Single ventricle physiology

What the Anesthesiologist Should Know before the Operative Procedure

Patients with a single cardiac ventricle typically undergo staged cardiac procedures culminating in a Fontan operation. While important variations in the cardiovascular anatomy and physiology affect the operative procedure and timing, the overarching goals are to assure that the transition from fetal circulation results in stable perfusion of both the systemic and pulmonary circulations; that the loading conditions on the ventricle are minimized to preserve function of the single ventricle and that the pulmonary vascular resistance will fall to the lowest possible value.

Each stage of this reconstruction poses unique physiologic challenges for the anesthesiologist. Infants and children with single ventricle physiology may require a variety of non-cardiac surgeries in which the principles described in this chapter apply, but the focus here will be on the cardiac surgeries.

The greatest disparities in the range of physiology and surgical therapy occur with the initial stage when anatomic variables require individualized approaches to achieve the overarching hemodynamic objectives. For example, the Fontan operation was originally described for tricuspid atresia (TA) where there is a single left ventricle. However, the most common single ventricle malformation is hypoplastic left heart syndrome (HLHS), where there is a single right ventricle. In the former, the initial surgery is predicated on the extent of pulmonary blood flow.

Neonates with TA and no ventricular septal defect (VSD) will require a systemic to pulmonary shunt (e.g., Blalock-Taussig) to provide pulmonary blood flow. Neonates with TA and VSD are assessed individually to determine whether the resulting flow through the right ventricular outflow tract to the pulmonary arteries is appropriate.

As with all single ventricle patients, the ideal physiology at this stage results in a balance where pulmonary blood flow and systemic blood flow are approximately equal. With admixture of equal proportions of pulmonary and systemic venous return, this translates into a target systemic oxygen saturation in the range of 80%. Systemic oxygen saturation levels in this range are entirely satisfactory in supporting growth and development in the first few years of life.

Higher saturation levels, whether due to anatomic reasons or ventilatory management impose significant burdens on the ventricle and may also have a detrimental long-term impact on pulmonary vascular resistance, both of which work counter to the goals above. For example, during induction of anesthesia and intubation of a neonate with single ventricle, it is often easy to ventilate with supplementary oxygen in such a way that pulmonary vascular resistance falls and oxygen saturation rises well into the 90’s. To achieve this oxygen saturation, the ratio of pulmonary to systemic blood flow (Qp:Qs) must be well above 4:1.

In order for the single ventricle to preserve systemic perfusion under these conditions, it has to increase its volume output more than two-fold. Ultimately, it will fail to be able to meet systemic perfusion demands and systemic hypotension will result. Since the goal is to allow the PVR to fall normally to a small fraction of the systemic vascular resistance, the anatomic connection that provides pulmonary blood flow must be restrictive and result in a Qp:Qs near 1 for the first few months of life while the PVR falls.

While the anatomic and surgical challenges of HLHS are significantly more complex, the physiologic principles and goals are exactly the same. In HLHS, the systemic circulation depends upon the ductus arteriosus. Restriction to pulmonary blood flow most commonly occurs in relation to obstruction to pulmonary venous return as it courses across the inter-atrial septum on its way back to the single (right) ventricle. Since flow into the pulmonary arteries is not restrictive, neonates with HLHS are particularly prone to pre-operative management that results in excessive Qp:Qs.

When systemic perfusion is jeopardized, the addition of small amounts (2-4%) carbon dioxide to the inspired gas mixture can be extremely effective in restoring systemic blood flow. Initial palliation for HLHS, whether by operative reconstruction using a Norwood procedure or a hybrid transcatheter/surgical variation, targets the same physiologic goals: provide unobstructed systemic flow from the right ventricle to the aorta, relieve obstruction to pulmonary venous return at the atrial septum and restrict pulmonary arterial flow to achieve a Qp:Qs near 1.

Although there are numerous other malformations that result in single ventricle, they are less common and generally fall into one of the two paradigms above. Either they need a reliable restrictive source of pulmonary blood flow or they require more complex interventions to achieve unobstructed, reliable systemic blood flow.

If the initial approach achieves stated goals, the PVR will fall sufficiently in the first 3-4 months of life to allow a staged conversion to Fontan circulation to begin. These operations shift the source of pulmonary blood flow from a systemic, high pressure vessel through a restrictive connection to the central veins at low pressure. The physiologic benefit that this offers to the ventricle is that the volume output it is required to generate is reduced from Qp plus Qs to Qs alone. In other words, when the pulmonary and systemic arterial circulations are connected in parallel, as required in all initial interventions, the single ventricle must double its volume output even when the Qp:Qs ratio achieves an optimal value near 1.

When the arterial source of pulmonary blood flow is ligated, the ventricular volume output demand immediately falls to that required for the systemic circulation alone. Pulmonary blood flow is provided by a diversion of systemic venous return. To be successful, the impediments to pulmonary blood flow must be minimal. These impediments include PVR, pulmonary venous obstruction at the atrial septum or atrioventricular valve, and the diastolic properties of the single ventricle itself. When completed, the Fontan results in all caval return flowing directly into the pulmonary arteries. As such, cardiac output is completely dependent upon the volume of venous return that can “passively” traverse the pulmonary vascular bed.

