(Circulation. 1996;93:585-593.)
© 1996 American Heart Association, Inc.
Articles |
Enhanced Regional Deformation at the Anterior Papillary Muscle Insertion Site After Chordal Transsection
From the Departments of Medicine and AMES-Bioengineering, University of California San Diego, La Jolla, Calif.
Correspondence to James W. Covell, MD, University of California, San Diego, Department of Medicine 06l3J, 9500 Gilman Dr, La Jolla, CA 92093.
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Abstract |
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Background Clinical and experimental studies of mitral valve replacement have shown a depression of ventricular function after chordal transsection; most recent studies have proposed that this is secondary to a depression of local function near the papillary muscle insertion site. However, there is no direct experimental evidence for changes in local fiber shortening in the wall of the left ventricle overlying the papillary muscle. Accordingly, we investigated the effect of chordal transsection on left ventricular shape and on three-dimensional regional deformation of the myocardium near the insertion of the anterior papillary muscle.
Methods and Results In six open-chest dogs, two sets of three transmural columns of radiopaque markers were implanted in the anterior wall, one set at the tip of the papillary muscle (basal) and one at the site of papillary muscle fiber insertion (apical). A Björk-Shiley mitral valve was placed in the left atrium adjacent to the native valve. Markers were then tracked with biplane cineradiography, and deformation was quantified with the use of finite strain analysis. Chordal transsection resulted in reduced left ventricular end-systolic pressure and slowed relaxation. After chordal transsection, outward displacement of the ventricular wall and transverse shearing deformation were observed in the area of the papillary muscle during isovolumic contraction. Circumferential and radial strains during ejection were maintained at our basal site and enhanced on our apical site.
Conclusions Chordal transsection led to enhanced local shortening and wall thickening and regional strain nonuniformity. These results indicate that chordal transsection induces an unloading of myocardium at the papillary muscle insertion site and that the resulting heterogeneity of regional function is the mechanism for the reduced global function and slowed ventricular relaxation.
Key Words: mitral valve • chordal transsection • myocardial contraction
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Introduction |
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TopAbstractIntroductionMethods Results Discussion References |
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The importance of the mitral apparatus to left ventricular function has been suggested in many clinical studies of mitral valve replacement.1 2 3 4 5 6 7 8 9 10 These studies documented that surgical mitral valve replacement with chordal preservation maintained postoperative ejection performance, while left ventricular ejection performance was reduced in patients undergoing mitral valve replacement with chordal transsection.1 2 7 9 Moreover, experimental studies11 12 13 14 15 16 have described a decrease in global left ventricular function after chordal transsection that supports the clinical findings. Several experimental13 14 17 and clinical studies5 7 8 18 that used echocardiography or piezoelectric dimension gauges to assess global left ventricular dimensions have reported a decline in segmental function in the area of the papillary muscle as well as a depression of global function. Indeed, Pitarys et al8 and Corin et al18 have shown that segmental function is reduced only in the area of the papillary muscle after chordal transsection. A variety of studies have also reported changes in global ventricular shape after chordal transsection.9 15 16 Others have shown that there is a local shape change in the left ventricular wall overlying the papillary muscle.8 18 Only one study to date has directly examined local dimension changes in the wall of the left ventricle overlying the papillary muscle: In this study Yun et al17 showed that preload recruitable stroke work calculated from left ventricular pressure and a local wall thickening gauge is depressed after chordal transsection.
Although the exact mechanism of the global depression of function observed after chordal transsection remains unclear, most evidence points to a local depression of function near the papillary muscle. Both loss of isovolumic recruitment of fiber length9 11 15 19 originally proposed by Rushmer20 and increased systolic stress secondary to local or global systolic shape changes have been proposed to explain this depression. Unfortunately, no study to date has directly examined local function at the site of the papillary muscle insertion before and after chordal transsection. This type of study seems essential to directly test the proposal that there is a local depression of function. Indeed, in the absence of direct myocardial injury this depression seems unlikely. Thus, in the present study, we examined the effects of transsection of the chordae of the anterior papillary muscle (APM) on the three-dimensional regional deformation of the myocardium near the insertion of the papillary muscle. This approach allowed us to directly measure strains across the left ventricular wall.21
In contrast to previous studies after chordal transsection, we found enhanced regional circumferential systolic shortening and increased wall thickening in the area of the papillary muscle insertion. These results support the hypothesis that after chordal transsection, myocardium near the papillary muscle is unloaded and local shortening is enhanced. We propose that the depression of global function observed after chordal transsection is secondary to local heterogeneity of function. This proposal is supported by the slowed ventricular relaxation observed in this and other studies.
