine undergoes metabolism during its transit through the sampling catheter.12 Furthermore, during enhanced release of endogenous adenosine (such as occurs during VF, protocol 1), myocardial interstitial concentrations of adenosine are severalfold higher than those in the venous effluent because of the coronary endothelium, which serves as a metabolic and physical barrier to the exchange of adenosine between the interstitium and effluent.22 In contrast, when exogenous adenosine is administered (protocol 2), the concentration gradient of adenosine is inverted (effluent > interstitium)22 because of the uptake and metabolism of adenosine by endothelial cells. Therefore, since infusion of adenosine during defibrillation (protocol 2) was designed to approximate the CS effluent concentration measured after 5 minutes of VF, the interstitial myocardial concentrations of adenosine achieved during protocol 2 were probably less than those actually achieved at 5 minutes of VF, and this factor may account for the finding that adenosine did not adversely affect DFT in all dogs.
The rise in adenosine concentration in CS effluent during VF was
paralleled by a rise in norepinephrine and epinephrine effluent levels.
We initially considered it necessary to infuse exogenous catecholamines
during assessment of the effects of adenosine on DFT (to reproduce
conditions at 5 minutes of VF). In 21 dogs, isoproterenol was therefore
infused with adenosine and dipyridamole (protocol 2A, Fig 1).
Eleven
other dogs did not receive isoproterenol. No statistical difference was
observed in the effects of adenosine on DFT between the two groups of
dogs. We interpreted these results as indicating that sufficient
amounts of endogenous catecholamines, necessary to exert its
facilitatory effect on DFT, were released after 30 seconds of VF.
Therefore, it is important to emphasize that the data for control DFT
(protocols 2A and 2B) also reflect the facilitatory effects of
endogenous catecholamines on DFT.
The unfavorable effects of adenosine on DFT were observed during both transthoracic and transmyocardial defibrillation. These effects, although unequivocal in both circumstances, were more pronounced during transmyocardial defibrillation. The explanation for this discrepancy is not readily apparent. Differences between the two protocols included a different waveform for defibrillation, damped sine wave (transthoracic) versus trapezoidal wave (transmyocardial), and current-based defibrillation (transthoracic) versus energy-based defibrillation (transmyocardial). In contrast to current-based defibrillation, the energy-based method depends on the resistance of the defibrillation load. However, transmyocardial resistance was equivalent in all phases of protocol 2B, eliminating this consideration as a potential explanation.
The cardiac effects of adenosine are mediated by the cell surface A1 and A2 adenosine receptors. Activation of the A1 receptor in ventricular tissue antagonizes the stimulatory actions of catecholamines on the slow inward calcium current (ICa) and on the transient inward current (ITI).23 This effect is mediated by an inhibitory G-protein that decreases intracellular cAMP. The hypothesis that the effects of adenosine on DFT observed in this study were also mediated by the A1 receptor was supported by the complete reversal of the effects of adenosine by the alkylxanthine CPT, a competitive adenosine antagonist with an A1/A2 affinity of 140.24 25
Although alkylxanthines also inhibit phosphodiesterase and the release
of catecholamines from nerve terminals and the adrenal
medulla,21 these actions are unlikely to account for the
effects of CPT observed in this study, for several reasons. First, as
indicated above, phosphodiesterase inhibition occurs at levels far
greater than that required to antagonize the A1
receptor.21 Second, as shown in this study, CPT had little
or no effect on heart rate (Fig 2C), results consistent with a
minimal
effect on phosphodiesterase inhibition and catecholamine release.
Third, although CPT has some effect on the A2 receptor,
partially reversing adenosine- and dipyridamole-induced
hypotension, its effects on the A1 receptor are more
pronounced (ie, reversing heart rate effects, Fig 2B).
Dipyridamole was used in this study to potentiate the effects of adenosine through its inhibition of cellular adenosine uptake.17 26 Although dipyridamole inhibits prostacyclin and also elevates cAMP levels by inhibiting phosphodiesterase,26 the evidence strongly suggests that the primary effects of dipyridamole in this study were related to its inhibition of adenosine transport. This is supported by the abolition of the effects of dipyridamole (and thus adenosine) on defibrillation threshold by CPT. If dipyridamole resulted in an elevation of cAMP and therefore lowered DFT, the effects of adenosine on threshold observed in this study would have been masked. Finally, possible reflex sympathetic activation due to dipyridamole- (and adenosine-) induced hypotension would have been expected to attenuate an elevation of DFT due to the myocardial effects of adenosine.
To determine the mechanism of action of adenosine on DFT, its effects were determined in the same dog during both the innervated and denervated states. In contrast to its adverse effect on DFT in the innervated condition, its lack of effect during the denervated state indicates that its effects on DFT are probably mediated through an antiadrenergic or cAMP-dependent mechanism. These actions are consistent with the known antiadrenergic effects of adenosine in ventricular myocardium, which include negative inotropy, termination of catecholamine-mediated ventricular tachycardia, and suppression of catecholamine-mediated His-Purkinje system automaticity.27 28
One may question whether adenosine elevates defibrillation threshold or merely abolishes the facilitatory effects of catecholamines on threshold. This is essentially a semantic argument, however. As shown in this study, there is an enhanced release of endogenous catecholamines at 30 seconds of VF. Therefore, a "control" defibrillation trial, which consists of several inductions of ventricular fibrillation and subsequent defibrillation, is performed under conditions consistent with elevated endogenous catecholamine levels. As a result, "control" DFT under these conditions is lower (because of the effects of catecholamines) than a theoretical control threshold determined in the absence of catecholamines. Since this latter condition never occurs under physiological experimental conditions, we believe it is conceptually more meaningful to interpret the effects of adenosine on DFT as those consistent with elevation.
The antiadrenergic effects of adenosine enumerated above are consistent with its role as a homeostatic metabolite that functions to maintain an optimal O2 supply-demand ratio within the myocardium. By functioning as a negative feedback inhibitor of ß-adrenergic stimulation, mediated at both prejunctional and postjunctional levels,7 adenosine protects the myocardium against excessive O2 demand. The deleterious effects of adenosine on defibrillation shown in this study, however, are a "paradoxic" consequence of the otherwise protective role associated with the antiadrenergic mechanism of action of adenosine. Thus, it appears that the spectrum and consequences of the myocardial antiadrenergic effects of adenosine are more complex than previously appreciated. The results of this study suggest that release of adenosine during prolonged VF mediates a significant increase of DFT through an antiadrenergic mechanism. Reversal of the effects of adenosine with an A1 receptor antagonist should facilitate defibrillation by lowering threshold.
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Acknowledgments |
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This work was supported in part by a grant from the National Institutes of Health (RO1-44747) and the Michael Wolk Heart Foundation. Dr Lerman is an Established Investigator of the American Heart Association.
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Footnotes |
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Reprint requests to Bruce B. Lerman, MD, Division of Cardiology, New York Hospital–Cornell Medical Center, 525 E 68th St, Starr Pavilion, 4th Floor, New York, NY 10021.
Received June 23, 1994; accepted September 23, 1994.
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