Monitoring systemic venous oxygen saturations in the hypoplastic left heart syndrome

The Congenital Heart Surgeons' Society Presidents and Their Contributions

The Congenital Heart Surgeons' Society (CHSS) was founded by 16 congenital heart surgeons in 1973, who endeavored to share their clinical advances in an informal setting that would stimulate honest and forthright discussions. As the Society grew, prospective studies were organized from a centralized data center that was established and based first in Birmingham, Alabama, thence to Toronto, and recently in a collaboration between Toronto and the Cleveland Clinic. These studies formed the basis for a myriad of outcomes reports that favorably impacted surgical results. The Kirklin-Ashburn Fellowship was created and endowed by the membership which has been successful in training many congenital heart surgeons. The CHSS was then incorporated into a 501(c) (3) not-for-profit organization with bylaws, officers, and committees in 2002. Increased membership followed. The CHSS has become the face of congenital heart surgery in North America by affiliating with the World Journal for Pediatric and Congenital Heart Surgery, having one designated member on the American Board of Thoracic Surgery, and hosting joint meetings with the European Congenital Heart Surgeons Association. Since 2002, 11 presidents have been elected for two-year terms and have guided the advances that have been achieved by the CHSS. Their contributions and achievements are highlighted in chronological order.

Balancing a single-ventricle circulation: 'physiology to therapy'

The parallel supply of the pulmonary and systemic circuits complicates the management of single-ventricle lesions. Achieving a balance between the two limbs of the circulation forms the basis of optimizing the systemic oxygen delivery, with the oxygen availability being highly sensitive to alterations in pulmonary/systemic blood flow ratio (Q _p/Q _s). The identification of a 'balanced' circulation is challenging wherein various parameters should be evaluated in close conjunction with each other. The prompt identification of circulatory maldistribution should be backed up with a sound management strategy aimed at attaining an equitable systemic and pulmonary perfusion. Any degree of ventricular dysfunction compromises the total output (Q _p + Q _s) supplying the two circuits explaining the role of inodilators in improving the myocardial performance in addition to lowering the systemic vascular resistance and optimizing Q _p/Q _s in setting of a single-ventricle physiology. Moreover, the pulmonary circulation is modulated by a multitude of factors intricately linked to the single-ventricle lesion, including anatomical characteristics unique to the underlying lesion (branch pulmonary arterial and venous stenosis), preoperative interventions, associated aortopulmonary and venovenous collaterals, plastic bronchitis, pulmonary arteriovenous fistulae, underlying ventricular dysfunction,, and many others. The article highlights the physiology, diagnosis, therapeutic optimization of a single-ventricle circulation, and the peculiarities pertaining to the pulmonary circulation of the uni-ventricular lesions.

Resuscitation and perioperative management of the high-risk single ventricle patient: first-stage palliation

Infants born with hypoplastic left heart syndrome or other lesions resulting in a single right ventricle face the highest risk of mortality among all forms of congenital heart disease. Before the modern era of surgical palliation, these conditions were universally lethal; recent refinements in surgical technique and perioperative management have translated into dramatic improvements in survival. Nonetheless, these infants remain at a high risk of morbidity and mortality, and an appreciation of single ventricle physiology is fundamental to the care of these high-risk patients. Herein, resuscitation and perioperative management of infants with hypoplastic left heart syndrome are reviewed. Basic neonatal and pediatric life support recommendations are summarized, and perioperative first-stage clinical management strategies are reviewed.

