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Ann Thorac Surg 1995;60:1267-1273
© 1995 The Society of Thoracic Surgeons
Divisions of Cardiology and Cardiothoracic Surgery, Departments of Pediatrics and Surgery, The Children's Hospital of Philadelphia, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania
Accepted for publication June 7, 1995.
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Abstract |
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 TopFootnotes Abstract IntroductionMaterial and MethodsResultsCommentConclusionsReferences |
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Methods. We reviewed the results of echocardiography performed before and 8 ± 7 days after (1) Fontan operation for tricuspid atresia (n = 9), (2) hemi-Fontan operation for tricuspid atresia (n = 10), and (3) closure of a ventricular septal defect (n = 13). Measurements were made from images of the left ventricle at end-diastole: (1) apical, septal, and posterior wall thickness; and (2) long- and short-axis cavity diameters, cross-sectional areas, and ventricular volume. Posterior wall thickness to cavity dimension ratio was calculated.
Results. Wall thickness increased in all groups, with the greatest degree of increase after the Fontan operation. Cavity measures decreased most dramatically after the Fontan operation, with less dramatic and equivalent changes noted after the hemi-Fontan operation and ventricular septal defect closure. Posterior wall thickness to cavity diameter ratios were equivalent in all before operation, increased after operation, and were greatest after the Fontan operation.
Conclusions. Changes in ventricular geometry identified as an increase in wall thickness and a decrease in cavity dimension are most dramatic after the Fontan operation. Changes seen after the hemi-Fontan operation are of a milder degree, which may in part explain the excellent clinical course after this operation.
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Introduction |
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TopFootnotesAbstractIntroductionMaterial and MethodsResultsCommentConclusionsReferences |
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In the heart with a functional single ventricle, cavity volume and myocardial muscle mass are increased relative to normal [1–6]. Surgical approaches to this form of congenital heart disease include the Fontan operation or hemi-Fontan operation, procedures that both result in volume unloading of the heart. Despite the salutary effects of reduced ventricular work that accompany volume unloading, exaggerated alterations in ventricular geometry expressed as increased wall thickness and decreased cavity dimensions have been noted on echocardiographic evaluation after the Fontan operation, with concomitant clinical signs of diminished cardiac output [5, 7, 8]. Although changes in ventricular dimensions have been reported after the hemi-Fontan operation as well [9, 10], the clinical course after this procedure is typically smooth and without complications, unlike that after the Fontan operation.
After removal of a chronic volume load, regression of myocardial mass proceeds slowly relative to diminution in cavity size [11]. The persistence of increased muscle mass in the presence of acutely diminished ventricular volume after operation results in increased wall thickness and decreased cavity dimensions. This acute increase in the mass to volume ratio may result in a physiologic state that deleteriously influences myocardial performance in the early postoperative period. Variability in the degree of geometric change may therefore influence the clinical course and outcome in children after repair of congenital heart disease in which a volume load is acutely diminished.
The purpose of this study was to investigate and compare the dimensional changes in ventricular geometry that occur as a result of acute ventricular volume reduction after the Fontan operation and hemi-Fontan operation for functional single left ventricle. In addition, to determine whether this phenomenon is unique to the single-ventricle heart, we studied the dimensional changes of the left ventricle after closure of a large ventricular septal defect (VSD) with elimination of a large shunt and volume load in a two-ventricle system.
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Material and Methods |
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TopFootnotesAbstractIntroductionMaterial and MethodsResultsCommentConclusionsReferences |
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The Fontan operation consisted of a lateral tunnel using Gore-Tex (W.L. Gore & Assoc, Flagstaff, AZ) for channeling of systemic venous blood to the branch pulmonary arteries and augmentation of the confluence of the pulmonary arteries with pulmonary homograft. All operations in the Fontan group were performed before May 1989; after this period, we routinely used the hemi-Fontan operation in all patients, before and in anticipation of the Fontan operation.
All patients in the hemi-Fontan group had operation after May 1989. The hemi-Fontan operation consisted of a side to side association of the superior vena cava with the branch pulmonary arteries, occlusion of inflow of the superior vena cava into the right atrium, and augmentation of the confluence of the branch pulmonary arteries [12].
All patients with a large VSD had a right ventricular pressure of at least two-thirds systemic pressure and a Qp/Qs (pulmonary flow to systemic flow) ratio of at least 2:1 determined at previous cardiac catheterization. The VSD was closed with a right atrial approach using a patch in all. No patient had greater than a small, hemodynamically insignificant residual VSD (Doppler color jet diameter less than 3 mm) after repair.
