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EDITORIAL |
Japan
Hypoplastic left heart syndrome (HLHS) represents an anatomical spectrum of congenital hearts defects with varying degrees of underdevelopment of the left-sided heart structures.1 Before 1980, the condition was practically always fatal, usually in the first month of life.2 Since Norwood and colleagues3 reported successful surgical palliation (the Norwood procedure) in 1981, and multistaged reconstructive surgery involving the Norwood procedure (stage I) followed by bidirectional Glenn (stage II) or hemi-Fontan (stage II) and Fontan operations (stage III), the outlook for patients with HLHS has dramatically improved. The Norwood procedure must provide unobstructed systemic blood flow from the right ventricle (RV), relief of obstruction to pulmonary venous return, sufficient blood flow to the lungs in the absence of significant volume overload, and sufficient coronary circulation. Balancing pulmonary and systemic flows is critical to success. Despite successful reconstructive surgery, most deaths occur in the first 24 to 48 h postoperatively due to hemodynamic instability secondary to the unpredictable rapid fall in pulmonary vascular resistance.4 Maldistribution of cardiac output associated with the systemic-to-pulmonary shunt has been implicated as a major cause of early death after stage I palliation.5,6 A review of 122 postmortem cases after the Norwood procedure at the Children’s Hospital, Boston, indicated that the most prevalent causes of death were impairment of coronary perfusion, excessive pulmonary blood flow, and obstruction to pulmonary blood flow.7 This maldistribution of blood flow was implicated in more than 60% of postoperative deaths.
Therefore, over the last decade, many efforts to achieve a balanced circulation during the early postoperative period have focused on limiting pulmonary blood flow and improving systemic blood flow. These measures have included reducing the size of the systemic-pulmonary shunt, ventilator manipulations according to delicate blood gas analyses, the use of hypoxic admixtures and systemic vasodilators, monitoring mixed venous oxygen saturation, and home surveillance.6,8–14 Furthermore, Ungerleider and colleagues15 reported routine use of a mechanical ventricular assist device following the Norwood procedure. Recently, several experienced centers have achieved operative survival for the Norwood procedure of between 63% and 94%. However, this procedure still remains a challenging step with high mortality in many institutions with a small surgical volume, and such delicate postoperative management requires much manpower on a 24-hour basis.6,8,13,15–18
RIGHT VENTRICLE-PULMONARY ARTERY SHUNT IN FIRST-STAGE PALLIATION OF HLHS
The right ventricle-pulmonary artery (RV-PA) shunt was first introduced by Norwood and colleagues3 in 1981, the shunt materials used were relatively large for neonates: 8-mm non-valved PTFE tubes in 2 patients and 12-mm valved conduits in another 2. These patients all died within 11 hours after surgery, either from excessive pulmonary blood flow or from RV failure. In Japan, Kishimoto and colleagues19 revived the RV-PA shunt procedure using a xenopericardial valved conduit in 7 patients and reported stable postoperative hemodynamics with a high diastolic blood pressure. However, their long-term results were unsatisfactory and no other surgeons used the procedure. These findings led us to attempt an RV-PA shunt with a small non-valved PTFE conduit for first-stage palliation.20 We constructed a non-valved PTFE shunt between the RV and the PA as an alternative pulmonary blood source. This first attempt was performed on an 8-day-old girl with aortic atresia, mitral atresia, and hypoplasia of the ascending aorta, using a 4-mm PTFE graft, in 1998. Coronary hypoperfusion due to diastolic runoff through the systemic-pulmonary shunt has been proposed as a cause of late mortality after the Norwood operation, and this is the major disadvantage of systemic-pulmonary shunts.21 The fundamental advantage of an RV-PA shunt was that elimination of diastolic run-off into the pulmonary circulation results in high diastolic pressure, which was constantly over 40 mm Hg throughout the entire postoperative period, and coronary perfusion was independent of the pulmonary-systemic flow ratio (Qp/Qs). Independent coronary perfusion resulted in a very stable postoperative course. The postoperative hemodynamics were clearly different between the RV-PA shunt and the systemic-pulmonary shunt. The RV-PA shunt provided adequate pulmonary blood flow and also stable systemic circulation after stage I reconstruction, without requiring delicate manipulations of the systemic and pulmonary vascular resistance.
The volume load is directly associated with RV performance in patients with HLHS following stage I palliation. A single RV with a volume load imposed by a systemic-pulmonary shunt shows a significant increase in ventricular end-diastolic volume and ventricular mass, but a decrease in ejection fraction.22 In addition, the systemic tricuspid valve is exquisitely sensitive to volume loading of the RV, especially when required to eject at systemic arterial pressure. Another advantage of the RV-PA shunt is a significantly lower Qp/Qs associated with a smaller RV end-diastolic dimension and tricuspid valve annulus compared to the systemic-pulmonary shunt, because blood flow to the lung in the RV-PA shunt is only during systole. The decrease in volume load on the RV associated with a smaller ventricular dimension and tricuspid annulus may be advantageous to prevent progression of congestive heart failure and tricuspid regurgitation, and consequently may contribute to reduce interim mortality.
There are many advantages of the RV-PA shunt, but there are also disadvantages in this procedure. The effects of a right ventriculotomy for placement of the RV-PA shunt on the systemic RV function are of great concern. The necessity for a right ventriculotomy, which may affect contractile function of the systemic ventricle, is a potential disadvantage of the RV-PA shunt. Another concern related to ventriculotomy is ventricular arrhythmia. Many papers report no adverse effect on RV function and no significant arrhythmia after an RV-PA shunt;23 however, long-term follow-up is necessary. Limited growth of the pulmonary vasculature is also an important disadvantage of the RV-PA shunt, which may be due to the following factors: first, the Qp/Qs provided by the RV-PA shunt is lower than that in patients with a systemic-pulmonary shunt; second, a non-valved RV-PA shunt has some flow reversal to the RV, which causes a decrease in the effective pulmonary blood flow.23 Finally, the RV-PA shunt becomes obstructed over time.20 Because of these hemodynamic characteristics of the RV-PA shunt, PA growth is not sufficient before stage II. The results of analysis of the effects of PA size on clinical outcome after the Fontan operation have been conflicting.24,25 However, lower pulmonary resistance has been said to be a more important factor than PA size to establish Fontan circulation. In patients with an RV-PA shunt, the pulmonary vascular bed may be protected from excessive pulmonary blood flow, which results in lower pulmonary resistance than that in patients with a systemic-pulmonary shunt, and Fontan circulation could be established in patients with HLHS following stage I palliation with an RV-PA shunt, despite smaller PA size.
Recently, many centers have adopted stage I palliation with an RV-PA shunt and reported a significant improvement in stage I mortality and morbidity. In some centers, mortality in stage I palliation has dropped below 10%. However, controversies remain regarding the systemic- pulmonary shunt vs. RV-PA shunt in the long-term results of HLHS. Tabbutt and colleagues26 from Philadelphia Children’s reported their recent series of 149 patients to compare the systemic-pulmonary shunt and RV-PA shunt in stage I palliation and found no difference in overall mortality between the groups. The mortality tended to be higher in so-called high-risk patients with a systemic-pulmonary shunt compared to an RV-PA shunt. Clearly a more extensive experience is necessary before a definitive conclusion can be drawn. However, the techniques to manage HLHS continue to evolve.
REFERENCES
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