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Methods |
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The University of California, San Diego, is accredited by the American Association for Accreditation of Laboratory Animal Care (AAALAC). All experiments were conducted according to AAALAC guidelines for the use of animals in research and were approved by the local faculty animal use committee.
Surgical Preparation
Six adult mongrel dogs weighing 27 to 35
kg were
anesthetized with sodium pentobarbital, intubated, and
ventilated with oxygen. Anesthesia was maintained with
additional barbiturate injections (50 to 100 mg/h). In each study, the
chest was opened via median sternotomy and left fourth intercostal
space thoracotomy. The pericardium was opened and the heart suspended
in a pericardial cradle. We elected to study the effects of sectioning
of the anterior left ventricular chordae tendineae for
several reasons. Hansen et al19 reported that the chordae
of the anterior and posterior mitral leaflets had an additive influence
on global left ventricular systolic
performance, but the contribution of the anterior chordae was
more important. Furthermore, Yun et al17 showed that the
influence of the anterior chordae on local left ventricular
systolic function was greater than that of the posterior
chordae at both papillary muscle insertion sites. In addition, it is
technically difficult to implant bead sets into the posterior wall of
the left ventricle. To aid in locating the anterior papillary muscle
and positioning marker beads, we inserted a finger into the left
ventricle via the left atrial appendage. A purse-string suture
(R-833 polyester fiber) was placed around the base of the left atrial
appendage for hemostasis during insertion of the finger. The positions
of the tip and insertion of the papillary muscle were outlined on the
epicardial surface with a marking pen. Columns of gold beads (1-mm
outer diameter, approximately 2-mm spacing) were inserted across the
ventricular wall from the epicardial surface using a
trocar. Three columns were placed at the tip of papillary muscle (basal
site) and three at the papillary muscle insertion (apical site). After
marker implantation, larger (1.6 mm) lead beads were sewn to the
epicardial surface above each column, at the apex of the left ventricle
(apex bead), and at the bifurcation of the left main coronary
artery (base bead). A Konigsberg micromanometer was
inserted into the left ventricular apex and matched to a
fluid-filled, 120-cm 7F pigtail catheter passed from the femoral
artery and positioned inside the left ventricle.
A second purse-string suture was placed to secure a Björk-Shiley mitral valve using the method of Ohtani et al.22 In brief, a No. 5 silk suture threaded on a semicircular (80-mm diameter), 16-gauge needle was passed below the coronary sinus through the right atrium, pushed out from the superior medial aspect of the right atrium, and anchored at the base of the left atrium. A 25-mm Björk-Shiley mitral valve secured within a grooved Teflon ring that was attached to a 2-mm ODX 40-cm long stainless steel rod was inserted into the left atrium through the proximal purse-string suture (polyester) and secured in place with the second purse-string suture (silk).
Data Acquisition
Three-dimensional bead positions were
detected with biplane
cineradiography. Our cameras recorded cinefilm
asynchronously at 120 frames per second. Data were recorded with
the respirator off at end expiration for approximately 5 to 8 seconds.
During each run, the following were recorded on an 8-channel chart
recorder (Gould Instruments): ECG, aortic pressure, left
ventricular pressure, left ventricular dP/dt,
and cinemarks for correlation of film frame and
hemodynamic events. Satisfactory dP/dt data were
obtained for five of the six dogs.
