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Ann Thorac Surg 1995;60:492-493
© 1995 The Society of Thoracic Surgeons
Division of Cardiothoracic Surgery, University of California, San Francisco, 505 Parnassus Ave, San Francisco, CA 94143-0118
To the Editor:
Failure of hemi-Fontan physiology in the two patients reported by van de Wal and associates [1], among other factors, may be related to failure of relief of systemic outflow obstruction. The first patient in their report had Taussig-Bing physiology with coarctation of the aorta, in which case one should suspect the presence of subaortic obstruction. The second patient had double-inlet left ventricle with transposed great arteries and a rudimentary right ventricle communicating with the left ventricle through a bulboventricular foramen (BVF).
Subaortic stenosis in single-ventricle equivalents with pulmonary overcirculation takes several forms. In the most common type, namely double-inlet left ventricle, the stenosis either is isolated to the BVF, is present in association with obstruction within the outflow chamber, or is combined with aortic annular hypoplasia. In patients with tricuspid atresia and transposition of the great arteries, the stenosis is usually at the level of a restrictive BVF. In the less common form of double-inlet ventricle, where the dominant ventricle is a morphologically right ventricle, the obstruction is usually located at the level of the hypoplastic subaortic infundibulum or is caused by a common atrioventricular valve displaced into a hypoplastic left ventricular outflow tract.
It is well established that subaortic stenosis is a risk at some stage in patients in whom the aorta arises from a rudimentary right ventricle that communicates with a functionally single ventricle through a BVF. The development of BVF obstruction is associated with pressure overload, hypertrophy, and fibrosis of the functionally single ventricle [2]. The probability of its appearance is increased by smallness of the BVF (an initial BVF index of less than 2 cm2/m2 poses a high risk for development of BVF obstruction) and probably by the association of distal aortic arch obstruction. The mechanism of late obstruction in most cases is failure of the BVF to grow in proportion to somatic growth [3]. Subaortic stenosis also can be due to obstruction within the outflow chamber [4] or encroachment of straddling atrioventricular valve tissue or other fibrous tissue on the BVF [5]. Routine use of intraoperative transesophageal echocardiography clearly demonstrates that development of subaortic obstruction, at the BVF level or beyond, may occur instantaneously whenever major changes in ventricular volume load take place. In the first patient in Van de Wal and associates' report, a bidirectional cavopulmonary shunt was converted into an aortopulmonary shunt, so that increased aortic inflow may have led to relative subaortic obstruction. In the second patient, a drastic volume reduction (secondary to conversion of a generous central aortopulmonary shunt to a bidirectional cavopulmonary shunt) may have resulted in subaortic obstruction. Subaortic obstruction in such circumstances is a potentially reversible phenomenon that generally cannot be demonstrated at postmortem examination.
In addition, the first patient might have had severe ventricular hypertrophy and impaired ventricular compliance due to the long-term effect of a tight pulmonary artery band, which acts as a fixed resistor. The subsequent construction of a generous central aortopulmonary shunt, as evidenced by high diastolic run-off, has a high likelihood of creating relative myocardial ischemia by decreasing coronary perfusion pressure and by adding an extreme volume load to an already pressure-overloaded and hypertrophied ventricle, thereby negatively influencing the myocardial metabolic supply/demand ratio. The pulmonary circulation of the second patient, by means of a generous central aortopulmonary shunt, for a long period was exposed to central aortic hemodynamics with its inherent high shear stress on the pulmonary endothelium; therefore, this patient was at high risk of development of pulmonary arteriolar disease with reactive pulmonary vascular physiology. Moreover, as evidenced by an extremely low systemic diastolic blood pressure (25 mm Hg), this patient was exposed to myocardial ischemia with potential for subendocardial myocardial infarction.