When the Fontan procedure was performed in a single step, a significant number of patients exhibited life threatening low cardiac output syndromes due to acute changes in the diastolic ventricular function that accompanied the rapid reduction in ventricular volume load. While the risk of this phenomenon can be reduced by postponing Fontan surgery until 2-3 years of life, it was not eliminated. As a result, the Fontan procedure is typically staged with a superior vena cava-pulmonary artery (SVC-PA) connection initially (e.g., bi-directional Glenn or hemi-Fontan), which preserves venous return to the ventricle during the acute, transient change in diastolic function by direct supply of the inferior vena cava blood without having to overcome the resistance of the pulmonary vascular bed.

Since the proportion of venous return from the superior and inferior vena cavae is approximately equal in young infants, this procedure typically results in little change in a systemic oxygen saturation. The Fontan can be completed by an inferior vena cava-pulmonary artery connection once compliance improves as the ventricle has had ample opportunity to remodel at a lower end-diastolic volume. This approach has dramatically reduced the mortality of the Fontan procedure and also allows much earlier relief of the extra volume load on the ventricle with an SVC-PA anastomosis in the first 6 months of life.

Anesthetic and peri-operative management for all cava-pulmonary artery procedures should focus on promoting pulmonary blood flow (PBF). These center around maintaining or augmenting intravascular volume to provide sufficient “driving” pressure; lowering PVR and optimizing ventricular performance with particular attention to diastolic function.

1. What is the urgency of the surgery?

What is the risk of delay in order to obtain additional preoperative information?

Urgent indications for surgery are almost exclusively confined to the initial neonatal interventions. While a substantial proportion of single ventricle patients require the ductus areteriosus to provide either pulmonary or systemic blood flow, prompt recognition of these malformations and institution of a prostaglandin E (PGE) infusion nearly always stabilizes the circulation temporarily.

Because of gross disparities in ventricular size, these malformations are often recognized on fetal ultrasounds. Among neonates who are not diagnosed prenatally, those with ductus-dependent PBF are often recognized to be hypoxemic in the nursery.

However, the findings of those with ductus-dependent systemic circulation can be more insidious, and they may not present until they have obvious shock. Nevertheless, PGE infusions usually prove effective in these circumstances and the neonates can be stabilized and transported to congenital heart centers for complete evaluation and treatment.

• Emergent

– A very small minority of neonates with single ventricle are not responsive to PGE treatment or have anatomic variants that are not amenable to survival without immediate intervention in the first few minutes of life. The most common examples are neonates with HLHS and intact inter-atrial septum. Because there is virtually no route of egress from the pulmonary veins, gas exchange is negligible and they present with profound hypoxemia and hypercarbia despite heroic attempts to ventilate the lungs. They must receive an immediate intervention to open the atrial septum. This can be achieved with an interventional cardiology or cardiac surgery approach. Because of the particularly high risk in this cohort, a few centers have begun to perform transcatheter fetal interventions to open the atrial septum.

• Urgent

– Neonates who have ductus-dependent circulations fall into this category. While they require continuous PGE infusions and intensive care observation, there is time to conduct complete evaluation of their cardiovascular system and other organ systems for co-existing disease. The initial stage interventions can be planned appropriately and timed to occur within the first few days of life. Occasionally, an infant who would be a candidate for SVC-PA anastomosis will require surgery urgently when the arterial source of PBF is acutely compromised and significant hypoxemia ensues.

• Elective

– All Fontan completion operations are completely elective. Given the extent to which systemic output is critically dependent upon optimal pulmonary blood flow, this operation should not be performed when physiologic conditions are unfavorable. Cardiologists often avoid referring patients for Fontan surgery during seasons when respiratory viruses are prevalent so that early morbidity is not increased by a modest change in PVR caused by an intercurrent respiratory infection. With the rare exception noted above, SVC-PA anastomosis operations are also elective. In light of the benefits accrued by reducing ventricular volume load early and the resilience this anatomic arrangements offers in preserving cardiac output, this operation is usually performed as soon as feasible.

2. Preoperative evaluation

Preoperative evaluation starts with a thorough evaluation of the cardiovascular system. In neonates presenting with single ventricle malformations, this can often be accomplished with a comprehensive history, physical exam and echocardiogram. Despite huge advances in fetal echocardiography, prenatal diagnoses must be confirmed with a postnatal evaluation, even when a newborn appears to need an emergent intervention. For those diagnosed postnatally, the details of initial presentation provide important clues as to whether systemic or pulmonary blood flow is at risk.

Pulse oximetry and systemic blood pressure in all four extremities indicate current physiologic status. Echocardiography provides both anatomic details about the malformation as well as important physiologic information. Doppler measurements provide key measures that help the anesthesiologist anticipate the physiologic tendencies of any individual child with single ventricle.

For example, in HLHS, color Doppler measurement flow direction in the ductus arteriosus should reveal right-to-left flow in systole as the right ventricle provides systemic perfusion and left-to-right flow in diastole indicating that the PVR is lower than the systemic vascular resistance (SVR). Absence of flow reversal suggests high PVR. Pulse Doppler measurement of the pressure gradient at the inter-atrial septum should show some restriction to flow left-to-right across this septum as blood returns to the right atrium and ventricle. Without some restriction to flow at the atrial septum, the physiology will tend to very high Qp:Qs where the right ventricle struggles to provide sufficient systemic output to support metabolic demands.