Experimental Protocol
Before chordal transsection, we
recorded two cine runs in a
steady hemodynamic state at low (<8 mm Hg) and at
high (
8 mm Hg) left ventricular
end-diastolic pressure (EDP) using warm homologous
blood transfusion as needed to elevate EDP. Data obtained prior to
chordal transsection at low EDP in four dogs in this study were
reported previously23 ; data at high EDP matching the EDP
after transsection are reported here. A control left
ventricular injection of contrast (Hypaque, Wintrop
Pharmaceuticals) via the pigtail catheter revealed that there was no
regurgitation beyond the normal mitral valve. After the
control data were recorded, the anterolateral papillary muscle
chordae tendineae were cut using Love-Kerrison Rongeur Forceps (3-mm
bite) inserted directly through the left ventricular wall
(Fig 1A
). Warm homologous blood was infused to keep the
left ventricular peak systolic pressure more than
90 mm Hg and to match the left ventricular
end-diastolic pressure (within 3 mm Hg) with
pretranssection values. After transsection, the forceps were removed
and the left ventricular wall repaired with 2-0 silk
mattress sutures. A repeat left ventriculogram revealed no
regurgitation beyond the Björk-Shiley valve. We
again recorded two cine runs in steady state at low and high EDP.
After the study, the heart was fixed by high-pressure retrograde
aortic root perfusion with buffered glutaraldehyde
(2.5%), which closed the aortic valve and perfused the
coronary vessels.24 All hearts were removed and
stored in 10% buffered formalin (Fisher Scientific).
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In postmortem examination, we confirmed that all the chordae tendineae attached to the APM were cut in each dog. Hearts were positioned on a cutting jig,21 and tissue blocks containing the markers and having edges parallel to the in situ circumferential and longitudinal directions were removed. In each block, the junction of the papillary muscle with the left ventricular wall was grossly identified and the blocks were carefully examined under fluoroscopy to determine the placement of markers relative to the papillary muscle border.
Data Analysis
The three-dimensional coordinates of the
implanted beads
were reconstructed from the biplane images at end diastole,
end isovolumic contraction, and end systole. End diastole
was taken as the time of the peak QRS wave of the ECG. The
micromanometer and aortic pressure profiles were
carefully matched with the catheter in the left ventricle before it was
withdrawn into the aortic root. Then, pressure at the nadir of the
dicrotic notch from the fluid-filled aortic root catheter was used
to estimate end-systolic pressure on the
micromanometer tracing. In each selected beat, bead
positions at aortic valve opening (deformed configuration) were
compared with bead positions at end diastole (reference
configuration) to calculate isovolumic contraction strain, and bead
positions at end systole (deformed configuration) were compared with
end isovolumic contraction (reference configuration) to calculate
ejection strain. Moreover, bead positions at end diastole
after chordal transsection (deformed configuration) were compared with
those at end diastole before chordal transsection
(reference configuration) to assess end-diastolic
remodeling strain. Finite (large) deformation theory was used to
calculate strains after the method described by Waldman et
al.25 Groups of four beads formed tetrahedra with bases
nearly parallel to the epicardial surface (three beads differing in
depth by no more than 1.5 mm) and heights of between 1.9 and 4.1 mm.
Holmes et al23 reported previously that a boundary between
the myocardial wall and the papillary muscle could be distinguished
grossly and was also revealed in muscle fiber angle measurements. Thus,
we believed that the homogeneous strain theory was
inappropriate at the papillary muscle border and excluded tetrahedra
spanning the border between inner wall and papillary muscle from
analysis. Tetrahedra within the myocardial wall were grouped
into outer and inner halves of the wall at the basal and apical sites.
Marker tetrahedra were used to calculate six independent finite
strains. These strains were expressed in a local coordinate system
derived from the apex and base markers and the three epicardial beads.
In this system, the first axis is circumferential, the second axis is
longitudinal, and the third axis is radial. The three normal strains
describe shortening or lengthening along the circumferential
(E11), longitudinal (E22), or radial
(E33) axes. The three shear strains
(E12, E13, E23)
describe changes in angle between pairs of axes that were mutually
perpendicular in the reference configuration. We used three distances
as the indices of local shape change as shown in Fig 1B
. L
indicated
the distance from base to apex; L1 indicated the distance from base to
a perpendicular from the deepest bead at the basal site; and S
indicated the perpendicular distance from the base-apex axis to the
deepest bead at the basal site.
Statistical Analysis
Data from tetrahedra in the outer and
inner halves of the wall
in the area of APM insertion were averaged for each dog. All values are
reported as mean±SD unless otherwise noted. Changes in regional
strains and shape indices were analyzed using ANOVA with a
repeated measures design (SuperANOVA version 1.1, Abacus
Concepts). When significant differences were detected by ANOVA,
contrasts were performed to determine which individual differences were
statistically significant; in these cases, Bonferroni's correction for
multiple comparisons was used. End-diastolic remodeling
strain data were compared with 0 using a one-sample t
test (StatView, version 4.0, Abacus Concepts). An effect with
P<.05 was considered statistically significant.