Two-day control of pulmonary blood flow with an adjustable systemic-pulmonary artery shunt

Our laboratory has developed an adjustable systemic-pulmonary artery shunt (AS) to provide improved control of pulmonary blood flow (PBF) after neonatal palliation of hypoplastic left heart syndrome (HLHS). Six piglets of 6-10 kg underwent left thoracotomy and placement of a 3.5 mm polytetrafluoroethylene (PTFE) shunt from the left subclavian artery to the left pulmonary artery (LPA). The LPA was ligated at its origin. An AS was placed on the PTFE graft after both anastomoses had been performed. A flow probe was placed on the LPA distal to the shunt insertion. The AS was adjusted every 2 hours (0.1 mm increments over 18 minutes) from fully open to an estimated 60% flow reduction throughout the 44-hour test period, similar to delayed sternal closure (DSC). At any shunt setting, standard deviations of normalized blood flow in each piglet were ranged from 1.34% to 8.05% indicating consistent and stable relationship between shunt setting and flow over the DSC. These data lend strong evidence that the device would perform successfully in a human infant during the DSC. Clinical trials are necessary to determine whether mechanical control of PBF results in improved clinical outcomes.

Phenoxybenzamine in the Treatment of Hypoplastic Left Heart Syndrome: A Core Review

Perioperative management of neonates after the Norwood procedure is extremely complex. Limited reserve of the neonatal single ventricle and the parallel arrangement of the pulmonary and systemic circuits result in a tenuous balance between pulmonary and systemic blood flows. Precise manipulations of both pulmonary and systemic vascular resistance are necessary to prevent excessive pulmonary blood flow at the expense of systemic oxygen delivery. An emerging treatment strategy aimed at improving early mortality is the intraoperative administration of phenoxybenzamine, a profound systemic vasodilator. Maximum systemic vasodilation is thought to reduce afterload of the single ventricle and produce a more stable parallel circulation by ameliorating the postoperative fluctuations in systemic vascular resistance. Although this strategy has gained popularity at many centers, it is not without scrutiny. The following review provides an overview of the pharmacology of phenoxybenzamine, the surgical and physiologic implications of the Norwood procedure, and a discussion of the pros and cons of phenoxybenzamine administration.

Postoperative management: the role of mixed venous oxygen saturation monitoring

The results of the Norwood operation have improved considerably over the last decade, due in part to improvement in postoperative care. Mixed venous oxygen saturation (MvO2) monitoring has been an important addition to postoperative management. Our use of MvO2 monitoring in Norwood patients has included 96 infants operated from 1996 to the present. This strategy has proven to be technically straightforward and adds information not provided by monitoring systemic saturation alone. MvO2 has a nadir at 6-12 hours after surgery and below a value of 30% is associated with anaerobic metabolism. It identifies patients at risk for early mortality. It also allows evaluation of management of treatment strategies that evolve over time and of specific interventions in individual patients. Optimizing MvO2 constitutes an important goal of postoperative management after the Norwood procedure.

Single-ventricle physiology: perioperative implications

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.

Alteration of the critical arteriovenous oxygen saturation relationship by sustained afterload reduction after the norwood procedure

OBJECTIVES

Hemodynamic vulnerability after the Norwood procedure for hypoplastic left heart syndrome results from impaired myocardial function, and critical inefficiency of parallel circulation. Traditional management strategies have attempted to optimize circulatory efficiency by using arterial oxygen saturation (SaO(2)) as an index of pulmonary/systemic flow balance, attempting to maintain SaO(2) within a theoretically optimal critical range of 75% to 80%. This optimal range of SaO(2) has not been verified clinically, and strategies targeting SaO(2) may be limited by the fact that SaO(2) is a poor predictor of systemic oxygen delivery. We have previously reported higher venous saturation (SvO(2)), lower arteriovenous oxygen content difference, lower systemic vascular resistance, lower pulmonary/systemic flow ratio, and improved survival with the perioperative use of phenoxybenzamine and continuous monitoring of SvO(2). In this investigation, we tested the hypothesis that intense afterload reduction with phenoxybenzamine would modify the SvO(2)-SaO(2) relationship by preventing deterioration of systemic oxygen delivery at high SaO(2).

