Asian Cardiovasc Thorac Ann 2000;8:339-343
© 2000 Asia Publishing EXchange Pte Ltd
Modified Blalock-Taussig Shunt in Neonates: Determinants of Immediate Outcome
M Sanjeeva Rao, MCh,
Anil Bhan, MCh,
Sachin Talwar, MS,
Rajesh Sharma, MCh,
Shiv Kumar Choudhary, MCh,
Balram Airan, MCh,
Anita Saxena, DM,
Shyam Sunder Kothari, DM,
Rajnish Juneja, DM,
Panangipalli Venugopal, MCh
Department of Cardiothoracic and Vascular Surgery
Cardiothoracic Sciences Centre
All India Institute of Medical Sciences
New Delhi, India
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For reprint information contact: Anil Bhan, MCh Tel: 91 11 686 4851 Fax: 91 11 686 2663 email: anil_bhan{at}hotmail.com Department of Cardiothoracic and Vascular Surgery, Cardiothoracic Sciences Centre, All India Institute of Medical Sciences, Ansari Nagar, New Delhi 110029, India.
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Abstract
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Forty-six neonates with various conditions of low pulmonary blood flow received a modified Blalock-Taussig shunt with a polytetrafluoroethylene graft. Ages ranged from 2 to 30 days (mean, 14.1 days). Hospital mortality was 10.9%. Shunt block requiring reoperation occurred in 3 patients. Incremental risk factors for early mortality were found to be restrictive atrial septal defect, univentricular physiology, and postoperative reintervention. It was found that a Blalock-Taussig shunt could be performed in the neonatal period with a predictable outcome. Single-ventricle physiology and restrictive atrial septal defect were the major determinants of outcome in this subgroup.
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Introduction
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In spite of the increased trend towards definitive surgical repair of complex congenital heart defects in neonates, palliative systemic-to-pulmonary artery shunts are required in some critically ill patients.1–4 The Blalock-Taussig shunt is regarded as a safe, reliable, and effective means of increasing pulmonary blood flow in these patients.2,5 The Great Ormond Street modification using thin polytetrafluoroethylene (PTFE) conduit, most closely approaches the ideal shunt and is considered to be the optimal procedure for small infants and neonates.2,5–8 The smaller subclavian artery of most neonates has been postulated to be a flow regulator allowing shunt flow to increase with growth until the conduit is the smallest vessel in the circuit.9 We reviewed our experience of the modified Blalock-Taussig shunt in neonates to identify factors determining the immediate outcome after surgery.
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Patients and Methods
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Between January 1991 and October 1997, 46 neonates (39 males and 7 females) underwent a modified Blalock-Taussig shunt at this institute. Their ages ranged from 2 to 30 days (mean, 14.1 days); 36 were under 2 weeks of age. Weights ranged from 2.0 to 3.5 kg with a mean of 2.8 kg. The diagnosis was made by echocardiography in all patients; only 6 required cardiac catheterization. Because of the heterogenicity of their diagnoses, the patients could be classified into 3 morphological groups as shown in Table 1
. Thirty-seven patients had a patent ductus arteriosus (PDA) and 3 had a closing ductus arteriosus as indicated by intermittent Doppler signals on echocardiography. Forty-four patients had confluent pulmonary arteries, 1 had bifurcation stenosis, and 1 had stenosis of the origin of the left pulmonary artery. Of the 20 patients with single-ventricle morphology, 8 had a restrictive type of atrial septal defect (ASD), defined as turbulent signals across the ASD on Doppler echocardio-graphy. Sixteen patients required preoperative intervention in the form of prostaglandin E1 therapy (11), ventilatory support (4), and pulmonary balloon valvulotomy (1).
The modified technique for a Blalock-Taussig shunt described by de Leval and colleagues9 was performed using a standard posterolateral thoracotomy in the fourth intercostal space. There was minimal dissection of the subclavian artery to allow adequate exposure for placement of a partial exclusion clamp as proximal as possible at its widest part. The pulmonary artery was dissected sufficiently to avoid compromise of the upper lobe branch by the anastomosis. Heparin was administered in a dose of l mg•kg–1 before clamping the subclavian artery. A thin Gore-Tex PTFE tube graft (WL Gore, Elkton, MD, USA) was chosen according to the size of the pulmonary artery. A graft of 4 mm in diameter was used in 31 patients, 13 received a 3.5-mm graft, and 2 had a 5-mm graft. One end of the graft was trimmed obliquely and anastomosed end-to-side to the subclavian artery, using a continuous 7/0 polypropylene suture. The other end of the graft was trimmed straight and after clamping and incising the pulmonary artery, distal anastomosis was carried out using a continuous 7/0 polypropylene suture. The length of the graft was adjusted so that it lay straight along the mediastinum without kinking and without distortion of the pulmonary artery. The graft was deaired from the distal anastomosis before releasing the subclavian clamp. Heparin was not reversed. Forty-one of the 46 patients had a right-sided shunt as we prefer to perform the shunt on the side of the superior vena cava. In patients undergoing univentricular repair later in life, the shunt was divided, the pulmonary arteriotomy was extended, and the superior vena cava was anastomosed to it. In this way, the entire operation was right-sided and distortion of the pulmonary artery was avoided. In patients with tetralogy of Fallot, a right shunt is easier to take down and the phrenic nerve is better protected. However, when the left pulmonary artery was small, the shunt was deployed on the left side.