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Results |
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Hemodynamics and Local Shape Change
Hemodynamic indices before and after chordal transsection are shown in Table 1
. Heart rate did not
change after chordal transsection. When compared at matched
end-diastolic pressure, left ventricular
end-systolic pressure decreased significantly after chordal
transsection from 138±39 to 118±30 mm Hg. Although maximal dP/dt
did
not change, minimal dP/dt fell by an average of 20% after chordal
transsection (P<.05). Diastolic changes were
assessed by comparing dimensions at the same
end-diastolic pressure before and after chordal
transsection (Table 2). Chordal transsection did not
affect apex-base length (L) and S at end diastole.
However, L1 at end diastole increased significantly by an
average of 2.8 mm after chordal transsection, indicating a shift of the
tip of the papillary muscle toward the apex. There were significant
effects of chordal transsection on cardiac dimensional changes during
systole. With chordae intact, dimensional changes were small during
systole. After chordal transsection, apex-base length (L), L1, and
S increased significantly during isovolumic contraction. During the
ejection phase, there were no further changes in dimensions after
chordal transsection.
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Strain Data
Tetrahedra were selected and grouped by
transmural location as
described in "Methods." There were 11±3 tetrahedra (range,
6 to
15) in the outer wall and 13±8 tetrahedra (range, 7 to 28) in the
inner wall at the basal site. At the apical site, there were 10±9
tetrahedra (range, 2 to 26) in the outer wall and 12±13 tetrahedra
(range, 2 to 38) in the inner wall. It was difficult to obtain
tetrahedra that met our selection criteria with all four beads in the
papillary muscle at either apical or basal sites. We were able to
achieve this in only three hearts and have not included these data
here.
The average systolic strain data at each site and each depth at
baseline and after chordal transsection are shown in Tables 3
and 4 and Fig 2. In analyzing
the data, we have separated the total systolic strain into two
components: isovolumic and ejection strains. Isovolumic strain
describes the deformation occurring between end diastole
(reference configuration) and aortic valve opening (deformed
configuration). Ejection strain describes the deformation occurring
between aortic valve opening (reference configuration) and aortic valve
closure (deformed configuration).
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Isovolumic Contraction Strains
Although there were some
significant longitudinal (difference
between basal and apical site) and transmural (between outer and inner
wall) variations in isovolumic strains detected with ANOVA, isovolumic
contraction strains prior to chordal transsection were quantitatively
small and represented only a small portion of total
systolic strain.
After chordal transsection, there were large increases
in longitudinal
lengthening (+E22) at the inner wall at both sites and
twofold to threefold increases in positive longitudinal-radial
shear in the inner wall. These changes are shown in Table 3 and
summarized in Fig 2.
Ejection Strains
During control, wall thickening strain
(E33) and
longitudinal-radial shear strain (E23) were both
greater in the inner wall than the outer wall. There were no
significant differences of ejection strains between the basal and
apical sites during control.
After chordal transsection, inner wall
circumferential shortening
strain (E11) and radial strain (E33) at the
apical site were greater than at the basal site. There was a transmural
increase in E11 and E33 only at the apical site
(Table 4).
In summary, with rare (and quantitatively
small) exceptions,
systolic finite strains (both isovolumic and ejection) in the
outer wall did not change with chordal transsection. This was not the
case for the inner wall, where there were significant changes in
systolic strains with chordal transsection as well as regional
differences. During isovolumic contraction, all strains and all changes
with chordal transsection were small except longitudinal-radial
shear strain (E23). Large positive values of
E23 in the inner wall during isovolumic contraction were
observed after chordal transsection. During the ejection phase, there
were no significant differences between basal and apical sites in the
control condition. However, chordal transsection influenced ejection
strains differently at the basal and apical sites. Both circumferential
shortening strain (E11) and wall thickening strain
(E33) were enhanced at the apical site and unchanged at the
basal site (Fig 2).