METHODS

Seventy-one consecutive neonates undergoing the Norwood procedure with and without phenoxybenzamine were studied. Perioperative hemodynamic management targeted SvO(2) greater than 50%. Hemodynamic data were prospectively acquired for 48 hours postoperatively and analyzed to assess the effect of phenoxybenzamine on the relationship between SaO(2) and SvO(2) and other hemodynamic indices. Sixty-two patients received phenoxybenzamine 0.25 mg/kg on cardiopulmonary bypass; 9 who did not served as controls.

RESULTS

In control patients, SvO(2) peaked at an SaO(2) of 77%, with reduced SvO(2) at SaO(2) > 85% and SaO(2) < 70% (P <.01), while arteriovenous oxygen content difference increased with SaO(2) greater than 80% (P <.001). In patients receiving phenoxybenzamine, the SvO(2) increased linearly with SaO(2) greater than 65% (P <.001), and arteriovenous oxygen content difference was constant at all SaO(2) (P = ns). The SvO(2) was higher, and the arteriovenous oxygen content difference lower, across the whole SaO(2) range with phenoxybenzamine (P <.0001).

CONCLUSIONS

A critical range of SaO(2) for optimizing systemic oxygen delivery was confirmed in control patients, and was effectively eliminated by phenoxybenzamine, specifically by eliminating the systemic hypoperfusion associated with high SaO(2). This effect allows higher SaO(2) to be included in a rational hemodynamic strategy to improve systemic oxygen delivery in the early postoperative management of patients receiving intense afterload reduction with phenoxybenzamine. The predictability of SvO(2) from SaO(2) is low in both groups, emphasizing the importance of SvO(2) measurement in these patients.

Neonatal physiology of the functionally univentricular heart

The neonate with functionally univentricular physiology presents unique challenges to the cardiac team. An integrated approach that applies working knowledge of cardiac anatomy, cardiopulmonary physiology, and basic principles of intensive care is essential to guide management of each individual patient. This requires cooperative and constructive involvement of a surgical, medical, nursing and respiratory care team experienced in the management of such patients. In the neonate with this physiology, systemic oxygen delivery is optimized by manipulating pulmonary and systemic resistances, augmenting total cardiac output, and utilizing strategies for ventilation that preserve optimal pulmonary recruitment.

Single-ventricle physiology

The patient with single-ventricle physiology presents a significant challenge to the intensive care team at all stages of management. An integrated approach that applies a working knowledge of cardiac anatomy, cardiopulmonary physiology, and the basic principles of intensive care is essential to guide management for each individual patient. This management requires cooperative and constructive involvement of surgeons, cardiologists, and intensivists, as well as a nursing and respiratory care team experienced in the management of single-ventricle patients. The outcome of each stage of palliation for single-ventricle lesions should continue to improve as new ideas are developed and as older ideas are subjected to rigorous scientific analyses.

Modified Norwood operation for hypoplastic left heart syndrome

BACKGROUND

We examined early results in infants with hypoplastic left heart syndrome undergoing the Norwood operation with perioperative use of inhaled nitric oxide and application of extracorporeal membrane oxygenation.

METHODS

Medical records were reviewed retrospectively.

RESULTS

Between April 1997 and March 2001, 50 infants underwent a modified Norwood operation for hypoplastic left heart syndrome. Mean age at operation was 7.5 +/- 5.7 days, and mean weight was 3.1 +/- 0.5 kg. Five infants had a delayed operation because of sepsis. The mean diameter of the ascending aorta by echocardiography was 3.6 +/- 1.8 mm. Ductal cannulation was used to establish cardiopulmonary bypass in all patients. Mean circulatory arrest time was 39.4 +/- 4.8 minutes. The size of the pulmonary-systemic shunt was 3.0 mm in 6 infants, 3.5 mm in 37, and 4.0 mm in 7. Infants with persistent hypoxia (partial pressure of oxygen < 30 mm Hg) received nitric oxide after they were weaned from cardiopulmonary bypass. Extracorporeal membrane oxygenation was initiated in 8 infants in the pediatric intensive care unit primarily for low cardiac output and in 8 in the operating room because of the inability to separate them from cardiopulmonary bypass. The 30-day mortality rate was 22% (11 of 50 patients), and the hospital mortality rate was 32% (16 of 50 patients). Mean follow-up was 17 months. Ten patients (20%) underwent stage-two repair, with one operative death. One survivor had a Fontan procedure, and 2 underwent heart transplantation, with one death.