Echocardiographic Measurements
The echocardiograms were performed on a Hewlett-Packard Sonos 500 or Sonos 1000 system using either 5-MHz or 3.5-MHz transducer frequencies and were recorded on 
-in VHS videotape. Imaging included subcostal sweeps, apical views, and parasternal views. Some patients received oral/rectal chloral hydrate at a dose range of 60 to 110 mg/kg for sedation before study. Echocardiograms performed after the operations were done at a mean of 8 ± 7 days (median, 6 days; range, 1 to 30 days). Echocardiograms were reviewed using an off-line analysis package (Digisonics) interfaced with a digitizing board (Summagraphics) and a personal computer. High-resolution two-dimensional images were searched for at end-diastole, and measurements were averaged over three to five cardiac cycles and recorded. The following measurements were made in each patient before and after operation.
WALL THICKNESS DIMENSIONS.
These consisted of the following: (1) apical wall thickness (apical view), (2) septal wall thickness at the level of the tips of the papillary muscles (short-axis parasternal view), and (3) left ventricular posterior wall thickness at the level of the tips of the papillary muscles (short-axis parasternal view).
CAVITY DIMENSIONS.
Cavity dimensions were measured as follows: (1) long-axis left ventricular cavity length, measured from the level of the coaptation site of the mitral valve to the apex (apical view); (2) long-axis left ventricular cavity cross-sectional area (traced in the apical view); (3) short-axis left ventricular cavity anteroposterior diameter, measured at the level of the tips of the papillary muscles (parasternal short-axis view); and (4) short-axis left ventricular cavity cross-sectional area, traced at the level of the tips of the papillary muscles (parasternal short-axis view).
DERIVED PARAMETERS.
(1) The ratio of posterior wall thickness to left ventricular short-axis cavity diameter was used as an estimate of the mass to volume ratio before and after operation in each of the three groups [13]. (2) Left ventricular volume was estimated for each patient before and after operation by applying a modified biplane area-length method formula, where volume = 0.85 x left ventricular cavity cross-sectional area long axis (apical view) x left ventricular cavity cross-sectional area short axis (parasternal view)/long-axis cavity length (apical view). The calculated volume using this formula has been shown to correlate with, but probably underestimates, the true volume [14]. Because it is a derived index that uses three of the direct measurements, the percentage change of this value was thought to be a good indicator of the three-dimensional cavity volume change that may occur after operation.
Statistics
Values are expressed as mean ± standard deviation. Measurements were indexed as follows [15]: wall thickness measurements to the square root of the body surface area (BSA), ventricular cavity linear dimensions to the cube root of the BSA, and cross-sectional areas and ventricular volume to BSA. Because the interval between the preoperative and postoperative studies was short, no significant change in BSA was assumed to occur. Wilcoxon signed rank test was used to look for differences between measurements made before and after the operation. As a gauge of the degree of change after each specific surgical intervention, the percentage change of each measure was calculated. Kruskal-Wallis analysis was used to test for differences in the percentage changes noted among the three groups for each variable measured. If a significant difference was noted, multiple pairwise comparisons were made using Dunn's test. A p value less than 0.05 was considered significant.
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Results |
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TopFootnotesAbstractIntroductionMaterial and MethodsResultsCommentConclusionsReferences |
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Change in Wall Thickness After the Procedure
A significant increase in mean wall thickness was noted after the Fontan operation and after VSD closure at the apex, interventricular septum, and posterior wall (Table 1
). In the patients who underwent the hemi-Fontan operation, although mean wall thickness increased slightly in all three regions measured, the change was not significant.
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). The smallest percentage change in wall thickness occurred after the hemi-Fontan operation. At the apex, a difference was noted in the percentage changes among the three groups (p = 0.003). The percentage increase in wall thickness was similar for the Fontan operation and VSD closure; both were markedly greater than that after the hemi-Fontan operation (p < 0.05).
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At the posterior wall, a difference in the percentage increase was noted among the groups, although the disparity was of a lesser degree than at the other regions (p = 0.032). The percentage change was significantly greater for the Fontan group than for the hemi-Fontan group (p < 0.05). No statistical difference was present in the percentage change between the Fontan group and the VSD group or between the hemi-Fontan group and the VSD group.
Change in Cavity Dimensions and Volumes After Operation
Preoperative measures of left ventricular long-axis length, short-axis diameter, and long- and short-axis cross-sectional areas were greatest for the Fontan operation group and smallest for the VSD closure group (Table 1
). The preoperative ratio of left ventricular short-axis diameter to long-axis length, a reflection of the shape of the ventricle before surgical volume unloading (ratio = 1 is a sphere; ratio less than 1 is ellipsoid), was significantly greater for the patients with tricuspid atresia than for patients with two ventricles undergoing closure of a VSD (Fontan group, 0.90 ± 0.17; hemi-Fontan group, 0.99 ± 0.27; VSD group, 0.77 ± 0.13; p < 0.01).