End-Diastolic Remodeling Strains
We found a small shift in
the position of the papillary muscle
(only L1 lengthening, from 31.4±4.8 to 34.2±6.1 mm) at end
diastole after chordal transsection. However, chordal
transsection caused no significant remodeling deformation within the
tissue (remodeling strains E11,
E22, and E33 were not significantly
different from zero in a one-sample t test at matched
end-diastolic pressure) (Table 5).
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Discussion |
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TopAbstractIntroductionMethods Results Discussion References |
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The importance of the mitral apparatus to left ventricular function has been suggested in several clinical studies1 2 7 9 that showed that mitral valve replacement with chordal transsection depressed postoperative ejection performance compared with mitral valve replacement with chordal preservation. In the present study, sectioning of the chordae tendineae to the anterior papillary muscle in the dog resulted in a decease in left ventricular systolic pressure. This decrease in systolic pressure was associated with a local systolic outward displacement of the ventricular wall in the area of the papillary muscle and enhanced circumferential shortening and wall thickening in the myocardium overlying the papillary muscle insertion site. Although previous studies have shown bulging in the wall over the papillary muscle, this is the first study to document enhanced function and transverse shearing deformation in the inner wall at the site of the papillary muscle insertion and a shift in the diastolic position of the papillary muscle. Since systolic wall motion abnormalities produced, for example, by changing the local activation sequence have been shown to produce decreases in pressure generation,26 27 28 we propose that the alteration of performance that occurs with chordal section is due to heterogeneity of shortening caused by local "unloading" of the myocardium at the papillary muscle insertion site and not (as suggested by previous studies) due to a local depression of regional function.
Effects of Chordal Transsection During
Diastole
To examine whether there were changes in the diastolic
configuration of the papillary muscle, marker positions were compared
before and after chordal transsection at the same
end-diastolic pressure. At end diastole,
we found that the dimension L1 increased significantly after
chordal transsection. This motion of the papillary muscle toward the
apex is consistent with severing the connection between the
papillary muscle and the base of the heart and suggests that the
chordae tendineae are normally under tension even during
diastole (Fig 3).
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Earlier studies reported that
base-apex distance at end
diastole increased significantly after chordal
transsection,14 15 16 29 but
we did not find a significant
change in L. This difference may be explained by the fact that only the
anterior chordae were transsected in our study, while all chordae
attached to the anterior and posterior papillary muscles were disrupted
in earlier studies. We found no significant change in the perpendicular
distance from the base-apex axis to the deepest bead (S) at end
diastole. Salter et al29 and Sarris et
al14 also found that the minor-axis
end-diastolic dimension did not change after chordal
transsection. Shintani and Glantz16 reported that
septum–free wall and anteroposterior left ventricular
dimensions at end diastole increased significantly after
chordal disruption, but EDP was allowed to increase in their study. Yun
et al15 found that the end-diastolic
septal-lateral axis dimension increased significantly, but
anterior-posterior axis dimension did not change significantly
(P=.06) at matched EDP. However, even these latter two
studies found a more ellipsoidal shape at end diastole
despite the increased minor axis dimension.15 16 On
the
basis of our results and the ellipsoidal shape change in earlier
studies, we conclude that the end-diastolic shape
change after chordal transsection occurs primarily along the major
axis. Our results indicate that the majority of this change occurs at
the base of the heart between the papillary muscle insertion and the
mitral annulus. This shape change after chordal transsection may be
caused by the loss of the tethering effect of the chordae, which are
normally parallel to the base-apex axis when the mitral valve is
open (Fig 3). We found no significant end-diastolic
remodeling strains after chordal transsection. This would indicate that
the increase in L1 reflected a transposition (rigid body motion) of the
papillary muscle without deformation of the overlying inner wall
tissue. This also supports our impression that the tip of the papillary
muscle is not well integrated with the overlying
myocardium. Moreover, the absence of significant
diastolic remodeling strains argues against
diastolic changes in local muscle fiber length as the cause
of changes in regional function after chordal transsection.
Effects of Chordal Transsection During Isovolumic
Contraction
We found that the shape of the left ventricular wall
in the area of papillary muscle insertion changed substantially during
isovolumic contraction. Increased base-apex distance and local
bulging were observed at end isovolumic contraction after chordal
transsection (Fig 4), and these changes continued to end
systole. If chordae tendineae tension increased rapidly during
isovolumic contraction, the loss of this force after chordal
transsection would explain the shape changes we observed. Both lateral
bulging and increased base-apex distance are consistent
with the orientation of the chordae when the mitral valve is
closed.
