Episode Transcript
Hi, my name is Peggy Vogt. I am a pediatric cardiac anesthesiologist at Children’s Healthcare of Atlanta. Today, I will be discussing single ventricle physiology and the single ventricle palliative pathway including perioperative anesthetic considerations. Single ventricle physiology is a very complex topic. Anesthesia residents and general pediatric anesthesiologists are the target audience for this podcast, not pediatric cardiac anesthesiologists. The learning objectives for this podcast are gaining an increased understanding of single ventricle physiology and the surgical palliation pathway.
There are multiple anatomic variations that may require the single ventricle palliation pathway. For instance, unbalanced atrioventricular septal defect, double outlet right ventricle, pulmonary atresia with intact ventricular septum, and hypoplastic left heart syndrome. Two of the most common lesions are hypoplastic left heart syndrome and tricuspid atresia. Tricuspid atresia leads to an underdeveloped right ventricle and falls under the umbrella term hypoplastic right heart syndrome. Overall, single ventricle lesions within congenital heart disease are rare. However, it results in significant morbidity despite advances in medical and surgical treatment.
HLHS and HRHS - definition, incidence, and comorbid conditions
The Society of Thoracic Surgeons anatomically defines hypoplastic left heart syndrome or HLHS as "a spectrum of cardiac malformations with normally related great arteries…characterized by underdevelopment of the left heart with significant hypoplasia of the left ventricle including atresia, stenosis or hypoplasia of the aortic or mitral valve, or both valves, and hypoplasia of the ascending aorta and arch.” In contrast, hypoplastic right heart syndrome may have a small right ventricle, abnormal tricuspid and pulmonary valves, and hypoplasia of the main pulmonary artery.
HLHS accounts for 2-3% of all congenital heart disease resulting in a prevalence rate of two to three cases per 10,000 live births in the United States. It is the most common form of functional single-ventricle heart disease. HLHS is usually an isolated congenital defect but can be associated with conditions including Turner syndrome (XO), Jacobsen syndrome (deletion of distal 11q), Trisomy 13, and Trisomy 18.
2. Overview of single ventricle circulation
In either hypoplastic left or right heart syndrome, there is only a single ventricle that must provide adequate blood flow to both the pulmonary and systemic circulations. This single ventricle is both pressure and volume overloaded.
Blood flow in HLHS is complex. It can be helpful to follow the path of red blood cells as they are oxygenated in the lungs and as they travel to the systemic circulation to deliver oxygen to the tissues. Try to visualize the path of the red blood cells and the level and direction of shunting. Oxygenated blood from the lungs enters the left atrium and flows through an obligatory atrial septal defect where it mixes with systemic venous blood entering the right atrium. The mixture of oxygenated and deoxygenated blood enters the single right ventricle and is pumped out into the pulmonary arteries. So how does blood reach the systemic circulation? The patent ductus arteriosus must remain patent after birth to allow blood from the pulmonary arteries to reach the aortic arch and, thus, the systemic circulation.
Hypoplastic right heart syndrome, or HRHS is similar: systemic venous blood flow enters the right atrium and flows to the left atrium through an atrial septal defect. This deoxygenated blood mixes with the oxygenated blood in the left atrium as it returns from the lungs via the pulmonary veins. This mixed blood flows to the left ventricle where it is pumped by the single ventricle into the aorta to reach the systemic circulation. So how does blood reach the pulmonary circulation? Once again, the ductus arteriosus is essential and must remain patent. It allows blood from the aortic arch to reach the pulmonary artery and the lungs, oxygenating the blood. The mixed blood that is pumped into the aorta but does not flow through the PDA will reach the systemic circulation.
For both lesions, life after birth requires two things: adequate atrial blood mixing and a patent ductus arteriosus. Atrial-level blood mixing through an atrial septal defect or patent foramen ovale is required to ensure that adequately oxygenated (but mixed) blood travels systemically. Patients with restrictive ASDs have inadequate mixing, inadequate systemic oxygenation, and may rapidly deteriorate after birth. If a patient has a restrictive ASD, then catheter-based techniques can be used to expand the atrial defect until the first stage of surgical palliation. This procedure is called a balloon atrial septostomy. In rare cases, emergency surgery called an atrial septectomy may be performed in which the surgeon removes the atrial tissue. Patients with single ventricles also require a patent ductus arteriosus (PDA) to ensure either systemic or pulmonary blood flow. Normally, the ductus arteriosus naturally starts to close within the first 48 hours after birth. Single ventricle patients are ductal dependent and require a prostaglandin infusion and/or stenting to maintain ductal patency until surgical palliation.