CONCLUSIONS

Early application of extracorporeal membrane oxygenation for hemodynamic instability and selective use of nitric oxide for persistent hypoxia in the immediate postoperative period may improve survival of patients with hypoplastic left heart syndrome. Renal failure requiring hemofiltration during extracorporeal membrane oxygenation (p < 0.05) and cardiopulmonary arrest in the pediatric intensive care unit (p < 0.05) were predictors of hospital mortality.

Hemodynamic effects of inspired carbon dioxide after the Norwood procedure

BACKGROUND

Mortality in the early postoperative period after the Norwood procedure remains substantial. Inspired carbon dioxide (CO2) has been suggested to improve hemodynamic status in this setting. Inspired CO2 can be delivered by one of two strategies, ie, with or without an accompanying increase in minute ventilation. The hemodynamic effects of these two strategies have not previously been studied in a controlled fashion.

METHODS

Seventeen infants (median age, 9 days; range, 4 to 49 days) undergoing Norwood procedures were prospectively enrolled in this crossover study. Patients were studied while sedated, paralyzed, and mechanically ventilated 1 day to 6 days after operation. The inspired oxygen fraction was kept constant (mean value, 0.24 +/- 0.01). Measurements were made at five time points: 1 = baseline; 2 = inspired CO2 with increased ventilation; 3 = baseline; 4 = inspired CO2 alone; and 5 = baseline. Mixed venous oxygen saturation was monitored using indwelling lines in the superior vena cava.

RESULTS

Inspired CO2 with increased ventilation produced a rise in mean airway pressure with no change in arterial CO2 tension or pH. This strategy had no effect on hemodynamic status or oxygen delivery. Inspired CO2 alone produced a rise in arterial CO2 tension and a fall in arterial pH (respiratory acidosis). This strategy resulted in significant improvement in both variables of systemic oxygen delivery: mixed venous oxygen saturation increased from 48% +/- 2% to 56% +/- 2% (p < 0.05), and arteriovenous oxygen saturation difference decreased from 3% +/- 2% to 26% +/- 2% (p < 0.05).

CONCLUSIONS

Inspired CO2 after the Norwood procedure can improve oxygen delivery. This improvement occurs only if minute ventilation is kept constant. There is no improvement if minute ventilation is increased. Clinical use of inspired CO2 may be limited by the accompanying fall in pH. Differentiation of cerebral from total-body effects of inspired CO2 will require further study.

Postoperative management after the Norwood procedure

Staged reconstruction has become the preferred approach to hypoplastic left heart syndrome at many centers in the United States. The overall results of this strategy are most adversely affected by a high mortality at the initial stage, the Norwood procedure. The hemodynamic instability of a single ventricle providing blood flow in parallel to the systemic and pulmonary circulations combined with the stresses of cardiopulmonary bypass and circulatory arrest result in a precarious postoperative condition. Diligent perioperative management at this stage is essential to survival. To help simplify the complexity of single-ventricle physiology, this article describes a mathematical model that identifies the key elements that affect systemic oxygen delivery. The importance of balancing the circulation is underscored. The value of monitoring both systemic arterial and venous oxygen saturations to assess systemic-to-pulmonary blood flow ratio is derived from this mathematical model and confirmed experimentally and clinically. Recent research using animal models of single-ventricle physiology is also described. Using these concepts and information, techniques for achieving adequate systemic oxygen delivery are discussed. Copyright 1998 by W.B. Saunders Company

Venous saturation and the anaerobic threshold in neonates after the Norwood procedure for hypoplastic left heart syndrome

BACKGROUND

Reduction in oxygen delivery can lead to organ dysfunction and death by cellular hypoxia, detectable by progressive (mixed) venous oxyhemoglobin desaturation until extraction is limited at the anaerobic threshold. We sought to determine the critical level of venous oxygen saturation to maintain aerobic metabolism in neonates after the Norwood procedure (NP) for the hypoplastic left heart syndrome (HLHS).

