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Eur J Cardiothorac Surg 2005;28:56-60
© 2005 Elsevier Science NL

Does size matter? Larger Blalock–Taussig shunt in the modified Norwood operation correlates with better hemodynamics

Joachim Photiadis a , * , Michael Hü;bler b , Nicodème Sinzobahamvya a , Stanislav Ovroutski b , Brigitte Stiller b , Roland Hetzer b , Andreas E. Urban a , Boulos Asfour a

a German Pediatric Heart Institute, Sankt Augustin, Germany
b German Heart Institute Berlin, Berlin, Germany

Received 29 December 2004; received in revised form 14 February 2005; accepted 31 March 2005.

* Corresponding author. Address: Deutsches Kinderherzzentrum Sankt Augustin, Arnold Janssen-Strasse 29, 53757 Sankt Augustin, Germany. Tel.: +49 2241 249603; fax: +49 2241 249 602. (Email: photiadis{at}gmx.de).


   Abstract
Top
 Abstract
1. Introduction
 2. Methods
 3. Results
 4. Comment
 References
 
Objective: Excess pulmonary to systemic blood flow ratio (Qp/Qs) correlates with hemodynamic instability and mortality after modified Norwood operation. Studies suggest that maximal oxygen delivery occurs at a Qp/Qs of around 1. The use of a rather small modified Blalock–Taussig shunt (MBTS) is believed to achieve this goal. However, optimal MBTS size with respect to postoperative hemodynamics remains unclear. Methods: Between 2/2002 and 2/2004, 20 consecutive patients underwent Norwood operation; there were 19 operative survivors: nine with a normalized MBTS area (NSA) ≥3.3mm2/kg (group 1) and 10 with NSA<3.3mm2/kg (group 2). Mean arterial pressure (MAP) and common atrial pressures (CAP), arterial and superior vena cava oxygen saturations, urinary output and inotropes recorded for the postoperative hours 0, 6, 12, 18, 24 and 48 were analyzed. Results: Hospital mortality was 11.1% (1/9) in group 1 and 30% (3/10) in group 2 (P=0.6). For group 1 significantly higher MAP of 52±1.3 versus 46±0.8mmHg (P<0.001), higher urinary output of 6.2±0.5 versus 4.2±0.5ml/kg per h (P<0.01), lower CAP of 8±0.3 versus 10±0.4mmHg (P<0.001), and lower heart rate of 145±2.6 versus 160±1.6bpm were recorded than for group 2. In group 1, lower doses of adrenaline (0.03±0.01 versus 0.15±0.01µg/kg per min, P<0.05) and noradrenaline (0.01±0.01 versus 0.13±0.04µg/kg per min, P<0.01) were needed. Although Qp/Qs was more often calculated to be >1.5 in group 1 (51 versus 31%), arteriovenous oxygen difference and oxygen excess factor were not significantly different, indicating similar oxygen delivery. Conclusions: Monitoring of the central venous oxygen saturations and application of afterload reduction in cases of high Qp/Qs allows the insertion of a larger MBTS without association with lower oxygen delivery. In fact, better hemodynamic status with less inotropic support was noted with a larger MBTS early after Norwood operation.

Key Words: Norwood operation • Shunt size • Hemodynamics • CHD • Hypoplastic left heart

Abbreviations: CAP = common atrial pressure • CO2 = carbon dioxide • CPB = cardiopulmonary bypass • DILV = double inlet left ventricle • DKS = Damus–Kaye–Stansel operation • ECMO = extracorporeal membrane oxygenation • FiO2 = fraction inspired oxygen • HLHS = hypoplastic left heart syndrome • IAA = interrupted aortic arch • LV = left ventricle • MAP = mean arterial pressure • MBTS = modified Blalock–Taussig shunt • MV = mitral valve • NO2 = nitrogen • NSA = normalized shunt area, defined as cross-sectional shunt area divided by patient's body weight • PVR = pulmonary vascular resistance • Qp/Qs = pulmonary to systemic blood flow ratios • SaO2 = arterial oxygen saturation • SvO2 = central venous oxygen saturation • SVR = systemic vascular resistance • TA = tricuspid atresia