Awareness of the serious side effects of aortic outflow obstruction in univentricular hearts on the outcome of Fontan physiology has prompted the adoption of a policy of relieving the obstruction in the newborn period even before the appearance of hemodynamically or anatomically significant stenosis. Early palliation should be directed at relieving aortic obstruction, protecting the pulmonary circulation, and preventing the deleterious effects of chronic pressure and volume overload on the ventricle. With others, I firmly believe that subaortic or BVF obstruction in univentricular physiology is managed optimally by an aortopulmonary anastomosis with or without aortic arch reconstruction and construction of an aortopulmonary shunt [6, 7]. This approach bypasses all levels of obstruction to aortic outflow, whether caused by hypoplasia of the aortic annulus or beyond. In addition, it provides limited, but adequate flow to the pulmonary artery, and therefore allows adequate systemic oxygenation, yet minimizes the chance of development of pulmonary vascular disease. This physiology should be transformed into hemi-Fontan physiology early in life (as soon as the pulmonary vascular resistance decreases) to preserve systemic ventricular function.
``Unexplained'' increased pulmonary vascular resistance is rare. Increased pulmonary vascular resistance in the hemi-Fontan or Fontan circulation is almost always related to either cardiac or pulmonary causes, or both. Possible mechanisms in the first category consist of impaired ventricular compliance, ventricular hypertrophy, aortic outflow obstruction (with high likelihood to varying degrees all applicable to the presented patients), or atrioventricular valve regurgitation. Possible mechanisms in the second category consist of pulmonary vascular disease, hypercarbia, pulmonary edema (with high likelihood to varying degrees all applicable to the presented patients), phrenic nerve palsy, or stenosis in the surgically inaccessible segments of the branch pulmonary arteries. Based on all aforementioned considerations, I wholeheartedly agree with Dr Karl and associates that failure of preoperative evaluation, but above all suboptimal surgical management before the hemi-Fontan stage, rather than extraterrestrial pathophysiology may have been responsible for the reported results.
References
Pediatric Heart Center, Wilhelmina Children's Hospital, PO Box 18009, CA 3501 Utrecht, the Netherlands
To the Editor:
We thank Dr van Son for raising the important issue of systemic outflow obstruction in the setting of a Fontan or hemi-Fontan physiology.
We are well aware of the potential problem of subaortic obstruction when creating a Fontan circulation and attentively evaluate its presence. It has been our policy to perform a Damus-Kaye-Stansel procedure in all cases of univentricular hearts where there is a likelihood or evidence of systemic ventricular outflow obstruction [1–3]. We believe that subaortic obstruction did not play a role in the cases we reported [4].
Our first patient with double-outlet right ventricle and a subpulmonary ventricular septal defect had undergone coarctation repair soon after birth. The aortic arch in this case was by no means hypoplastic. Before a partial cavopulmonary connection was performed, cross-sectional and Doppler echocardiographic examination, direct pressure measurements, and cineangiography at cardiac catheterization revealed no subaortic obstruction. Furthermore, as a routine, we measure pressures in all cardiac chambers when coming off bypass. In this patient no systolic gradient was measured between the left ventricle and aorta after bypass, and the left ventricular end-diastolic pressure was 10 mm Hg.
With regard to the second patient with double-inlet left ventricle, hypoplastic right ventricle, and transposed great vessels, preoperative echocardiography, direct pressure measurements, and cineangiography at cardiac catheterization likewise identified no subaortic gradient. On coming off bypass, aortic systolic and ventricular systolic pressures were equal.
Based on the above information as well as the postmortem findings, it is our strong belief that subaortic obstruction was not the cause for the problems encountered in both our patients. Although it appears attractive to postulate subaortic obstruction as the cause of death in these 2 patients, the evidence is lacking. Increased pulmonary vascular resistance is more likely to have been the case.
The precise physiologic mechanisms affecting pulmonary flow after the Fontan operation are not entirely understood. Under ideal circumstances, blood flow through the lungs probably is maintained by a combination of right atrial pressure (with or without atrial contraction), changes in intrathoracic pressure during respiration, and creation of a potential space by emptying of the left (pulmonary venous) atrium, thus supporting a transpulmonary gradient [5]. After coming off bypass, both our patients had an elevated transpulmonary gradient in keeping with an elevated pulmonary vascular resistance.
The issue of resuscitation after cavopulmonary connection is not addressed by van Son. He discusses quite extensively the preoperative diagnosis, but there is no mention of the perioperative management. How do they manage patients who run into problems from an increased pulmonary vascular resistance that was underestimated preoperatively, or have they not encountered such cases as we and Karl and associates [6] have?
References
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