In neonates with TA, echocardiography is used to determine whether there is a ventricular septal defect capable of providing pulmonary blood flow independent of the ductus arteriosus. If so, pulse Doppler can be employed to assess whether this source of PBF is likely to be sufficient or even excessive. Although the physiologic goal remains achieving a Qp:Qs near 1, the surgical options to achieve that objective run the gamut from no neonatal intervention to supplementing PBF with a systemic-to-pulmonary shunt to limiting PBF with a pulmonary artery band.

Preoperative evaluation of infants and children for SVC-PA anastomosis and Fontan completion typically requires additional cardiac diagnostic information. A few mmHg pressure difference in the systemic venous circulation after these interventions can make the difference between a good outcome and significant morbidity. Systemic venous pressure lower than 15 mmHg is generally well tolerated, while pressure above 19 mmHg is much more likely to be associated with significant morbidity. Non-invasive cardiac imaging modalities are not yet capable of measuring PVR and diastolic ventricular function with sufficient precision to distinguish good candidates for these procedures, so cardiac catheterization is necessary to measure PVR, ventricular end-diastolic pressure and minor restrictions across the atrial septum.

• Medically unstable conditions warranting further evaluation include:

Neonates demonstrating physiology at the extremes of Qp:Qs are less medically stable with either significant hypoxemia or evidence of insufficient systemic perfusion. In either case, prompt surgical intervention is warranted.

• Delaying surgery may be indicated if:

A neonate who presented with a severe shock state or cardiac arrest has sustained vital organ injury that would benefit from a period of recovery before being subjected to the rigors of a major cardiac surgery. Otherwise, surgery may be delayed to allow thorough evaluation of any evidence of a serious genetic syndrome or other major malformation that would preclude meaningful survival. Because of its salutary effect on ventricular volume loading, there are few non-cardiac indications to delay SVC-PA anastomosis short of acute infectious illnesses. Since cardiac output is likely to be preserved, some advocate the operation even in the face of moderate ventricular dysfunction in hopes that relief of the volume overload will improve performance. However, Fontan completion should be delayed if there are any indications that PVR or ventricular diastolic function will result in systemic venous pressure elevations above 18-19 mmHg as they are much more likely to result in low cardiac output syndromes and serious morbidity.

3. What are the implications of co-existing disease on perioperative care?

Perioperative evaluation

Coexisting conditions fall into two broad categories: those that are secondary to the cardiac disease and those that are primary related to malformations of other organ systems and syndromes. Neither HLHS nor TA are frequently associated with genetic syndromes, but they can sporadically occur with a variety of malformations and should be thoroughly evaluated in the newborn period. Secondary organ injury can occur on initial presentation, particularly in neonates who were not suspected of having heart disease until they developed profound hypoxemia or shock. Heterotaxy syndrome is a rare disorder occasionally associated with complex single ventricle malformations as well as intestinal malrotation and other developmental abnormalities in the abdominal viscera. Over time, single ventricle patients acquire a variety of secondary organ system manifestations that will be described below.

Perioperative risk reduction strategies

The neonate who presents with secondary organ injury should have a period of supportive intensive care to enable the injury to resolve unless satisfactory resuscitation to a stable hemodynamic state cannot be achieved without an intervention. Infants for SVC-PA anastomosis or Fontan completion should not undergo those procedures until acute intercurrent respiratory illnesses are resolved. Indeed Fontan procedures may be best avoided entirely during influenza season.

b. Cardiovascular system

Acute/unstable conditions

The substantial proportion of neonates with single ventricle who require the ductus arteriosus to provide either systemic or pulmonary blood flow are acutely unstable until they are recognized to have the heart malformation and PGE instituted. Despite PGE infusion a small minority will remain unstable when the infusion is not effective or they have an anatomic variant such as HLHS with intact atrial septum that requires emergent intervention. Following the initial stage, infants can become unstable if the source of pulmonary blood flow is acutely compromised (e.g., Blalock-Taussig shunt thrombosus).

Baseline cardiac dysfunction – goals of management

The initial stage of management for any single ventricle patient requires a systemic connection to the pulmonary circulation and thus creates an excess volume load for the ventricle. These neonates are often placed on digoxin and diuretic and occasionally angiotensin converting enzyme (ACE) inhibitor therapy to ameliorate mild congestive heart failure. ACE inhibitors may be continued following Fontan completion as a means to try and optimize ventricular compliance and lower end-diastolic pressure.

c. Pulmonary

Pleural effusions

While pleural effusions can complicate any stage of repair, they are most common following Fontan completion. In the early postoperative period, they can compress the lung, compromise gas exchange and pulmonary blood flow thereby jeopardizing cardiac output. Active surveillance for this sequella is critical as is drainage of symptomatic patients.

ARDS

In the small cohort of neonates with HLHS and severely restrictive interatrial communication, pulmonary venous pressure can be extraordinary resulting in extreme pulmonary congestion and very low pulmonary compliance. This condition can only be treated by relief of the interatrial obstruction. Even when that is successfully accomplished, it may take days for the pulmonary function to resolve.