   1. Introduction
Top
 Abstract
 1. Introduction
2. Methods
 3. Results
 4. Comment
 References
 
With improved surgical techniques and better understanding of the hemodynamics after the modified Norwood operation early mortality in hypoplastic left heart syndrome (HLHS) has fallen; however, it remains 10–20%, even at experienced centers [1,2]. Mortality correlates with inadequate systemic oxygen delivery caused by an imbalance of pulmonary and systemic blood flow ratio (Qp/Qs) or low cardiac output, or both [3]. Efforts to achieve balanced circulation have concentrated on control of pulmonary vascular resistance (PVR) by manipulation of the fraction of inspired oxygen (FiO2), CO2 and NO2 [4]. Total resistance of the pulmonary circulation is composed of the resistance of the pulmonary vascular bed and of the modified Blalock–Taussig shunt (MBTS) itself. The use of a vasodilator will result in reduction of systemic vascular resistance (SVR) and PVR. As the PVR falls, the MBTS becomes the most important resistor in the pulmonary circuit and Qp/Qs is mainly determined by SVR and MBTS resistance. This reflects the importance of MTBS size for optimal hemodynamics after the Norwood procedure. By decreasing the size of the MBTS pulmonary over-circulation is reduced and cardiac output is diverted to the systemic circulation, resulting in better systemic oxygen delivery and improved outcome [5,6]. On the other hand, after a period of cardiac and circulatory arrest increased afterload for the single ventricle suppling the systemic and pulmonary circulation is hazardous and will further decrease its already limited reserve. Moreover, the use of a small MBTS can cause postoperative hypoxemia in patients with preoperatively restrictive persistent foramen ovale and concomitant pulmonary vascular disease [7,8]. This retrospective study aims at describing the postoperative hemodynamics with respect to MBTS size.


   2. Methods
Top
 Abstract
 1. Introduction
 2. Methods
3. Results
 4. Comment
 References
 
2.1. Patients
Between February 2002 and December 2003, out of 20 consecutive patients having undergone Norwood operation for HLHS or Damus–Kaye–Stansel operation (DKS) for complex forms of single ventricle with systemic outflow obstruction, there were 19 operative survivors. Thirteen patients operated on at the German Pediatric Heart Institute, Sankt Augustin, and six at the German Heart Institute Berlin (Table 1 ) therefore were eligible for postoperative hemodynamic studies.


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  		Table 1. Patient demographics
 
2.2. Definition of groups according to normalized BT shunt area
Shunt diameter [3.5 (n=9), 4 (n=8) and 5mm (n=2)] was chosen according to the surgeons' preference. Normalized BT shunt area (NSA) was defined as the cross-sectional area of the MBTS divided by the patient's body weight, e.g. a 3kg neonate receiving a 3.5mm diameter MBTS would have NSA=(3.5mm/2)2x{pi}/3kg=3.21mm2/kg. Mean NSA was 3.4±0.1mm2/kg. The median NSA (3.3mm2/kg) of the entire study group was chosen as cut-off to build two groups. Nine patients with an NSA≥3.3mm2/kg were assigned to group 1 and 10 with an NSA<3.3mm2/kg to group 2.

2.3. Surgical management
Perioperative management and surgical conduct were similar at both institutions following a multidisciplinary protocol. This included appropriate preoperative stabilization, patch augmentation of the aortic arch using pulmonary homograft material, atrial septectomy, and placement of an MBTS (Goretex®, W.L. Gore and Associates, Inc., Flagstaff, AZ) shunt between the innominate artery and the right pulmonary artery. All patients were cooled on cardiopulmonary bypass (CPB); 0.5mg/kg phentolamine was administered to facilitate cooling and rewarming. Arch reconstruction was performed with continuous antegrade cerebral perfusion via the MBTS to limit duration of complete circulatory arrest. After completion of operation, the heart was assisted with partial CPB as long as necessary to achieve serum lactate levels below 4mmol/l and normal sinus rhythm. Rarely, sequential atrioventricular pacemaker stimulation was instituted before the patient was weaned from CPB. Modified ultrafiltration was always applied. Operative techniques and CPB times were not statistically different for the two groups of patients.