Staged surgical palliation
Surgical palliation of HLHS currently consists of four staged procedures. They need to be spaced over time to allow the patient to adapt to the significant physiologic changes following each procedure. We will discuss palliation of HLHS, which is more common than HRHS, and has a different approach during only the first of the three stages of surgery.
The first stage for palliation of HLHS is called the Norwood procedure and is generally performed in neonates that are 3-7 days old to allow time for the pulmonary vascular resistance to decrease. It consists of 4 surgical steps performed on cardiopulmonary bypass:
1. Construction of a neo-aorta (utilizing the pulmonary valve, main pulmonary artery, and ascending aorta) to provide unobstructed blood flow from the single ventricle to the systemic circulation.
2. Ligation of the ductus arteriosus
3. Creation of a surgical shunt to provide stable blood flow to the pulmonary circulation. These shunts prevent large uncontrolled shifts of pulmonary blood flow. There are three major types of shunts. The modified Blalock-Taussig-Thomas shunt, or BTT shunt, is usually made from a synthetic conduit that connects the subclavian artery to the pulmonary artery (a “classic” BTT shunt uses the patient’s subclavian artery to directly connect to the pulmonary artery). A Sano shunt is an armored synthetic conduit that connects the single ventricle to the pulmonary artery. Finally, a central shunt is a synthetic conduit that connects the aorta directly to the PA. A modified BTT shunt or a Sano shunt are the most commonly placed shunts to provide pulmonary circulation in Norwood physiology. The type of shunt placed is determined by surgical and institutional experience. Recent literature shows no difference in outcomes in modified-BTT shunts vs Sano shunts once the patients reach the second stage of palliation.
4. Creation of an unrestrictive atrial septal defect (a common atrium) to optimize venous and oxygenated mixing.
Of note, some surgical centers prefer a non-bypass cardiac cath lab “hybrid” procedure, during which the ductus arteriosus is stented and the ASD is enlarged using catheterization techniques, and two bands are placed around the proximal branches of the left and right pulmonary arteries to limit excessive pulmonary blood flow. The hybrid procedure is especially useful in neonates that are not good candidates for cardiopulmonary bypass.
Blood flow post-Norwood is as follows: venous blood flow from the vena cava enters the right atria, mixes with oxygenated blood from the left atria, enters the right ventricle, and is then pumped through the neo-aorta. If the patient has a Sano shunt, the blood is directed to the pulmonary arteries through the shunt from the right ventricle. If the patient has a modified-BTT, blood exits the right ventricle through the neo-aorta, flows through the aortic arch, and when it reaches the subclavian artery (usually on the right side) it can flow to the pulmonary arteries or continue systemically. At the same time, oxygenated blood from the lungs enters the left atria, mixes with blood from the right atria through the unobstructed ASD, and also enters the right ventricle to be pumped through the neo-aorta where it can pass through the shunt or go to the systemic circulation. After recovering from surgery, the systemic SpO2 is generally 75%-85% on room air (21% FiO2) due to continued atrial mixing.
The second stage of palliation for HLHS is the bidirectional Glenn procedure, where the superior vena cava is anastomosed end-to-side to the pulmonary artery (typically right), and the first stage shunt is removed. This second stage of palliation is typically performed at three to six months of age. The goal of the second stage is to route 50% of the systemic venous return, via the superior vena cava, directly to the pulmonary arteries. This reduces the workload of the single ventricle, making it more efficient and focusing its output primarily on pumping blood to the systemic circulation. The Glenn connects the superior vena cava to the right pulmonary artery and blood flows to the left and right pulmonary artery (hence the term bidirectional Glenn). After the bidirectional Glenn procedure, pulmonary blood flow is no longer pulsatile flow from the modified BTT or Sano shunt. Instead, pulmonary blood flow is dependent on passive venous return from the SVC. Blood flow post-Glenn is as follows: venous blood flow from the superior vena cava passively drains into the right pulmonary artery. Venous blood from the inferior vena cava enters the right atrium, mixes with oxygenated blood from the left atrium, enters the right ventricle, and then is pumped out through the neo-aorta. At the same time, oxygenated blood from the lungs enters the left atria, mixes with venous blood in the common atrium, and also enters the right ventricle to be pumped through the neoaorta to the systemic circulation. The patient will still experience persistent cyanosis (SpO2 of 75%-85%) due to the ongoing flow of deoxygenated blood from the IVC to the right atria, which is mixed with oxygenated blood from the left atria. The mixed blood is pumped by the single ventricle, the right ventricle, to the systemic circulation. After the Glenn procedure, it is important for the anesthesiologist to remember that paradoxical emboli are still a risk from any blood that returns via the IVC.
