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Ann Thorac Surg 1999;67:161-167
© 1999 The Society of Thoracic Surgeons

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Original Articles

Phenoxybenzamine improves systemic oxygen delivery after the Norwood procedure¹

^a Cardiothoracic Surgery, Department of Surgery, Medical College of Wisconsin, Children’s Hospital of Wisconsin, Milwaukee, Wisconsin, USA
^b Department of Anesthesia, Medical College of Wisconsin, Children’s Hospital of Wisconsin, Milwaukee, Wisconsin, USA
^c Department of Pediatrics, Medical College of Wisconsin, Children’s Hospital of Wisconsin, Milwaukee, Wisconsin, USA
^d Department of Critical Care, Medical College of Wisconsin, Children’s Hospital of Wisconsin, Milwaukee, Wisconsin, USA
^e Department of Pediatric Cardiology, Medical College of Wisconsin, Children’s Hospital of Wisconsin, Milwaukee, Wisconsin, USA

Address reprint requests to Dr Tweddell, Children’s Hospital of Wisconsin, 9000 W Wisconsin Ave, PO Box 1997, Milwaukee, WI 53201
e-mail: jstwedde{at}mcw.edu

Presented at the Thirty-fourth Annual Meeting of The Society of Thoracic Surgeons, New Orleans, LA, Jan 26–28, 1998.

	Abstract

Top Footnotes Abstract Introduction Material and methods Results Comment Acknowledgments References

 
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.

	Introduction

Top Footnotes Abstract Introduction Material and methods Results Comment Acknowledgments References

 
Achieving adequate systemic oxygen delivery during the early postoperative period after the Norwood procedure frequently is complicated by excessive pulmonary blood flow at the expense of systemic blood flow and by the limited reserve of the neonatal single ventricle [1]. Efforts to achieve a balanced circulation during the early postoperative period have centered on increasing pulmonary vascular resistance to more closely match systemic vascular resistance (SVR) by adding CO₂ to the inspired gas and by decreasing the size of the systemic to pulmonary artery shunt [2–4]. Despite these efforts, the early mortality rate after the Norwood procedure has remained between 20% and 25%, even at experienced centers [5–7].

Recognizing that circulatory balance could not be ascertained without the measurement of both arterial and venous oxygenation, we began routinely placing intravascular optical catheters in the superior vena cava after the Norwood procedure. Our initial experience with the intravascular optical catheter, using a conventional postoperative Norwood procedure management protocol, identified an early period during which there was an increased ratio of pulmonary to systemic flow (Qp/Qs) with inadequate systemic oxygen delivery. We hypothesized that the -blocker phenoxybenzamine (POB) would help balance the circulation by lowering the SVR and stabilizing systemic vasoconstrictor responses. We report herein the early postoperative hemodynamic profiles of newborns who underwent the Norwood procedure with or without POB.

	Material and methods

Top Footnotes Abstract Introduction Material and methods Results Comment Acknowledgments References

 
To determine the effect of the -blocker POB on the outcome of patients undergoing the Norwood procedure, we analyzed the outcome of 15 consecutively seen patients who underwent the Norwood procedure between July 1996 and April 1997 at the Children’s Hospital of Wisconsin. The start date of inclusion was selected to coincide with the routine placement of 4-French intravascular optical catheters (Abbott Laboratories, North Chicago, IL) directly into the superior vena cava at the time of operation (Fig 1). To decrease the risk of thrombosis, no other intravenous lines were placed in the superior vena cava.

View larger version (28K): [in this window] [in a new window]   Fig 1. The 4-French oximetric catheters were placed routinely through a small incision directly into the superior vena cava and secured with sutures reinforced with felt pledgets. The catheters were positioned such that only 3 to 5 mm was within the vessel.

The superior vena cava oxygen saturation was used to approximate the systemic venous oxygen saturation (SvO₂) and was the primary guide used in postoperative management. Intravascular optical catheters were placed in 5 of 7 control patients and in 8 of 8 patients who received POB. In 2 control patients, 1 with a bilateral superior vena cava and 1 with a small single right superior vena cava, intravascular optical catheter placement was not attempted. Standardized postoperative management was aimed at achieving adequate systemic oxygen delivery, defined as an SvO₂ of 50% or greater, and a balanced circulation, defined as a Qp/Qs of 0.8 to 1.2. Standardized inotropic support included the routine use of milrinone (a 50-µg/kg loading dose given before weaning from cardiopulmonary bypass followed by a continuous infusion of 0.5 µg · kg^-1 · min^-1) and dopamine (3 µg · kg^-1 · min^-1). Additional inotropic support was provided as necessary with epinephrine (0.05 to 1.0 µg · kg^-1 · min^-1), and additional vasodilator therapy was provided as necessary with nitroprusside (0.5 to 5 µg · kg^-1 · min^-1). Intentional hypercapnea escalating to the addition of CO₂ (1% to 4%) was used if necessary to limit excessive pulmonary blood flow.