Oximetric catheters (4F, Edwards Life Sciences, Irvine, CA, USA) were placed through the common atrium into the superior vena cava to allow continuous monitoring of systemic venous oxygen saturation (SvO2) [2]. An additional line was placed in the common atrium for pressure monitoring and infusion of inotropic drugs. The sternum was routinely left open, usually for 2 days until stable hemodynamic conditions were achieved.

2.4. Postoperative management
All patients received dopamine (3–6µg/kg per min) and phentolamine (2–8µg/kg per min). Milrinone (0.3–0.8µg/kg per min), epinephrine (0.05–0.2µg/kg per min) and norepinephrine (0.05–0.2µg/kg per min) were added, if supplementary inotropic support became necessary. Postoperative management included target mean arterial blood pressure (MAP) of around 50mmHg, common atrial pressures (CAP) of 8–12mmHg, hematocrit between 45 and 55%, urinary output greater than 1ml/kg per h, SvO2 greater than 50%, and arterial oxygen saturation (SaO2) between 75 and 80%. Patients received adequate sedation using a continuous infusion of fentanyl (5–10µg/kg per min) and midazolam (1–4µg/kg per min) until chest closure. Ventilator settings were adjusted to maintain normocapnia with the lowest FiO2 possible to achieve adequate arterial and venous oxygen saturation. Qp/Qs was calculated according to the Fick method, assuming a pulmonary venous saturation of 97%. Oxygen excess factor, which has been shown to correlate with systemic oxygen delivery [9], was calculated as SaO2 divided by the AV saturation difference. After removal of the oximetric catheter, the left to right shunt was estimated according to clinical signs of heart failure and afterload reduction therapy with carvedilol (0.1–1.5mg/kg per day) and captopril initiated if necessary.

2.5. Hemodynamic data collection and statistical analysis
Preoperative and perioperative data were collected retrospectively. Mean systemic arterial pressure, heart rate, mean common atrial pressure, urinary output, blood gas analysis, SaO2 and SvO2, standard base excess (BE), serum lactate, and dosages of epinephrine, norepinephrine, milrinone and phentolamine were collected to be studied as hemodynamic data at 0, 6, 12, 24, and 48h after operation or until the patient expired or extracorporeal membrane oxygenation (ECMO) was initiated (Table 2 ). The hour ‘0’ corresponds to the time of the patient's arrival in the intensive care unit. Data were summarized as mean±SEM. Preoperative and operative characteristics of group 1 and group 2 patients and variables for survivors and non-survivors were compared by independent Student's t-test for parametric data analysis. Levene's test was used to test for equality of variances. For non-parametric data the Mann–Whitney U test, Fisher's exact test, or the {chi} 2 test was used, as appropriate. Analyses were performed using the statistical software package SPSS 11.0 (SPSS, Inc., Chicago, IL). Differences were considered statistically significant at a P value of ≤0.05.


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  		Table 2. Number of postoperative hemodynamic parameter available for each time point
 

   3. Results
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
4. Comment
 References
 
3.1. Availability of hemodynamic data
Table 2 shows the number of hemodynamic parameters available at each time. A total of 52 samples in group 1 and 53 in group 2 were collected for each hemodynamic parameter and were analyzed (Table 2). The unavailability of data was due either to early death (n=3) or to the use of extracorporeal membrane oxygenation (ECMO, n=1).

3.2. Preoperative conditions and surgical variables
There were no significant differences in the preoperative conditions of age, weight, preoperative use of inotropes and intubation or in the surgical variables (Table 1).

3.3. Hemodynamic data analysis
Reviewing the whole study period, the MAP was higher (P<0.001) and the CAP lower (P<0.001) with lower doses needed of epinephrine (P=0.01) and norepinephrine (P=0.005) in group 1 than in group 2 (Table 3 ). Urinary output was higher (P=0.001), despite similar doses of furosemide, in group 1. No significant differences were noted for doses of milrinone and phentolamine between the groups.


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  		Table 3. Postoperative patients' hemodynamics
 
Significantly lower fraction inspired oxygen (P=0.01) was needed to achieve a similar SaO2, reflecting a less restricted pulmonary blood flow in group 1. Even if Qp/Qs was more often calculated to be >1.5 in group 1 (P=0.06), the arteriovenous oxygen difference and oxygen excess factor were not significantly different between the groups; indicating similar oxygen delivery.