Pulmonary A-V malformations

Microscopic connections between distal pulmonary artery branches and pulmonary veins represent a cause of progressive hypoxemia, particularly in patients with SVC-PA anastomosis. The presence of AVM’s can be inferred from unusually rapid transit on PA angiography and direct measurement of pulmonary vein oxygen saturation. If hemodynamics permit, the only treatment is Fontan completion to restore hepatic venous return to the PA.

Reactive airway disease (asthma)

Although there is no causal association between single ventricle and asthma, the latter is sufficiently common in children that some patients have both conditions. Careful expectant management of asthma is important particularly at the Fontan stage where small adverse impact on PVR is important. Asthma medications should be continued perioperatively in these children.

d. Renal-GI:

Perioperative evaluation

Primary renal and GI abnormalities rarely occur with HLHS or TA. However, they may be part of syndromes in which these or other single ventricle malformations occur. All neonates should have basic serum chemistries to identify renal dysfunction. Further diagnostic evaluation using ultrasound or other modalities is indicated when patients exhibit stigmata of multiple congenital malformations. Heterotaxy syndrome, evident by complex cardiac single ventricle malformations, is commonly associated with intestinal malrotation and polysplenia or asplenia and thus warrants full evaluation of abdominal viscera.

Neonates who present in shock also may have secondary renal or hepatic dysfunction. They should routinely be screened with blood chemistry panels to detect organ injury prior to major interventions.

Perioperative risk reduction

Primary anatomic malformations of the renal or GI systems rarely have functional significance and thus do not require specific risk reduction strategies. Intestinal malrotation may require a Ladds procedure to reduce the risk of volvulus, but this can be considered electively after the neonate has fully recovered from the initial cardiac intervention. Heterotaxy patients with functional asplenia receive lifelong antibiotic prophylaxis.

Newborns who have sustained renal or hepatic dysfunction as a result of shock should be allowed to recover as long as the cardiovascular system can be stabilized with PGE and other supportive measures.

e. Neurologic:

Acute issues

Whether from perinatal trauma or presentation in shock, newborns can sustain acute neurologic injury including intracranial hemorrhage and/or hypoxic-ischemic encephalopathy. As long as the circulation can be stabilized, a thorough evaluation including EEG, CT scan, and possibly MRI is necessary to determine need for treatment, prognosis and the timing of any intervention. In evaluating these newborns, it is important to know that a significant number have MRI abnormalities without antecedent history of shock or trauma, possibly due to abnormal cortical blood flow patterns in utero. Devastating neurologic injury may cause a family to decline aggressive management of the cardiac malformation. In the face of significant hemorrhage, interventions that require anticoagulation may be postponed several days, although there is little scientific evidence to inform a decision as to how long to wait.

Chronic disease

Single ventricle lesions may be part of a constellation of malformations that comprise a genetic or other recognized syndrome. Rarely these syndromes include extensive neurologic involvement that carries a bleak prognosis for cognitive development. In these circumstances, full neurologic evaluation is critical to aid the decision as to whether to intervene in the cardiac condition. More commonly, syndromes are associated with measurable but less extreme developmental delay. Nevertheless, this prognostic information should be provided to the family.

f. Endocrine:

N/A

g. Additional systems/conditions which may be of concern in a patient undergoing this procedure and are relevant for the anesthetic plan (eg. musculoskeletal in orthopedic procedures, hematologic in a cancer patient)

N/A

4. What are the patient's medications and how should they be managed in the perioperative period?

Prior to the initial procedure, patients who have ductus dependent systemic or pulmonary circulation are maintained with a continuous infusion of PGE. These must be maintained until the intervention designed to provide a stable source of systemic or pulmonary blood flow. Newborns receiving PGE require intensive care observation both because of the vulnerable nature of their circulation should the infusion be interrupted as well as some of the side effects, most importantly apnea, that can accompany these infusions.

Following the initial intervention, the single ventricle has a significant excess volume load which can predispose them to congestive heart failure. As a result, these neonates are commonly supported with digoxin and diuretic therapy. A small proportion will have evidence of significant ventricular dysfunction on echocardiography and they will typically receive an ACE inhibitor in addition to augment ventricular performance. Neonates with prosthetic systemic to pulmonary artery shunts will typically receive anti-platelet therapy with aspirin to reduce the risk of shunt thrombosus. With the exception of anti-platelet therapy, these medications should be continued up to the day of surgery. Surgeons have varying preferences regarding recovery from anti-platelet therapy, so it is important to collaborate on a plan for these medications.

Once the SVC-PA anastomosis has been performed, the ventricular volume load returns to normal and the need for supportive cardiac medications diminishes in most infants. Some may continue to receive diuretic therapy, particularly if they have manifested pleural effusions. A few infants who continue to demonstrate diminished ventricular function will continue to receive an ACE inhibitor after this procedure. These medications should be continued up to the day of surgery.

h. Are there medications commonly seen in patients undergoing this procedure and for which should there be greater concern?

Among the various medications that these infants may receive for chronic co-existing conditions, perhaps the most important are anti-epileptic medications. In concert with neurologists, a plan must be developed to avoid a significant reduction in therapeutic level following cardiopulmonary bypass. This usually involves parenteral supplementation or substitution designed to maintain a therapeutic level until routine oral treatment can be resumed.

i. What should be recommended with regard to continuation of medications taken chronically?