METHODS

A prospective perioperative database was maintained for demographic, hemodynamic, and laboratory data. Invasive arterial and atrial pressures, arterial saturation, oximetric superior vena cava (SVC) saturation, and end-tidal CO2 were continuously recorded and logged hourly for the first 48 postoperative hours. Arterial and venous blood gases and cooximetry were obtained at clinically appropriate intervals. SVC saturation was used as an approximation of mixed venous saturation (SvO2). A standard base excess (BE) less than -4 mEq/L (BElo), or a change exceeding -2 mEq/L/h (deltaBElo), were used as indicators of anaerobic metabolism. The relationship between SvO2 and BE was tested by analysis of variance and covariance for repeated measures; the binomial risk of BElo or deltaBElo at SvO2 strata was tested by the likelihood ratio test and logistic regression, with cutoff at p < 0.05.

RESULTS

Complete data were available in 48 of 51 consecutive patients undergoing NP yielding 2,074 valid separate determinations. BE was strongly related to SvO2 (model R2 = 0.40, p < 0.0001) with minimal change after adjustment for physiologic covariates. The risk of anaerobic metabolism was 4.8% overall, but rose to 29% when SvO2 was 30% or below (p < 0.0001). Survival was 100% at 1 week and 94% at hospital discharge.

CONCLUSIONS

Analysis of acid-base changes revealed an apparent anaerobic threshold when SvO2 fell below 30%. Clinical management to maintain SvO2 above this threshold yielded low mortality.

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

OBJECTIVE

Circulatory maldistribution is believed to be a major cause of early death after first-stage palliation for hypoplastic left heart syndrome. Flow reversal in the reconstructed aorta may reflect the pulmonary/systemic blood flow ratio. The purpose of our study was to investigate the utility of arterial PO (2), arterial oxygen saturation, and a newly developed Doppler-derived flow index in predicting the pulmonary/systemic flow ratio after first-stage palliation for hypoplastic left heart syndrome.

METHODS

Twenty-four infants who underwent first-stage palliation for hypoplastic left heart syndrome or a variant were studied. Superior vena cava blood samples were drawn to estimate the mixed venous saturation and permit calculation of the pulmonary/systemic blood flow ratio. Fifty-four samples were evaluated within the first 24 hours after surgery. Simultaneous blood draw and Doppler echocardiography were performed with interrogation in the distal aspect of the arch reconstruction. The ratio of the Doppler velocity-time integral of retrograde flow to the velocity-time integral of forward flow was calculated and compared with the pulmonary/systemic blood flow ratio

RESULTS

The median mixed venous saturation for the 54 samples was low (38.5%; range, 18%-64%). The median calculated pulmonary/systemic blood flow ratio was 1.4:1 (range, 0.3:1 to 4. 2:1). Pulse pressure, mixed venous saturation, and arterial PO (2) were not statistically significant predictors of the measured pulmonary/systemic blood flow ratio. Although both aortic oxygen saturation (R (2) = 0.84, P <.01) and Doppler flow reversal ratio (R (2) = 0.94, P <.001) were significantly associated with the measured pulmonary/systemic blood flow ratio, the model coefficient of determination was greatest for Doppler flow reversal ratio.