The control patients received conventional postoperative management as outlined earlier. Phenoxybenzamine was administered according to a protocol approved by the U.S. Food and Drug Administration and the institutional review boards of the Children’s Hospital of Wisconsin and the Medical College of Wisconsin, and informed written consent was obtained for each patient. The POB protocol was adapted from the one used by Mee and associates at the Cleveland Clinic (personal communication). Eight patients received POB, 0.25 mg/kg, at the commencement of cardiopulmonary bypass; no changes were made in their postoperative management. In 5 of the 8 patients who received POB, a continuous infusion at a rate of 0.25 mg/kg every 24 hours was maintained for up to 48 hours in the immediate postoperative period. The decision to use a continuous infusion in addition to the bolus dose was based on the target of achieving adequate systemic oxygen delivery (SvO₂ of >50%) and balanced circulation (Qp/Qs of 0.8 to 1.2).

Data were collected prospectively using a standardized form and compared between the control group and the POB group. Preoperative data included the anatomic diagnosis, weight, and age at the time of operation as well as the presence of additional congenital abnormalities. Operative data included the duration of circulatory arrest and shunt size. Postoperative data consisted of the SvO₂, arteriovenous oxygen content difference (AVO₂), Qp/Qs, mean arterial blood pressure (MAP), SVR index (SVRI), arterial oxygen saturation (SaO₂), and hemoglobin. The hemodynamic variables (AVO₂, Qp/Qs, and SVRI) were calculated using standard formulas [11]. To calculate the SVRI, an assumed oxygen consumption of 180 mL · min^-1 · m^-2 was used [12].

Data were recorded at hourly intervals for the first 48 hours after operation in all patients. To identify more closely interactions of hemodynamic parameters, an eight-channel analog-to-digital converter (DAP 102, Microstar Labs, Belleview, WA) and a data acquisition system (Dasy Lab, Omega Engineering, Stamford, CT) based on an IBM personal computer were used to continuously record the MAP, SaO₂ SvO₂, inspired oxygen concentration, end-tidal CO₂, and central venous pressure in 10 of the 15 patients.

Preoperative and operative patient characteristics of the two groups were compared using the Student’s t-test or the Mann-Whitney rank sum test. Hemodynamic data of the two groups were compared using two-way analysis of variance with a repeated-measures factor with the post hoc Bonferroni test for between-group comparisons (RM ANOVA) and by linear regression analysis with correction for repeated measures and inclusion of POB as an interactive term (STATA software, College Station, TX). Differences were considered statistically significant at a p value of < 0.05.

	Results

Top Footnotes Abstract Introduction Material and methods Results Comment Acknowledgments References

 
Preoperative and operative patient characteristics are summarized in Table 1. There was a borderline statistically significant difference (p = 0.06) in weight between the two groups. Shunt size, normalized as a ratio of either diameter or cross-sectional area to body weight, was not significantly different. Associated congenital anomalies included proximal esophageal atresia with a distal tracheoesophageal fistula combined with a diaphragmatic hernia in 1 patient and congenital complete heart block in 1 patient, both in the POB group. There were no early deaths (<30 days). There was one death before hospital discharge (postoperative day 50) caused by a reaction to the transfusion of a blood product; the hospital survival rate was 93%. There were no complications related to catheter use, such as bleeding or thrombosis. The patient with congenital complete heart block was managed with temporary pacing wires during the early postoperative period followed by the placement of a dual-chamber pacemaker before hospital discharge. There were no other arrhythmias that required treatment in either group and no electrocardiographic evidence of myocardial ischemia.

View larger version (39K): [in this window] [in a new window]   Fig 2. Superior vena cava saturation (SvO2) during the first 48 hours after the Norwood procedure. The SvO2 was significantly higher in the POB group than in the control group during hours 1 to 10 and hour 25 (p < 0.05, RM ANOVA).