• Cardiac:PGE should be maintained to the OR, otherwise cardiac medications should continue to the day of surgery

• Pulmonary: continued to the day of surgery

• Renal: diuretics may be withheld on the day of surgery as they are typically administered intravenously during the procedure

• Neurologic: must be maintained through the morning of surgery with careful plan to re-institute or substitute postoperatively

• Anti-platelet: usually stopped 5-7 days before surgery

j. How To modify care for patients with known allergies –

Significant medication allergies are uncommon in newborns and young children who typically undergo single ventricle surgery. Nor is latex allergy common, despite the multiple healthcare exposures these infants experience. The only allergy that presents with any frequency is antibiotic reactions to penicillins or cephalosporins. In such cases, clindamycin is substituted in accordance with AHA recommendations for bacterial endocarditis prophylaxis.

k. Latex allergy- If the patient has a sensitivity to latex (eg. rash from gloves, underwear, etc.) versus anaphylactic reaction, prepare the operating room with latex-free products.

N/A

l. Does the patient have any antibiotic allergies- – Common antibiotic allergies and alternative antibiotics]

N/A

m. Does the patient have a history of allergy to anesthesia?

Malignant hyperthermia

• Documented- avoid all trigger agents such as succinylcholine and inhalational agents:

– Proposed general anesthetic plan:

– Ensure MH cart available: MH protocol

• Family history or risk factors for MH:

• Local anesthetics/ muscle relaxants:

5. What laboratory tests should be obtained and has everything been reviewed?

Laboratory investigations of newborns with single ventricle are designed to achieve two objectives: determine the nature and extent of the cardiovascular disease and assess the primary and secondary involvement of other organ systems. Although physical exam and electrocardiography play an important role, echocardiography is the central pillar in the diagnosis of the anatomy and physiology of single ventricle malformations.

Cardiac catheterization is rarely necessary at this stage. Echocardiogram should be able to determine systemic and pulmonary venous connections; size and function of the atrioventricular valves; ventricular size and function; source of blood flow to the aorta and pulmonary arteries; and physiologic clues to areas of obstruction and relative pulmonary and systemic resistance. These assessments will help the anesthesiologist anticipate changes that might occur during the induction and maintenance of anesthesia.

Screening for other organ system involvement includes a careful physical exam looking for dysmorphic features that indicate syndromes or other isolated primary organ malformations. Blood chemistries are useful to determine whether secondary injury has occurred to kidneys, liver or other viscera. A baseline head ultrasound is standard as a screening assessment to look for malformations or intracranial hemorrhage.

Further testing is pursued when indicated by screening tests. For example, dysmorphic facial features, seizure activity, and/or abnormal head ultrasound would trigger further evaluation with EEG, CT scan or head MRI. Abnormal blood chemistries or echocardiography suggesting heterotaxy would trigger further diagnostic evaluation of the abdomen using ultrasound or other modalities. These extensive screening processes are vital to optimally plan timing and support during major cardiac surgery as well as providing prognostic information to help set the parents’ expectations and aid their decision making.

Infants presenting for SVC-PA anastomosis undergo a more targeted laboratory evaluation. While echocardiography is a part of this evaluation, the critical predictors of outcome are PVR and diastolic ventricular function, which require cardiac catheterization to measure with any precision. Specifically, good candidates will demonstrate PVR of 2 Wood units or less and a systemic ventricular end-diastolic pressure under 10 mmHg. Anatomic assessment of the pulmonary arteries is important to identify areas that might be amenable to surgical augmentation.

Unless previously identified derangements require follow-up, the blood tests at this stage are limited to a rudimentary blood count and electrolyte values. The expected hemoglobin levels are somewhat higher than normal due to hypoxemia and the electrolytes may exhibit alkalotic changes consistent with chronic diuretic administration. Coagulation studies are confined to the few infants who are on anticoagulant therapy.

Although arguably PVR and diastolic ventricular function are even more important for Fontan completion, cardiac catheterization is not necessarily repeated prior to this stage unless these values were known to be concerning. Cardiac MRI is increasingly employed prior to this stage as refinements in this modality now enable detailed delineation of the branch pulmonary artery anatomy and physiologic measures of pulmonary arterial and venous blood flow that can quantify the importance of collateral vessels. In addition, more precise quantification of valve regurgitation is possible. The same approach to blood tests is employed for this stage as in infants for SVC-PA anastomosis.

Intraoperative Management: What are the options for anesthetic management and how to determine the best technique?

Neonates with single ventricle undergo procedures that vary widely in complexity and thus anesthetic technique. Although the physiologic goals are the same for these neonates, anatomic variables require vastly different surgical management each of which has ramifications for anesthetic management. HLHS is typically treated with a Norwood operation, which is one of the most complex of any surgeries for congenital heart disease.

At the other extreme, a neonate with TA and a large VSD might only require pulmonary artery band to restrict pulmonary blood flow. Across this range, however, all these procedures require general anesthesia. Even hybrid cath/surgery approaches to HLHS entail a sternotomy for placement of bands on the branch pulmonary arteries. In some centers, neuraxial regional techniques are employed as an adjuvant to general anesthesia to augment postoperative pain management.