Long-axis ventricular length diminished significantly after the Fontan operation but not after the hemi-Fontan operation or VSD closure. Short-axis diameter decreased significantly after the Fontan operation and VSD closure, but not after the hemi-Fontan operation. Long-axis ventricular cavity cross-sectional area diminished significantly after operation in all three groups, as did short-axis ventricular cavity cross-sectional area.
The greatest degree of change in cavity dimensions was noted in the patients who underwent the Fontan operation, for all four indices measured (Fig 2). A dramatic difference was present in the percentage change among the three groups for the long-axis length (p < 0.0001). The percentage decrease in long-axis length was significantly greater for the Fontan group than for either the VSD or the hemi-Fontan group (p < 0.05). No difference in the percentage decrease for long-axis length was noted between the VSD and the hemi-Fontan groups.
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A significant difference was present among the three groups in the percentage change of the long-axis ventricular cavity cross-sectional area (p < 0.005). The percentage decrease in long-axis cross-sectional area was greater for the Fontan group than for the VSD and hemi-Fontan groups (p < 0.05). No difference in the percentage decrease for long-axis cross-sectional area was noted between the VSD and hemi-Fontan groups.
The degree of change was similar for all three surgical groups for the short-axis ventricular cavity cross-sectional area measurement (p = 0.07).
No difference in indexed ventricular volume was noted among the three groups before operation (p = 0.20); however, ventricular volume diminished significantly in all three groups after operation (Table 1). A significant difference was noted in the degree of change in ventricular volume among the groups (p < 0.05) (Fig 3). The percentage decrease in ventricular volume was greater for the Fontan group than for the hemi-Fontan group (p < 0.05), with no significant difference noted between the VSD and hemi-Fontan groups.
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). The preoperative mean posterior wall to cavity dimension ratios were similar for each of the patients in the three groups (p = 0.3). The ratio increased significantly for each group after operation (VSD p < 0.05; Fontan p < 0.01; hemi-Fontan p < 0.05). The postoperative ratio was significantly different among the groups (p < 0.005). Posterior wall to cavity diameter ratio was greater for patients after the Fontan operation than for patients having the hemi-Fontan operation (p < 0.05). No statistical difference was noted in the ratio between the patients who underwent the hemi-Fontan operation and VSD closure.
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Comment |
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TopFootnotesAbstractIntroductionMaterial and MethodsResultsCommentConclusionsReferences |
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Our data also confirm the previous observation that the single left ventricle is more spherical than the left ventricle in a two-ventricle system [16]. The dramatic changes in long-axis cavity measures after the Fontan operation suggest a unique feature of the left ventricle in tricuspid atresia, in that under conditions of maximal stimulus for geometric change after volume reduction, long-axis ventricular dimensions are more likely to diminish in the spherical cavity than they are in the ellipsoid normal left ventricle.
Left ventricular posterior wall thickness to short-axis cavity dimension ratio has been used as an index of the relative adequacy of myocardial hypertrophy in response to ventricular volume [13]. In our study, this ratio was identical in all three groups before operation, suggesting an equivalent stimulus to muscle hypertrophy in response to the volume loads imposed. After operation, the ratio increased in all three groups; however, the greatest degree of ``mismatch'' in wall thickness to cavity dimension was present after the Fontan operation. Postoperative ratios were similar for patients who underwent the hemi-Fontan operation and VSD closure. The physiologic sequelae of wall thickness to cavity diameter mismatch (systolic or diastolic dysfunction) are therefore expected to be greatest after the Fontan operation.
Rationale for Differences in the Degree of Geometric Change After Volume Unloading
The finding of acute geometric change in patients undergoing VSD closure indicates that this phenomenon is a universal one, occurring to some extent after all forms of volume unloading. Factors influencing the varying degrees of geometric change may relate to intrinsic properties or plasticity of the myocardium, in addition to the filling forces contributing to the volume delivered to the ventricle after operation.
The plasticity of the myocardium in response to acute volume unloading may be influenced by the degree and duration of chronic hypoxemia and preoperative volume load, as well as the histologic makeup of the myocardium (ie, collagen, elastin content). Recent evidence suggests maturational differences, as well as differences in the biochemical response to volume load, in the elastin and collagen contents of the right and left ventricle [17, 18].
Assuming a similar myocardial substrate, the ventricular filling pathways and hence the forces contributing to the filling volumes are distinctly different after each of the three operations studied. After VSD closure, the complete cardiac output traverses the pulmonary vasculature with the aid of pulsatile right ventricular thrust. After the Fontan operation, the complete cardiac output must passively traverse the pulmonary vasculature without the impelling force of a ventricular pump. Hence, cardiac output is relatively diminished. Ventricular filling is less significantly impeded after the hemi-Fontan operation because only the superior vena caval flow traverses the pulmonary vascular bed, whereas inferior vena caval flow drains directly into the atria and ventricle. The impact on ventricular filling is therefore most dramatic after the Fontan operation. After the hemi-Fontan operation, a relative redistribution of blood flow from superior to inferior regions of the body may theoretically allow continued unimpeded filling of the ventricle through inferior vena caval flow, which does not obligatorily traverse the pulmonary vascular bed.