View larger version (39K): [in this window] [in a new window]   Fig 3. Arteriovenous oxygen difference (AVO2) during the first 48 hours after the Norwood procedure. The AVO2 was significantly narrower in the POB group than in the control group during hours 1 to 10, 31, and 32 (p < 0.05, RM ANOVA).

View larger version (36K): [in this window] [in a new window]   Fig 4. Pulmonary to systemic flow ratio (Qp/Qs) during the first 48 hours after the Norwood procedure. The Qp/Qs was significantly higher in the control group than in the POB group during hours 1 to 10, 30 to 32, and 36 (p < 0.05, RM ANOVA).

View larger version (39K): [in this window] [in a new window]   Fig 5. Mean arterial blood pressure (MAP) during the first 24 hours after the Norwood procedure. The MAP was significantly lower in the POB group during hours 1, 4 to 19, and 33 (p < 0.05, RM ANOVA).

View larger version (40K): [in this window] [in a new window]   Fig 6. Systemic vascular resistance index (SVRI) during the first 48 hours after the Norwood procedure. The SVRI was significantly higher in the control group during hours 1 to 15, 19, 22, and 31 (p < 0.05, RM ANOVA).

View larger version (37K): [in this window] [in a new window]   Fig 7. Multichannel recording of the arterial saturation (SaO2), mean arterial blood pressure (MAP), and superior vena cava saturation (SvO2) during the first 4 hours after the Norwood procedure in a control patient with aortic atresia. The Y-axis common to all three curves indicates both the percent saturation and the MAP. The X-axis indicates the time, with the vertical grid indicating 15-minute intervals. Two episodes of decreased SvO2 were identified, one beginning at 17:00 and a second beginning at 17:45. Changes in the SvO2 were mirrored by changes in the MAP. Initially, the changes in the SvO2 were mirrored by changes in the SaO2, but with a marked decline in the SvO2 such as that which occurred between 18:00 and 18:15. The SaO2 decreased as well. However, throughout the early postoperative period, the SaO2 remained within an acceptable range. These data indicate that the SaO2 cannot be relied on to indicate a balanced circulation. Acute changes in the SvO2 can occur that are not reliably identified by either an increase or a decrease in the SaO2. (ICU = intensive care unit.)

View larger version (42K): [in this window] [in a new window]   Fig 8. The mean arterial pressure (MAP) was compared with the superior vena cava saturation (SvO2), arteriovenous oxygen delivery (AVO2), systemic vascular resistance index (SVRI), and pulmonary to systemic flow ratio (Qp/Qs) using linear regression analysis. The net effect of phenoxybenzamine (POB) was to decrease the SVRI and stabilize the Qp/Qs over a wide range of blood pressure. Phenoxybenzamine changed the direction of the relation between systemic oxygen delivery and the MAP such that systemic oxygen delivery was positively correlated with the MAP in the patients who received POB. (See text for details.)

	Comment

Top Footnotes Abstract Introduction Material and methods Results Comment Acknowledgments References

Despite arterial saturation in the target range, excessive pulmonary blood flow and decreased systemic oxygen delivery marked the early postoperative period of the control group in this study. In addition, multiple episodes of acute elevations in the Qp/Qs with decreases in the SvO₂ were identified (Fig 7). The most effective therapy for these acute episodes, which are characterized best as acute elevations of the SVR, was additional sedation with benzodiazepines or opioids, but these interventions were not uniformly effective in stabilizing the SVR. Our findings and those of others indicate that the SaO₂ cannot be relied on to indicate adequate systemic oxygen delivery and a balanced circulation [13, 14]. Physical examination, particularly assessment of the peripheral pulses, also has limitations. Whereas pulses may be diminished with low total cardiac output, circulatory imbalance with a high Qp/Qs can be marked by high pulse amplitude as a result of aortopulmonary runoff.

Previous efforts at achieving a balanced circulation in the early postoperative period have focused on increasing the pulmonary vascular resistance to prevent excessive pulmonary blood flow, most commonly by limiting the fraction of inspired oxygen and inducing hypercapnia. Hypercapnia can be induced by decreasing minute ventilation, adding dead space to the ventilator circuit, and adding 1% to 4% of CO₂ to the inspired gas mixture [2–4]. The use of supplemental CO₂ was associated with improved survival after the Norwood procedure [4]. Smaller systemic to pulmonary artery shunts also have been used to limit pulmonary blood flow. The net result of increasing the pulmonary vascular resistance to achieve a balanced circulation is an increase in the total resistance, which impairs cardiac output and may have a negative impact on single ventricle function. In addition, the use of smaller systemic to pulmonary artery shunts may lead to unacceptable levels of cyanosis in young infants.