Regional anesthesia

At most, neuraxial techniques serve as an adjuvant to general anesthesia designed to enhance postoperative analgesia. While there are some case series demonstrating efficacy in small numbers of patients without adverse events, there are no randomized prospective trials that show benefit in important outcome measures. In the absence of clear benefit, most clinicians are hesitant to employ regional methods because of procedural anti-coagulation for cardiopulmonary bypass and confounding effects of perioperative risks with low cardiac output syndromes and neurologic injury.

Neuraxial

• Benefits

– Doppler flow studies demonstrate enhanced pulmonary blood flow following Fontan procedure with the negative intrathoracic pressure that accompanies spontaneous ventilation as compared to positive intrathoracic pressure that occurs with mechanical ventilation. Thus, there is a theoretical advantage to early extubation following Fontan surgery, and presumably SVC-PA anastomosis. Advocates of neuraxial techniques often point to this finding and credit these blocks as a means to achieving earlier extubation. However, carefully administered combinations of opioids and sedatives like dexmedetomidine can reliably achieve the same objective.

• Drawbacks

– Risks of epidural hematoma in anticoagulated patients.

General anesthesia

The objectives of general anesthesia have evolved considerably over time. In the 1980’s there were indications that neonates and young infants would benefit from high dose opioid anesthetic techniques designed to suppress endogenous stress responses. But in the following decade, more modest doses of opioids provided equivalent outcome despite a lesser ability to suppress endogenous stress hormones. At present, anesthetic management should be tailored to expected perioperative hemodynamic responses.

Neonates are prone to high or volatile pulmonary vascular resistance, particularly if there has been some degree of obstruction to pulmonary venous return. Hence newborns with HLHS are notorious for volatile postoperative hemodynamic changes. As a result, most centers would incorporate relatively high dose opioids in their anesthetic regimen and often continue these agents by infusion for at least the first 24 hours in order to minimize this volatility. At the other extreme, TA neonates who only require a pulmonary artery band can receive a more modest dose of opioids with a goal of tracheal extubation on the day of surgery. Other single ventricle variants are managed according to the complexity of the procedure they require and underlying hemodynamic proclivities.

Infants for SVC-PA anastomosis and young children for Fontan completion are typically admitted electively on the day of surgery. In the absence of airway, respiratory or cardiovascular contraindications, they usually benefit from sedative premedication. Except for those with significant myocardial dysfunction, most will tolerate a careful mask induction with sevoflurane. As noted previously, intravenous sedatives and opioids should be employed judiciously with a goal of being able to resume spontaneous ventilation and extubate as soon as other considerations permit (i.e., good cardiac performance and minimal ongoing hemorrhage) in the ICU.

6. What is the author's preferred method of anesthesia technique and why?

The anesthetic for neonates with single ventricle varies according to the complexity of the physiology and the planned surgical procedure. Irrespective of the anatomic variant, the single ventricle struggles to maintain sufficient systemic output in the face of a substantial additional volume demand imposed by providing the pulmonary circulation in parallel. As a result, most of these neonates are exquisitely sensitive to myocardial depressant effects of anesthetics (or other drugs). An opioid-based technique (e.g., fentanyl 20 mcg/kg) effectively blunts undesirable volatile PVR changes while preserving ventricular function and still allowing tracheal extubation in the first 24 hours as appropriate.

Infants for SVC-PA anastomosis and young children for Fontan completion typically receive oral sedative premedication with pentobarbital (4 mg/kg) or midazolam (0.5-1 mg/kg) prior to mask induction with sevoflurane. These are volatile-based anesthetics with smaller opioid doses (fentanyl 5-7 mcg/kg) to supplement analgesia. At the completion of the procedure, if there are no contraindications to immediate tracheal extubation (e.g., ongoing hemorrhage or significant myocardial dysfunction), a loading dose of dexmedetomidine (0.5-1 mcg/kg) is administered followed by an infusion (0.5-1 mcg/kg/hr). Once extubated, supplementary morphine doses (0.05-0.1 mg/kg) are administered as necessary for analgesia.

What prophylactic antibiotics should be administered?

In accordance with the guidelines of the American Heart Association, all single ventricle patients undergoing cardiac surgery receive antibiotic prophylaxis with a first generation cephalosporin. Those with cephalosporin allergy receive clindamycin. As the AHA acknowledges, there is growing consensus that vancomycin should be considered when a given patient is colonized with methicillin-resistant staph or there is an unusually high prevalence of MRSA in the institution.

What do I need to know about the surgical technique to optimize my anesthetic care?

Although surgical approaches to single ventricle vary at each stage, the anesthetic implications are most significant with the initial stage. Neonates with HLHS pose the greatest management challenges. They present to the OR with a widely varying Qp:Qs.

Those with little restriction to pulmonary blood flow will be extremely sensitive to ventilatory management that lowers PVR, as it diverts even more cardiac output to the pulmonary circulation and often results in systemic hypotension, metabolic acidosis and occasionally coronary ischemia. In this cohort, one must be extremely judicious in the use of supplementary oxygen and mechanical ventilation.

If available, addition of inspired carbon dioxide (2-4%) can be extremely effective in maintaining PVR and systemic blood flow. At the other extreme, HLHS variants with very restrictive pulmonary blood flow require increasingly aggressive management of supplementary oxygen and ventilation to lower PVR and provide enough PBF to oxygenate the systemic circulation. When pulmonary venous return is profoundly obstructed, the pulmonary venous congestion results in extraordinarily poor lung compliance further complicating this task.