3.4. Outcome
There were four deaths, representing 21% (4/19). Causes of death were low systemic oxygen delivery and/or low cardiac output, despite aggressive inotrope therapy. No arrhythmia that required treatment and no electrocardiographic evidence of myocardial ischemia were noted in these patients. In one patient ECMO was initiated 18h after surgery but was discontinued because of cerebral bleeding 42h after surgery. There was no significant difference in hospital mortality: 11.1% (1/9) in group 1 and 30% (3/10) in group 2 (P=0.6). Preoperative, operative and postoperative risk factors for hospital mortality are displayed in Table 4 . Significant risks for death assessed by univariate analysis were: elevated mean common atrial pressure, low systemic mean arterial pressure, low systemic venous and arterial oxygen saturation, higher lactate levels, and longer CPB times. All survivors reached second stage palliation, except for one in group 2, who died unexpectedly 3 months after surgery. Postmortem examination did not reveal shunt thrombosis or stenosis as cause of death.


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  		Table 4. Univariate risk factor analysis for hospital mortality
 
3.5. Limitations
The number of patients included in this study is small. To attain a larger cohort, patients' data were collected from two centers following the same standardized management protocol. The total number of patients operated on was different (n=13 versus 6) in the two centers during the study period and it was to be expected that differing experience and expertise of the participating teams might lead to differences in outcome. However, data analysis did not reveal significant differences with respect to outcome parameters between the centers.


   4. Comment
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
 4. Comment
References
 
Low cardiac output and an imbalance between pulmonary and systemic flow, with excessive pulmonary flow, are common causes of inadequate systemic oxygen delivery and account for most of the deaths early after the Norwood operation. Postoperative management should therefore aim at maximizing systemic oxygen delivery by balancing the pulmonary and systemic circulations [2]. To achieve this goal, in practice PVR and SVR are manipulated as required. Previous studies have focused on elevating pulmonary vascular resistance by ventilator manipulations to modify CO2, or by adding CO2 and/or NO2 to the inspired gas [4]. In our patients, CO2 was kept in the normal range and Qp/Qs was modified by changing systemic vascular resistance by adding vasodilators or norephinephrine as needed.

The object of this study was to test the hypothesis that larger MBTS size correlates with better systemic oxygen delivery, better postoperative hemodynamics and consequently better outcome, with lower morbidity and mortality.

Evaluation of shunt size for good postoperative hemodynamics at every time point until second stage palliation remains difficult. With a drop in pulmonary vascular resistance a formerly adequately sized shunt may become too large. Although most surgeons would use either a 3.5 or 4mm MBTS for Norwood palliation in a neonate, there is very little published work with special reference to the size of the MBTS. In our study, shunt size was chosen according to the surgeon's preference. We defined the patient as having a ‘large’ shunt if the cross-sectional area of the MBTS, normalized by body weight, was at least 3.3mm2/kg or more. With ‘large’ shunts we found similar systemic oxygen delivery reflected by similar values of arteriovenous oxygen saturations differences and oxygen excess factor. Postoperative hemodynamic status in patients with large shunts was better, mirrored by higher mean arterial pressures with lower ventricular filling pressures and lower doses of catecholamines. Mortality was lower in the large shunt group, but due to the limited number of patients the differences did not reach significance levels. Other groups preferred larger (3.5mm2/kg) or smaller MBTS sizes [2,6]. The hospital mortality rate reported in these studies was remarkably different: 8% of patients in the group using larger shunts and 32% in that preferring smaller shunts. A small shunt is assumed to reduce pulmonary blood flow and thereby increase systemic oxygen delivery in patients with presumed normal or near-normal pulmonary vascular bed. On the other hand, in cases of increased pulmonary vascular resistance an adequately sized modified Blalock–Taussig shunt may be too small to achieve sufficient oxygenation. Pulmonary vascular abnormalities are common in HLHS [10,11], affecting the pulmonary arteries, veins and lymphatics. The course after Norwood operation is reported to be complicated by persistent systemic desaturation, leading to death [7,8]. With this condition a larger shunt allows more blood flow to the pulmonary vascular bed and should therefore help to reduce the total afterload for the single ventricle. Higher mean arterial pressures with lower ventricular filling pressure and the lower doses of norepinephrine administered in group 1 of this study underline this hypothesis.