Physiologic Significance of Geometric Change in the Functional Single-Ventricle Heart
A sudden and dramatic increase in the proportion of ventricular wall thickness to end-diastolic volume may have deleterious consequences on the physiology after the Fontan operation. Flow across the pulmonary bed, and hence cardiac output after the Fontan operation, are exquisitely influenced by the systolic function and the diastolic properties of the ventricle.
Systolic dysfunction has been well documented after the Fontan operation [3, 5, 16, 19–21]. With an acute alteration in ventricular geometry, in particular with a decrease in the long-axis dimensions of the ventricle, the normal systolic downward motion of the atrioventricular valve annulus may be impaired, diminishing the suction effect influencing pulmonary venous flow and contributing to a decrease in the transpulmonary drive [22].
An inordinate increase in the mass to volume ratio after the Fontan operation may negatively influence ventricular diastolic function. Studies have demonstrated increased preoperative myocardial mass to be a risk factor in performance of the Fontan operation [23, 24]. Excessive myocardial mass may change the receptive properties of the ventricle by altering compliance and impairing passive filling [25]. Active relaxation phases of diastole have been found to be altered after the Fontan operation as well. Gewillig and co-workers [26] found that after acutely abolishing volume overload, and mimicking in a dog model the postoperative mass to volume mismatch seen in Fontan patients, isovolumic relaxation time as well as time to left ventricular minimum pressure increased significantly. Analyzing digitized angiographic frames during diastole in patients who underwent the Fontan operation, Penny and Redington [27] found that 12 of 15 patients had evidence of regional wall motion abnormalities of relaxation and suggested that incoordinate diastolic motion was a major contributor to diastolic dysfunction. Our findings of regional differences in the degree of wall thickness increases after the Fontan operation, in contrast to the uniform increase noted at the three regions after VSD closure, may cause the regional diastolic motion abnormalities reported.
Other Studies
Although few data exist concerning the state of ventricular mass and volume immediately after repair of VSD, long-term studies have shown a relative increase in muscle mass in conjunction with normal ventricular volumes 2 years after operation [28, 29]. Others have used echocardiography to investigate the early changes in ventricular geometry after volume unloading of the functional single ventricle and have found similar relative degrees of dimensional alterations as in our study. Berman and Kimball [9] looked at the cross-sectional ventricular areas in short axis before and after bidirectional cavopulmonary anastomosis and found a mean decrease of 22% in 14 patients with morphologic left ventricle, identical to the 14% decrease noted in our study. Seliem and colleagues [10] retrospectively studied 35 patients with hypoplastic left heart syndrome with single right ventricle and found an increase in anterior wall thickness of 13% and a decrease of 33% in ventricular volume after the hemi-Fontan operation. In our study of morphologic left ventricle, wall thickness increases ranged from 7% to 16% depending upon the region investigated, and volume diminished by 24% after hemi-Fontan. Chin and associates [8] investigated the changes in ventricular volume after the Fontan operation and found a decrease of 52% in patients with single right ventricle and of 40% in those with single left ventricle, similar to the decrease of 46% found in our analysis.
Limitations
Because this was a retrospective analysis, selection bias may be present in our subjects; the patients studied were only those for whom echocardiograms were available early after their operations. Bias may exist in that patients with echocardiograms exhibiting pleural and pericardial effusions were excluded, although some patients included in the analysis later experienced substantial effusions. Differences in age between the groups may in part explain some of the geometric changes seen and may in fact illustrate the differences in intrinsic myocardial properties mentioned above. However, these were not specifically tested for and are only speculative at this time.
All patients who underwent VSD closure and hemi-Fontan survived, whereas 3 patients died after the Fontan operation. Because of the relatively small number of patients analyzed, statistical inferences concerning a relation between the degree of geometric change and outcome could not be made, despite our clinical observation that outcome is poor in the presence of severe geometric change. The small numbers were also limiting in that a multivariate analysis of the many other potential factors affecting the degree of ventricular contraction was not feasible.
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Conclusions |
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TopFootnotesAbstractIntroductionMaterial and MethodsResultsCommentConclusionsReferences |
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Footnotes |
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TopFootnotesAbstractIntroductionMaterial and MethodsResultsCommentConclusionsReferences |
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TopFootnotesAbstractIntroductionMaterial and MethodsResultsCommentConclusionsReferences |
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