There are three primary variations to HLHS at the initial stage: Norwood operation (aortopulmonary anastomosis with reconstruction of the aortic arch, atrial septectomy and a Blalock-Taussig shunt); Sano modification (a RV-pulmonary conduit is substituted for the B-T shunt); hybrid procedure (transcatheter septostomy, stent of PDA and surgical application of bands to the branch PA’s). The Norwood operation andano Sano modification have no significant differences in terms of intra-operative expectations.

They require extensive reconstruction of the aortic arch and a systemic-to-pulmonary artery shunt. The major immediate physiologic goals are to balance Qp:Qs using ventilatory manipulations; support systemic cardiac output with some combination preload, inotropic support and vasodilators; and restore normal hemostasis with protamine and blood products.

The most common problems encountered are: excessive hemorrhage; inadequate PBF due to high PVR or anatomic distortions caused by the shunt; low cardiac output due to the usual factors (preload, afterload, contractility) or excessive run-off into the pulmonary circulation; and myocardial ischemia related to distortion of the ascending aorta in the arch reconstruction or low diastolic pressure due to semilunar valve insufficiency or pulmonary run-off. Arrhythmias usually reflect underlying problems with the hemodynamics or myocardial ischemia and they should trigger an evaluation aimed at correcting the primary cause.

The hybrid procedure achieves the same physiologic goals without the need for complex open heart surgery and cardiopulmonary bypass. The immediate goals are the same: balance Qp:Qs and support systemic output. The causes of insufficient PBF are high PVR or PA bands that are excessively tight or distorting. Myocardial ischemia and poor ventricular function can result from malposition of the PDA stent that compromises retrograde flow into the aortic arch and coronaries as well as inadequate diastolic pressure.

While there are two primary surgical approaches to SVC-PA anastomosis (i.e., Bi-directional Glenn and Hemi-Fontan), they do not have important implications for anesthesia management. The distinction resides primarily in the manner in which they set the stage for Fontan completion. The former will require an extracardiac conduit from IVC to the PA’s while the latter will enable an intra-atrial baffle that incorporates a strip of atrial tissue.

In theory, the former avoids suture lines in the atrium and may reduce ultimate development of atrial arrhythmias, while the latter has growth potential that may be beneficial with small children. The immediate goals following this intervention are to augment intravascular volume to provide sufficient venous pressure to promote pulmonary blood flow and to minimize PVR. Best results for the latter include careful re-expansion of the lungs to normal functional residual capacity and a ventilation strategy that lowers alveolar CO2 with minimal positive pressure on the alveolar capillary bed.

This goal is most efficiently achieved with relatively large tidal volumes (10-12 ml/kg) at low rate (18-20/ min) with long expiratory time. The most common intra-operative problems include: excessive hemorrhage; unexpected hyoxemia due to anatomic issues with the anastomosis, inadequate SVC pressure, increased PVR or a decompressing venous communication to the IVC; or low cardiac output. Barring significant problems with myocardial protection, the latter is uncommon given that this procedure reduces ventricular volume work by nearly 50%.

If hypoxemia persists despite measures to optimize intravascular volume, ventilation and the anatomosis, a cardiac catheterization may be required to investigate and address decompressing veins. Milrinone exerts beneficial effects on systolic and diastolic function as well as PVR and thus should be the cornerstone of pharmacologic support.

Fontan completion generally follows the path set by the surgical technique selected for the SVC-PA anastomosis. While this distinction may have ramifications for cardiopulmonary bypass methods, there are no significant physiologic distinctions at the conclusion of the procedure. Most surgeons will leave a small fenestration between the IVC-PA connection and atrium. This permits some right-to-left blood flow that preserves ventricular preload when there are impediments to PBF of any kind. As a result, systemic oxygen saturation will be modestly lower than normal.

As with the SVC-PA anastomosis , the most important physiologic goal is to maximize PBF, but now the entire cardiac output relies on this flow, so the stakes are higher. This objective is achieved with a thoughtful ventilation strategy and fluid administration to augment intravascular volume. One cannot overstate the requirements for intravenous fluid supplements as the venous capacitance expands at the pressure usually necessary to provide sufficient PBF. Given the subtleties in assessing systemic output, milrinone is routinely instituted before specific indications are manifest. In addition to low cardiac output, hemorrhage is the most common immediate intraoperative problem.

What can I do intraoperatively to assist the surgeon and optimize patient care?

In all three stages, optimal patient care requires ongoing communication with the surgeon. Many problems have more than one cause and remedy, some of which may conflict. For example, hypoxemia following the initial procedure may be due to elevated PVR, a technical shunt problem, inadequate systemic perfusion pressure or cardiac output causing low mixed venous saturation. The means to address these vary considerably, so it is important to work through them systematically with the surgeon.

What are the most common intraoperative complications and how can they be avoided/treated?

For better clarity, they are presented above with each stage of the procedure.

a. Neurologic:

N/A

b. If the patient is intubated, are there any special criteria for extubation?

N/A

c. Postoperative management

All single ventricle patients, regardless of the stage of intervention, require careful observation in an intensive care unit following these surgeries. Neonates following initial stage surgery remain intubated and sedated with opioids for at least 12-24 hours to allow the most volatile period in PVR to subside and ventricular function to recover.