The drawback of a larger shunt is pulmonary over-circulation, especially with the physiological fall of pulmonary vascular resistance in the first months of life. Increased systemic vascular resistance with concomitant chronic volume overload of the single ventricle may account for unexpected death even a long time after the Norwood operation [12,13]. In our study, significantly lower FiO2 was needed and Qp/Qs was more frequently found to be higher than 1.5 in group 1, indicating that in fact Qp was less restricted in this group, even early after Norwood operation. However, the AV difference and oxygen excess factor were similar in the two groups, indicating that the use of a larger shunt was not accompanied by lower systemic oxygen delivery. This was accomplished by the continuous monitoring of Qp/Qs, immediate identification of increased left to right shunt and prompt intervention to decrease pulmonary over-circulation. A reduction of left to right shunt was attained by increasing hemoglobin concentration [14] or by decreasing systemic afterload with {alpha}-adrenoceptor antagonists, angiotensin converting enzyme inhibitors and ß-adrenoceptor antagonists [2,15]. There was only one interstage death in group 2, underlining the fact that using a larger shunt with adequate afterload reduction to balance systemic and pulmonary circulation was not associated with a higher rate of interstage mortality.

Therefore, we conclude that the size of the modified Blalock–Taussig shunt influences hemodynamics after the Norwood procedure. Unlike theoretical models and other studies with larger shunts, our data did not show a diversion of the cardiac output away from the systemic circulation, associated with lower systemic oxygen delivery. With monitoring of the central venous oxygen saturation and prompt afterload reduction in cases of pulmonary over-circulation, better hemodynamic status with less inotropic support was achieved by the use of larger shunts early after Norwood operation. Although the numbers of patients are too small to demonstrate significant differences in mortality, given the better postoperative hemodynamics and given the fact that pulmonary vascular disease is common with hypoplastic left heart syndrome, the use of modified Blalock–Taussig shunts with a normalized shunt area of at least 3.3mm2/kg, accompanied by adequate afterload reduction therapy, should be recommended.


   Acknowledgments
 
We would like to acknowledge Anne Gale for her kind assistance in preparing the manuscript.


   Footnotes
 
{star} Presented at the joint 18th Annual Meeting of the European Association for Cardio-thoracic Surgery and the 12th Annual Meeting of the European Society of Thoracic Surgeons, Leipzig, Germany, September 12–15, 2004.


   References
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
 4. Comment
 References
 

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  4. Tabbutt S, Ramamoorthy C, Montenegro LM, Durning SM, Kurth CD, Steven JM, Godinez RI, Spray TL, Wernovsky G, Nicolson SC. Impact of inspired gas mixtures on preoperative infants with hypoplastic left heart syndrome during controlled ventilation. Circulation 2001;104(12):I159-I164.[Medline]
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  6. Ishino K, Stumper O, De Giovanni JJ, Silove ED, Wright JG, Sethia B, Brawn WJ, de Leval M. The modified Norwood procedure for hypoplastic left heart syndrome: early to intermediate results of 120 patients with particular reference to aortic arch repair. J Thorac Cardiovasc Surg 1999;117(5):920-930.[Abstract/Free Full Text]
  7. Brann S, Brawn WJ, Raafat F, Sreeram N. Neonatal pulmonary vascular disease in hypoplastic left heart syndrome. Ann Thorac Surg 1995;60:433-434.[Abstract/Free Full Text]
  8. Photiadis J, Urban AE, Sinzobahamvya N, Fink C, Schindler E, Schneider M, Brecher A, Asfour B. Restrictive left atrial outflow adversely affects outcome after the modified Norwood procedure. Eur J Cardiothorac Surg 2005; 27:962–967..
  9. Buheitel G, Scharf J, Hofbeck M, Singer H. Estimation of cardiac index by means of the arterial and the mixed venous oxygen content and pulmonary oxygen uptake determination in the early post-operative period following surgery of congenital heart disease. Intensive Care Med 1994;20:500-503.[CrossRef][Medline]
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