Our initial experience with the intravascular optical catheter identified the need for additional afterload reduction in the early postoperative period. Afterload reduction has two benefits: decreased total resistance results in increased total cardiac output and reduction of the SVR leads to a more balanced Qp/Qs, resulting in better partitioning of the cardiac output. Phenoxybenzamine, an irreversible -blocker, was selected because it provides a uniform and reliable reduction in the SVR. Its long half-life (24 hours) minimizes moment-to-moment variability in afterload reduction. No other changes were made in the standard postoperative management protocol.

Significant improvements in the early postoperative course of the patients who received POB included a Qp/Qs nearer to 1, a lower SVRI, a higher SvO₂, and a narrower AVO₂. These data indicate that afterload reduction with POB improved systemic oxygen delivery during the early postoperative period. This improvement was achieved with a mild reduction in the MAP. Therefore, this strategy achieved a balanced circulation by lowering the SVR rather than raising the pulmonary vascular resistance, and by providing a net increase in cardiac output. During postoperative hours 24 to 48, the POB and control groups had similar hemodynamic and oxygen delivery data. The merging of hemodynamic data during the second postoperative day after the acute intervention with POB indicates that the early differences between the two patient groups were based on the pharmacology of POB rather than on underlying physiologic or anatomic differences.

A standardized inotropic support and vasodilator protocol was used in both groups, although there was no attempt to ensure equal inotropic support in the two groups. We identified no adverse effects of POB. Specifically, we identified no episodes of arrhythmias and no evidence of myocardial ischemia. These findings are consistent with studies of POB during hypovolemic shock in which coronary blood flow was increased with POB administration even in the face of significant hypotension [15].

Our data indicate that the patients who received POB had a consistent decrease in the SVR, whereas the SVR fluctuated during the early postoperative period in the control patients. Phenoxybenzamine blocks -receptors that are important in the sympathetic response [16]. Elevation of the SVR to maintain blood pressure in the face of decreasing systemic cardiac output is a highly preserved cardiovascular reflex. In a patient who has undergone the Norwood procedure, this reflex elevation of the SVR leads to a further decrease in systemic perfusion as the Qp/Qs increases. A positive feedback loop develops and may contribute to early death. By blocking the vasoconstrictor component of the sympathetic response, -blockade may be uniquely suited to the postoperative management of these patients. The use of -blockade with POB balanced the parallel circulation over the range of hemodynamic variables seen in this study (Fig 8).

Limitations of this study
Although data collection was prospective, this study was neither blinded nor randomized. In studies of this type, it is possible that nonrandom selection and unblinded evaluation can lead to results unrelated to the intervention studied [17]. However, the study protocol was designed to identify variations in a uniform group of patients who underwent the Norwood procedure over a short period (18 months). Management of the patients was consistent between the control and POB groups. The only change in management over the period of this study was the introduction of POB. Given the consistent postoperative management, uniformity of the patient groups, short period involved, and minimal comparable data in the literature, we believe the results are valuable. Further studies are indicated to confirm these results.

Conclusions
Continuous monitoring of the SvO₂ after the Norwood procedure allows for early identification of inadequate systemic oxygen delivery. Maneuvers to improve systemic oxygen delivery and provide for a balanced Qp/Qs can be initiated before other indicators of low output become manifest. The addition of POB to the standard inotropic and vasodilator regimen improved early postoperative systemic oxygen delivery. Phenoxybenzamine fundamentally changed the relation between blood pressure, systemic oxygen delivery, and the Qp/Qs. The use of POB was associated with improved systemic oxygen delivery and stabilization of the Qp/Qs.

	Acknowledgments

Top Footnotes Abstract Introduction Material and methods Results Comment Acknowledgments References

 
We are grateful for the outstanding care provided to the patients in this study by the nursing staffs of the pediatric and neonatal intensive care units of the Children’s Hospital of Wisconsin.

	Footnotes

Top Footnotes Abstract Introduction Material and methods Results Comment Acknowledgments References

 
¹ This article has been selected for the open discussion forum on the STS Web site: http://www.sts.org/annals

	References

Top Footnotes Abstract Introduction Material and methods Results Comment Acknowledgments References

 

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