If the underlying hemodynamics are sound, ventilatory support is diminished gradually and they can be extubated. If the hemodynamics do not seem satisfactory as they resume the work of ventilation, further investigation as to the cause is warranted using echocardiography and occasionally catheterization. The most common early postoperative complications are hemorrhage; low cardiac output; and rarely sudden cardiovascular collapse. The cause of the latter can be particularly elusive, often presumed to follow an acute change in PVR or coronary blood flow.

Later in the hospital course, unexpected hypoxemia is the most common complication and regularly requires catheterization to assure there are no anatomic problems with the shunt or PA’s. Since the hybrid procedure does not entail a period of myocardial ischemia on CPB, recovery from this procedure is more rapid. If the neonate has gone into that procedure with a natural airway, it is often possible to accelerate postoperative recovery and extubate in the first few hours following this intervention.

Barring ongoing hemorrhage or hemodynamic issues, infants following SVC-PA anastomosis and Fontan completion are extubated immediately after surgery. Sedative/analgesic regimens must be designed to provide patient comfort without major respiratory depression given the critical importance of alveolar gas exchange to PVR.

A dexmedetomidine infusion with small incremental doses of opioids provides these conditions with great reliability. Pleural and pericardial effusions represent the most common postoperative complication manifest by these infants, particularly following Fontan completion. By compressing the lung, they can have a significant adverse impact on respiratory mechanics and PVR, ultimately compromising cardiac output. Thus it is important to drain significant effusions when they cause any symptoms. Ultimately, the body will accommodate to this extraordinarily unusual cardiovascular physiology and the effusions will subside.

What's the Evidence?

Gaynor, JW, Mahle, WT, Cohen, MI. “Risk factors for mortality after the Norwood procedure”. Eur J Cardio-Thorac Surg. vol. 22. 2002. pp. 82-9. (This paper discusses risk factors for mortality in patients with hypoplastic left heart syndrome. It looks at 158 patients over a 3.5-year period.)

Ohye, RG, Sleeper, LA, Mahony, L. “Comparison of shunt types in the Norwood procedure for single-ventricle lesions”. New Engl J Med. vol. 362. 2010. pp. 1980-92. (This randomized study compares a modified BT shunt to right ventricle to pulmonary artery shunt in patients undergoing a Norwood procedure. Transplantation free survival was better in the right ventricle to PA shunt group at 12 months but these patients required more unintended interventions and complications.)

Rychik, J, Bush, DM, Spray, TL. “Assessment of pulmonary/systemic blood flow ratio after first stage palliation for hypoplastic left heart syndrome: Development of a new index with the use of Doppler echocardiography”. J Thorac Cardiovasc Surg. vol. 120. 2000. pp. 81-7. (The purpose of the paper was to investigate the utility of arterial PO2 arterial saturation and a Doppler derived flow index to predict Qp:Qs ratio following first stage repair of patients with hypoplastic left heart.)

Donofrio, MT, Jacobs, ML, Spray, TL, Rychik, J. “Acute changes in preload, afterload, and systolic function after superior cavopulmonary connection”. Ann Thorac Surg. vol. 65. 1998. pp. 503-8. (This paper demonstrates that preload and afterload decrease following superior cavopulmonary correction.)

Salvin, JW, Scheurer, MA, Laussen, PC. “Factors associated with prolonged recovery after the Fontan operation”. Circulation. vol. 118. 2008. pp. S171-6. (This paper reports on 218 patients following their Fontan procedure and looks at the associated factors that prolong recovery.)

Gruber, EM, Laussen, PC, Casta, A. “Stress response in infants undergoing cardiac surgery: A randomized study of fentanyl bolus, fentanyl infusion, and fentanyl-midazolam infusion”. Anesth Analg. vol. 92. 2001. pp. 882-90. (This paper reports on fentanyl and midazolam administration with respect to their effects on the stress response in patients undergoing cardiac surgery.)

Hickey, PR, Hansen, DD, Wessel, DL. “Blunting of stress responses in the pulmonary circulation of infants by fentanyl”. Anesth Analg. vol. 64. 1985. pp. 1137-42. (Landmark paper which demonstrates that fentanyl can attenuate the pulmonary vasoconstrictive effect of endotracheal tube suctioning.)

Tabbutt, S, Ramamoorthy, C, Montenegro, LM. “Impact of inspired gas mixtures on preoperative infants with hypoplastic left heart syndrome during controlled ventilation”. Circulation. vol. 104. 2001. pp. I-159-64. (This study shows the effects of a hypoxic gas mixture and hypercarbic gas mixture on the pulmonary to systemic ratio.)

Naguib, AN, Winch, P, Schwartz, L. “Anesthetic management of the hybrid stage 1 procedure for hypoplastic left heart syndrome (HLHS)”. Pediatr Anesth. vol. 20. 2010. pp. 38-46. (This retrospective study reports on the institutional results and anesthetic management of 77 patients who underwent the hybrid approach to treatment of the left heart syndrome.)

Tokuhira, N, Atagi, K, Shimaoka, H. “Dexmedetomidine sedation for pediatric post-Fontan procedure patients”. Pediatr Crit Care Med. vol. 10. 2009. pp. 207-12. (This paper discusses the use of dexmedetomidine in 14 patients following the Fontan procedure.)

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