The third stage of palliation is the Fontan procedure, where the surgeon connects the IVC to the pulmonary artery, creating a total cavopulmonary connection. Therefore, the systemic venous return from both cava is directed into the pulmonary arteries. This is often performed using a large extra cardiac Gore-Tex tube graft. The surgeon also often creates a fenestration between the graft and the right atrium, called a “fenestrated” Fontan, which allows continuous preload to the single ventricle in times of poor forward flow through the lungs due to acute elevations of pulmonary vascular resistance.
The Fontan procedure is typically performed at 18 to 30 months of age or when the child reaches 15 kg. Pre-Fontan pulmonary vascular resistance should be low enough to tolerate the increase in pulmonary blood flow. With the systemic and pulmonary circulations now separated, cyanosis is improved. The pulmonary venous return to the single ventricle will be entirely dependent on passive venous flow through the pulmonary arteries and lungs returning to the left atria. Blood flow post-Fontan is as follows: venous blood flow from the SVC and IVC passively drain into the right pulmonary artery. Oxygenated blood from the lungs enters the left atria, mixes with blood in the common atrium, and enters the right ventricle to be pumped through the neoaorta to the systemic circulation. We will discuss the implications of this in a moment.
The fourth and final stage of the single ventricle surgical pathway is a heart transplant. It’s important to keep in mind that the previous surgeries are palliative, not definitive repairs, and Fontan physiology will eventually fail. The longevity of a patient with Fontan physiology will be determined by various factors, such as which ventricle is systemic and the degree of atrioventricular valve regurgitation. Systemic left ventricles tend to have more favorable outcomes than systemic right ventricle patients. During prenatal counseling, parents should be educated that their child will eventually need a heart transplant. Heart transplant in patients with Fontan physiology is a particularly challenging subset of heart transplant patients; however, with surgical advancements, the one-year post-transplant survival is 89%.
Norwood, Glenn, and Fontan physiology
While many of you listening to this podcast may never manage a neonate or infant with a single ventricle, discussion of the physiology promotes comprehension of important concepts of cardiopulmonary shunt physiology. Ideally, these patients should be cared for by a pediatric cardiac anesthesiologist.
Post-Norwood physiology can be particularly delicate. Patients will continue to have cyanosis due to atrial-level mixing of systemic and pulmonary venous blood, which is ejected by a single ventricle into both the pulmonary and systemic circulations. Their goal oxygen saturation levels are 75-85% on room air. The modified BTT or Sano shunt provides a somewhat fixed resistance as pulmonary blood flow is shunted from the systemic circulation into the pulmonary circulation. Maneuvers that greatly change pulmonary vascular resistance, or PVR, and systemic vascular resistance, or SVR, should be avoided. Large decreases in PVR (i.e. high FiO2 or hyperventilation) are harmful because they will lead to pulmonary overcirculation and systemic and coronary hypoperfusion. In addition, the single ventricle is still providing both pulmonary and systemic blood flow and is at high risk for volume overload or dysfunction. Mortality at this “interstage” is particularly high and has been reported up to 15 percent. Causes include anatomic obstruction (atrial, aortic), shunt thrombosis, imbalance of pulmonary and systemic blood flow, and coronary ischemia from “steal” or “run-off” of aortic diastolic pressure into the pulmonary circulation.
Glenn physiology is focused on continual atrial mixing and maintaining passive SVC flow to the pulmonary arteries for adequate blood oxygenation. If possible, spontaneous respirations should be maintained to optimize negative intrathoracic pressure and venous return. Volume administration increases pulmonary venous perfusion pressure to the lungs and high FiO2 decreases PVR. Both will facilitate pulmonary blood flow. Mild hypercarbia may also be beneficial to optimize cerebral blood flow and venous return from the SVC. Compared to the post-Norwood “interstage”, there is a relatively low mortality rate after Glenn procedures. As the children grow, they may become increasingly cyanotic, particularly with exercise – a sign that they may need to progress to Stage three.