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We found significant isovolumic longitudinal stretching
(+E22) and shear deformation (E23) in the inner
wall at both basal and apical sites after chordal transsection.
Longitudinal stretching (+E22) during isovolumic
contraction was consistent with significant lengthening of the
base-apex distance. During isovolumic systole before chordal
transsection, we found a negative longitudinal-radial shear strain
(E23) in the inner wall at the apical site and essentially
no shear at the basal site. However, we found large positive values of
E23 during isovolumic systole after chordal transsection,
consistent with observed motion of the papillary muscle toward
the apex (Fig 2 inset).
Effects of Chordal Transsection on Strains During
Ejection
Circumferential shortening (E11) and wall
thickening
(E33) were enhanced after chordal transsection at the
apical site, but no changes were observed at the basal site. We did not
find any decline of ejection strains near the papillary muscle after
chordal transsection despite systolic bulging in this area.
Because global left ventricular function and regional segmental wall motion were depressed after chordal transsection, earlier studies speculated that a loss of inward force generated by attachment of the papillary muscle to the annulus may decrease circumferential shortening.5 9 12 15 Rushmer20 speculated that in the normal heart, the early movement of the atrioventricular ring toward the apex produced by papillary muscle shortening during isovolumic contraction may increase preload in circumferential midwall fibers and enhance circumferential strain during the ejection phase via the Frank-Starling mechanism. A loss of this early systolic shape change after chordal transsection would induce a decline in circumferential strain during ejection. However, this hypothesis is not consistent with our data. We did not find any circumferential remodeling strain or change of circumferential strain during isovolumic contraction that might affect ejection strain via the Frank-Starling mechanism, nor did we observe a decline of ejection strains. Indeed, our data indicate that regional function in the area of papillary muscle insertion may be enhanced by the unloading effect caused by a loss of the connection of the tip of the papillary muscle to the base of the heart after chordal transsection. Our results appear inconsistent with earlier studies describing a reduction of regional left ventricular function in the area of papillary muscle insertion after chordal transsection.13 14 17 Hansen et al13 and Sarris et a14 evaluated regional systolic function at the base of the papillary muscle using the regional end-systolic pressure-dimension relation and concluded that there was a decline of regional end-systolic elasticity. Yun et al17 evaluated segmental left ventricular function using the slope of the segmental stroke work–end-diastolic wall thickness relation and found a decline in segmental function not only in the areas subtending papillary muscle insertions but also in remote left ventricular regions. However, all of these studies used uniaxial measurements to examine regional deformation. Since we observed large changes in transverse shear with chordal transsection, it is quite likely that the uniaxial measurements were affected by the shearing deformation.21 23 For example, the positive E23 shears observed at both sites would be reflected in the dimension measurements of Hansen et al and Sarris et al as a chord lengthening. We did not find a change in ejection strains at the base of the papillary muscle although strains at our apical site were enhanced. These data support our hypothesis that it is regional heterogeneity of function that is associated with the decrease in global left ventricular function. Because ejection pressure was depressed after chordal transsection, a part of the enhancement of circumferential shortening and wall thickening may derive not only from the direct unloading at the papillary muscle insertion site but also from reduction of afterload. However, the unloading effect was limited to the apical site, which argues against a global effect of pressure reduction.
To explain the different effect of chordal transsection at the apical papillary muscle site, we considered the muscular architecture of the left ventricle in the area of the papillary muscle insertion.30 At the apical site, papillary muscle fibers insert to all layers. In contrast, papillary muscle fiber insertions at the basal site are much shallower and are limited to inner wall, more longitudinal fibers. We propose that the deep insertion of papillary muscles at the apical site enhanced the unloading effect of chordal transsection.