CONCLUSION

Measures of arterial oxygen saturation and arterial PO (2) may be misleading in assessing the circulatory status of infants after first-stage palliation for hypoplastic left heart syndrome. Doppler echocardiography, through use of the Doppler flow reversal ratio, provides a more useful measure of pulmonary/systemic blood flow ratio. Low mixed venous saturation after surgery may be due to factors other than pulmonary overcirculation, such as ventricular dysfunction and low cardiac output.

Patients at risk for low systemic oxygen delivery after the Norwood procedure

BACKGROUND

Identification of patients at risk for inadequate systemic oxygen delivery following the Norwood procedure could allow for application of more intensive monitoring, provide for earlier intervention of decreased cardiac output, and result in improved outcome.

METHODS AND RESULTS

Superior vena cava saturation (SvO2) and arteriovenous oxygen content difference were prospectively monitored as indicators of systemic oxygen delivery and recorded hourly for the first 48 hours in 29 of 33 consecutive patients following the Norwood procedure. Risk factors were evaluated using multiple linear regression to determine their impact on SvO2 and arteriovenous oxygen content difference. Age less than 8 days, weight less than 2.5 kg, aortic atresia, and prolonged cardiopulmonary bypass time were risk factors for low SvO2 and wide arteriovenous oxygen content difference (p < 0.05). Phenoxybenzamine and increasing time after operation were associated with higher SvO2 and narrower arteriovenous oxygen content difference (p < 0.05). Thirty-day survival was 97% and hospital survival was 94%. The earliest death occurred on postoperative day 20. Survival to bidirectional cavopulmonary shunt was 77%. Preoperative mechanical ventilation was the only risk factor identified for late death.

CONCLUSIONS

Aortic atresia, low weight, younger age, and prolonged cardiopulmonary bypass, previously identified risk factors for mortality, were associated with decreased SvO2 and narrower arteriovenous oxygen content difference in the early postoperative period. The impact of this hemodynamic vulnerability on mortality was minimized by continuous SvO2 monitoring.

Recent advances in the surgical management of the single ventricle pediatric patient

A standardized approach to the patient with single ventricle anatomy (SVA) is presented in this article. Regardless of the specific anatomic subtype, patients with SVA share common risk factors for early and late mortality and morbidity. Management of the SVA patients requires a plan to avoid development of these risk factors. Neonatal palliation is directed at relieving any systemic obstruction and appropriate limitation of pulmonary blood flow. The application of a standardized approach to the neonate with SVA, followed by staged palliation to a completion Fontan procedure should result in improved early and late outcome.

Phenoxybenzamine improves systemic oxygen delivery after the Norwood procedure

BACKGROUND

Achieving adequate systemic oxygen delivery after the Norwood procedure frequently is complicated by excessive pulmonary blood flow at the expense of systemic blood. We hypothesized that phenoxybenzamine could achieve a balanced circulation through reduction of systemic vascular resistance.

METHODS

In this prospective, nonrandomized study, oximetric catheters were placed in the superior vena cava for continuous monitoring of systemic venous oxygen saturation. Postoperative hemodynamic variables were compared between 7 control patients and 8 patients who received phenoxybenzamine.

RESULTS

The hospital survival rate was 93% (14 of 15 patients). Improvements in postoperative hemodynamics in the phenoxybenzamine group included a higher systemic venous oxygen saturation, a narrower arteriovenous oxygen content difference, a lower ratio of pulmonary to systemic flow, and a lower indexed systemic vascular resistance. In the phenoxybenzamine group, mean arterial blood pressure was related directly to systemic oxygen delivery, in contrast to the control group, where mean arterial pressure was related directly to indexed systemic vascular resistance and the ratio of pulmonary to systemic circulation.

CONCLUSIONS

Continuous postoperative monitoring of systemic venous oxygen saturation in a patient who has undergone the Norwood procedure provides early identification of low systemic oxygen delivery and an elevated ratio of pulmonary to systemic circulation. In this pilot study, phenoxybenzamine appeared to improve systemic oxygen delivery during the early postoperative period after the Norwood procedure. Further studies are indicated to confirm these results.