Stable Fontan physiology requires adequate pulmonary preload, low PVR, and adequate pulmonary perfusion to maximize oxygenated blood return to the single ventricle. The patient’s systemic venous preload drives passive blood flow through the lungs. The blood returns to the common atrium and single systemic ventricle for output to the body. The single ventricle then must create enough pressure to eject out the aorta, transit the entire systemic capillary bed, fill the systemic veins, and then overcome PVR to ensure passive venous flow to the pulmonary arteries. Systemic venous pressures can become elevated as a result.
Spontaneous ventilation aids in the venous return to the lungs during inspiration and perfusion through the pulmonary vasculature during expiration. If possible, positive pressure ventilation should be avoided.
Post-Fontan procedure, the patient’s saturation should be closer to 100% but may peak in the mid-90’s due to the persistent right-to-left shunting of coronary sinus blood and fenestration flow into the right atrium. The fenestration allows blood flow from the Fontan-IVC connection to “pop-off” to the common atrium in times of high PVR or pulmonary hypertension. The fenestration provides consistent ventricular preload to the ventricle thus ensuring cardiac output. This small right-to-left shunt decreases systemic oxygen saturation but can be closed in the cardiac cath lab in the future. Any intrinsic pulmonary disease or abnormal pulmonary vascular anatomy can negatively affect oxygenation as well.
Anesthetic Considerations for HLHS Patients with a Fontan
As Fontan patients age, their cardiac output will decrease from systemic ventricular systolic or diastolic dysfunction. The ventricular function of Fontan patients is on overage decreased by about 30% at rest and 50% with exercise and have limited cardiac reserve. The single ventricle provides both systemic and pulmonary blood flow, so any changes in afterload can significantly affect systemic circulation and preload. For example, an increase in afterload can decrease cardiac output to the systemic circulation. The decrease in systemic blood flow will consequently decrease preload given its dependence on passive flow. On the other hand, a drop in afterload will result in blood pooling in the systemic vasculature and decreased venous return and perfusion through the lungs. Therefore, cardiac output depends on optimal afterload to provide optimal preload. A fenestration, as just discussed, can help maintain cardiac preload in the setting of increased PVR, but this is at the expense of some degree of hypoxemia due to an increase in venous admixture.
After the Fontan procedure, there is significant potential for additional long-term cardiac complications including arrhythmias, cyanosis, ventricular dysfunction ranging from restricted exercise capabilities to heart failure, and atrioventricular valve regurgitation. These patients are also at risk for kidney, liver, and pulmonary disease, protein-losing enteropathy, and thromboembolic events. Many of these sequelae are indicators for cardiac transplantation in the child or adult with “failing” Fontan physiology, and these patients can be gravely ill. Due to the high incidence of sequelae, any patient with Fontan sequelae, but especially ventricular dysfunction, should receive a consult from cardiac anesthesia and care at a major medical center prior to any further interventions.
Now I would like to summarize the key points:
1. Single ventricle patients are initially dependent on atrial mixing and a PDA for adequate pulmonary and systemic blood flow. Their physiology is a delicate balance between systemic and pulmonary vascular resistances, and interventions that make rapid changes in either should be avoided
2. The stages of the single ventricle palliation pathway include 1) Norwood with modified BTT/Sano shunt and/or hybrid procedure, 2) bi-directional Glenn procedure, and 3) Fontan procedure
3. Patients with pre- and post-Norwood procedure have a particularly delicate balance of pulmonary and systemic blood flow with a significant risk of mortality. Avoid interventions that result in large changes to systemic and pulmonary vascular resistances.
4. Patients with Glenn and Fontan circulations have pulmonary blood flow that is dependent on passive venous flow. Spontaneous respirations maximize pulmonary return and pulmonary perfusion. If positive pressure ventilation is required, a slow respiratory rate with minimization of mean airway pressure over time and avoidance of excessive PEEP is recommended.
5. Over-oxygenation in patients with Norwood procedures can be particularly harmful due to pulmonary overcirculation and coronary ischemia due to diastolic run-off. In contrast, patients with Glenn or Fontan procedures tolerate high FiO2 concentrations.
6. Patients with Fontan procedures are at high risk for complications, including cardiac complications, risk for kidney, liver, and pulmonary disease, protein-losing enteropathy, and thromboembolic events. Strongly consider cardiac anesthesia at a major medical center for any surgeries.
7. All single ventricle patients will eventually be listed for a heart transplant.
