Single-ventricle physiology: perioperative implications

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Abstract

Neonates with functional single ventricles have pulmonary and systemic circulations that are supplied in parallel, creating significant cyanosis and ventricular volume overload. The goal of palliative surgery, excluding transplantation, is to convert single-ventricle circulation from a parallel to a series arrangement. This will ultimately require a complete cavopulmonary anastomosis (Fontan-type procedure) in which vena caval blood is rerouted directly into the pulmonary circulation. Various factors require that this palliation occur in stages. Stage I surgery, which is often a Norwood procedure, is done in the neonatal period and stabilizes, but does not resolve, parallel circulation. The tenuous balance between pulmonary and systemic perfusion during this stage makes noncardiac surgery hazardous, and it should be restricted to urgent or emergent indications. Stage II surgery, or partial cavopulmonary anastomosis, relieves both parallel circulation and volume overload, but not cyanosis. Relatively stable hemodynamics during this stage create favorable conditions for elective surgery. Patients who have undergone stage III surgery, the Fontan-type repair, vary in age from toddlers to adults, and in physical status from well-compensated to significantly debilitated. Fontan patients require thorough preoperative assessment when elective surgery is contemplated. Optimal communication between surgeons, anesthesiologists, and cardiologists is essential when caring for the patient with single-ventricle physiology.

Section snippets

Definitions and descriptions of univentricular lesions

Single-ventricle lesions are congenital heart anomalies in which one of the two ventricular chambers is either absent or so severely hypoplastic that a biventricular repair is impossible. Typically the atrioventricular valve associated with the absent or hypoplastic ventricle is also absent or hypoplastic (ie, mitral or tricuspid valve atresia or hypoplasia). Similarly, the arterial outflow valve associated with the absent/hypoplastic ventricle may also frequently be affected (aortic or

The stage I or Norwood patient

In many forms of single-ventricle anatomy, early survival depends upon continued patency of the ductus arteriosus.28, 29 Ductal patency is typically maintained with intravenous prostaglandin E29, 30 to provide temporary relief from either inadequate pulmonary blood flow (eg, HRHS with tricuspid and pulmonary atresia) or systemic outflow obstruction (eg, HLHS with aortic atresia). While maintaining ductal patency can be an important temporizing measure in ductal-dependent scenarios,30 most

Stage I anatomy

There is significant variability in the extent of the associated lesions in HLHS, which may range from moderate hypoplasia to complete absence of the pertinent left-sided structures (aortic valve, mitral valve, and left ventricle). The overall circulatory situation before Norwood palliation is summarized in Fig.1 and includes the following defining features:

  • 1.

    Pulmonary venous return egresses from the left atrium into the right atrium via a patent foramen ovale or atrial septal defect because of

Norwood physiology

Figure 2 depicts the circulatory situation before Norwood palliation. Resistance to flow in the systemic circulation (Rs) is higher than pulmonary resistance (Rp), favoring flow through the lungs (Qp) over flow to the systemic circulation (Qs). The diagram illustrates that there is an inverse relationship between the magnitude of the relative resistances (Rs, Rp) of the two parallel circulations and the magnitude of flow (Qs, Qp) through them. Ventricular volume work will be most efficient

Stage I surgery

The goal of Norwood palliation is to stabilize the single right ventricle parallel circuit so that systemic oxygen delivery is adequate to allow postoperative recovery and growth until a systemic venous to pulmonary artery shunt (bidirectional Glenn or hemi-Fontan operation) can be safely performed. Optimizing systemic oxygen delivery is accomplished by preserving ventricular function and creating a balanced parallel circuit.27, 43, 44, 45, 54

The surgical concepts of the Norwood repair are

Immediate post-Norwood issues

Figure 4 illustrates the circulatory situation after Norwood palliation with a systemic to pulmonary artery shunt. Ideally, the sum of the resistances of the pulmonary vasculature (Rp) and the shunt (Rshunt) will equal the systemic vascular resistance (Rs) so that the two parallel circulations are balanced and ventricular volume work is minimized. However, in the immediate postoperative period the ventricle has just sustained the insult of deep hypothermic cardiopulmonary bypass (at 18–20°C)61,

Perioperative implications following stage I palliation

The parallel circulation is inefficient and hazardous and the Norwood patient remains a high-risk candidate for surgery. Risks inherent in the parallel single-ventricle circulation are the exquisite preload dependence of the ventricle and its intolerance to sudden increases in afterload. Recurrent aortic arch obstruction can occur at any time postoperatively and may go unnoticed55, 57 until the patient is subjected to an additional stressor such as a surgical procedure or an infection.