Cobbs et al31 described in detail the pathological findings among seven patients who died after mitral valve replacement and suggested that spontaneous rupture of the left ventricle could occur in some patients after excision of the mural leaflet and the papillary muscles. In that study, they speculated that there was abnormal longitudinal traction on the left ventricular endocardium that sometimes separated along natural lines of cleavage or foci of minor trauma after mitral valve replacement. Based on our results, division of chordae tendineae enhanced the longitudinal stretching in the inner wall over the papillary muscle and radial-longitudinal shear (pulling down the tip of papillary muscle toward the apex) during isovolumic contraction. Both these factors would tend to increase the chance of endocardial injury in the basal areas. In clinical settings, left ventricular wall stress is considerably greater in the chronically dilated heart particularly in patients with marginal cardiac reserve.32 Moreover, the shape change during systole after mitral valve replacement in clinical patients may be greater than in our results, because all chordae tendineae attached to both anterior and posterior papillary muscles are disrupted. These factors would all work to enhance the possibility of endocardial injury.
Effect of Chordal Transsection on Relaxation
Peak negative
dP/dt was reduced after chordal transsection, a
finding consistent with earlier studies.14 16 The
time constant of left ventricular relaxation is increased
with wall motion asynchrony or nonuniformity of regional
ventricular function.33 34 Therefore, Shintani
and Glantz16 speculated that wall motion asynchrony or
nonuniformity of left ventricular function produced after
chordal transsection slowed left ventricular relaxation.
Local hyperkinesia, nonuniformity of regional ejection strains, and the
reduction of minimal dP/dt after chordal transsection in our study
support their speculation.
Shintani and Glantz16 also reported a decrease of maximal dP/dt; this result is not consistent with our observation that maximal dP/dt did not change significantly. The differential effect of chordal uncoupling on maximal and minimal dP/dt may reflect the fact that only the anterior chordae were transsected in our study, while all chordae were disrupted in Shintani and Glantz's work. Sarris et al14 reported that the percent change in maximal dP/dt (-19%) after chordal transsection was smaller than the change in minimal dP/dt (-32%), in agreement with our results. Moreover, Yun et al17 showed that transsection of only the anterior chordae did not change maximal dP/dt despite a significant decrease of maximal dP/dt after transsection of both anterior and posterior chordae tendineae.
Study Limitations
Several potential limitations of this study
should be considered.
First, we used open-chest dogs in which the pericardium was also widely
opened. Both these factors might enhance the magnitude of the changes
seen in the current study. The depression of function observed with
anesthesia might enhance the gradients in function
observed. All chordae attached to both anterior and posterior papillary
muscles are disrupted in standard mitral valve replacement. In these
clinical settings, regional heterogeneity of function
may be enhanced or reduced compared with our study.
Another potential
difficulty is the method of cutting the chordae
tendineae. It is possible that the ring supporting the mitral
prosthesis could have restrained normal motion of the mitral
annulus. However, the ring in this case is anchored in atrial tissue
(
5 mm from the annulus) and seems unlikely to restrain annular
motion. Moreover, Rayhill et al35 have shown that the
presence of rigid ring fixation does not influence left
ventricular systolic performance.
Love-Kerrison Rongeur Forceps were inserted through the anterior wall
into the left ventricle. Although this procedure caused some myocardial
injury, we did not find a significant reduction of any ejection
strains; indeed, strains were enhanced at the apical site and unchanged
at the basal site. However, since the site of injury was basal to both
sites, it is possible that the injury enhanced the changes in inner
wall longitudinal shortening. We think that this is unlikely, since the
injury was somewhat lateral to the site of measurement and was also
transmural. No changes in outer wall strains were observed.
Summary
Transsection of the chordae to the anterior papillary
muscle in a
preparation where mitral regurgitation was prevented
led to a systolic bulge in the area of the muscle and a
diastolic apical displacement of the tip of the papillary
muscle. These changes were accompanied by enhanced systolic
deformation at the papillary muscle insertion site without changes at
an adjacent site. The data support the concept that the chordae are
under tension at all times including diastole. Moreover,
the results indicate that the loss of ventricular function
after chordal transsection is due to heterogeneity of
regional wall motion at the site of insertion of papillary muscle
fibers into the ventricular wall, and not to local
depression of regional function as implied by earlier studies.
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Acknowledgments |
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This study was supported by grants from the National Heart, Lung, and Blood Institute (HL-32583), the Health Research Council of New Zealand, and the Whitaker Foundation.
Received March 28, 1995; revision received August 10, 1995; accepted September 18, 1995.
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