The shunt

Stage II palliation (partial cavopulmonary anastomosis)

The stage I or Norwood palliation can only be considered a first step toward the eventual goals of successful single ventricle palliation. The parallel nature of stage I circulation leaves the patient with two fundamental problems: excessive ventricular volume loading and cyanosis.

The Fontan operation eliminates both of these issues, but it cannot be undertaken in the newborn. This must await the normal postnatal reduction of pulmonary vascular resistance, so that passive cavopulmonary

Stage II: the procedure

There is often confusion regarding the difference between the hemi-Fontan operation and the bidirectional Glenn. Originally used for palliation of tricuspid atresia, the “classic” Glenn procedure consists of the creation of an end-to-end anastomosis between the divided superior vena cava and the divided right pulmonary artery.80 This provides pulmonary blood flow in a manner distinctly different from that of systemic arterial to pulmonary artery connections (Blalock-Taussig or central shunts),

Stage II physiology

Following stage II palliation, patients remain cyanotic but tend to be hyperdynamic and well-perfused. Approximately half of the blood entering the heart comes from the pulmonary veins, the remainder from the inferior vena cava. All of it mixes in the single ventricle and is delivered to the systemic arterial circulation. The Qp:Qs is now approximately 0.5:1, and the systemic oxygen saturation on room air is typically in the range of 80% to 85% (see Fig 6). The excessive ventricular

Perioperative implications following stage II

Because of the relative hemodynamic advantages of the stage II ventricle, it is reasonable to consider proceeding with elective surgery during this stage, with several perioperative considerations in mind. Many of these relate to the passive nature of pulmonary blood flow present in these patients. As mentioned, volume status, venous return, pulmonary vascular resistance, and ventricular function become particularly important. Hypovolemia is a known risk of prolonged preoperative fasting but

Stage III: the Fontan operation

First described in 1971 by Fontan and Baudet for palliation of tricuspid atresia,4 a Fontan-type procedure is the final step in single-ventricle palliation. Following this procedure, blood flows in series through the systemic arterial circulation, the systemic venous system, the pulmonary circulation, then back to the functional single ventricle. This arrangement requires diversion of both superior and inferior vena cava blood directly to the lungs. Afterward, the cyanosis that persisted from

Fontan: the procedure

Although most patients who have undergone a Fontan-type palliation have similar physiology, several technical variants of the procedure have been described. Early versions of the Fontan procedure sought to utilize atrial systole to facilitate pulmonary blood flow and thus involved connections that included a significant portion of the right atrium within the circulatory path to the lungs. However, problems such as atrial arrhythmias, atrial thrombus formation, obstruction to pulmonary venous

Fontan physiology

The well-compensated Fontan patient has minimal or no cyanosis and is well-perfused. Qp:Qs is equal, and the resting cardiac index is normal (see Fig 8). Even without a fenestration, arterial oxygen saturation remains slightly below normal because of the persistent drainage of coronary sinus blood into the systemic arterial circulation. Compared to a two-ventricle in-series circulation, the ability to increase cardiac output is significantly limited because of the absence of a pulmonary

Perioperative implications following the Fontan procedure

Because of increases in survival after staged single-ventricle palliation, increasingly more patients will present for elective surgery status post Fontan procedure. The functional status and comorbidity found in these patients varies significantly, from the young patient who is well-compensated to the adult with failing Fontan physiology. It is wise to consult not only the participating cardiologist well in advance of the date of surgery, but the anesthesia team as well. The urgency of the

Conclusions

The patient with single—ventricle physiology presents unique and interesting challenges in the perioperative setting. In addition to the variation in anatomical subtypes associated with single-ventricle lesions, the staged nature of palliation contributes to the complexity of patient management. Proper management at one stage of palliation may involve manipulations that are inappropriate at another stage. For example, while supplemental oxygen can be hazardous to the pre-Norwood patient,

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