Postoperatively, all patients received mechanical venti-latory support for 18 to 24 hours. They were extubated when hemodynamically stable with satisfactory arterial blood gas levels. Postoperative shunt patency was assessed by detection of a shunt murmur and analysis of atrial blood gases. Echocardiographic assessment was under-taken in doubtful cases. Dopamine 5 µg•kg–1•min–1 was used to maintain systolic pressures over 80 mm Hg to facilitate shunt patency. Postoperative heparin (10 units•kg–1•min–1) was infused in patients with small pulmonary arteries, small conduits (3.5 mm), or a high hematocrit. All patients were discharged on aspirin 5 mg•kg–1 daily.
The chi-squared test was used to analyze the effects of morphology, restrictive interatrial communication, age, weight, and preoperative or postoperative intervention on the outcome of surgery. A p value of < 0.05 was taken as statistically significant.
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Results
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Five patients died, giving an overall operative mortality of 10.9% (Table 2). The first patient had over-shunting but after clipping the shunt, he could not be weaned from the ventilator. The second patient who had required preoperative support, succumbed to acute cardiac failure in the immediate postoperative period. The third patient had been reexplored for bleeding and later died of a severe chest infection. In the fourth patient who had a restrictive ASD, bifurcation stenosis of the pulmonary arteries was detected in the postoperative period. He underwent shunt revision for a blocked shunt and prostaglandin E1 therapy to maintain ductal patency, but he died of acute cardiac failure. The fifth patient succumbed to low output in spite of balloon atrial septostomy. All of the patients who died were under 2 weeks old, less than 3 kg, and with single-ventricle physiology. Four also had a restrictive ASD. Various risk factors for early mortality were analyzed (Table 3). Univentricular physiology, restrictive ASD, and post-operative reintervention were statistically significant risk factors for early mortality. Age and weight were not significantly associated with early mortality.
Eight patients needed reintervention in the form of shunt revision (3), clipping of the shunt (l), PDA ligation (l), balloon atrial septostomy for restrictive ASD (l), and reexploration for bleeding (2). There were 3 blocked shunts; all were detected on the first postoperative day, manifesting as decreased arterial oxygen tension and reduced tissue oxygen saturation. These 3 patients were less than 3.2 kg in weight with pulmonary atresia morphology; shunt sizes were 3.5, 4, and 5 mm. At reoperation, the cause of the shunt blockage was not clear. It was assumed to be due to technical factors and shunt revision was performed. One patient who had shunt revision succumbed to acute cardiac failure.
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Discussion
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Conventional shunt procedures to increase pulmonary blood flow in the newborn have been shown to have disadvantages.1–5 The Potts shunt is no longer performed because it is associated with excessive pulmonary blood flow and pulmonary hypertension and it is difficult to take down at the definitive operation. The Waterson shunt has the problem of excessive pulmonary blood flow (with marked preferential flow to the ipsilateral lung), congestive cardiac failure, pulmonary artery distortion, and elevated pulmonary vascular resistance. A small pulmonary artery contraindicates the use of a Glenn shunt. Limitations of the classical Blalock-Taussig shunt in newborns include kinking of the subclavian artery at its origin, pulmonary artery tenting, ischemic sequelae in the ipsilateral upper limb, and frequent early occlusion. Hence, it is not an ideal shunt for the newborn.2,6
Reports from several centers have shown advantages of the modified Blalock-Taussig shunt using a prosthetic graft as it preserves the integrity of the subclavian artery, thereby avoiding acute and chronic ischemic sequelae in the upper extremity. The incidence of kinking of the subclavian artery or pulmonary artery distortion is minimal.2,5,8–12 Several studies have convincingly shown growth of the pulmonary artery after a modified Blalock-Taussig shunt.2,5,8–12 However, others have noted significant pulmonary artery distortion in neonates undergoing this procedure.3 The major advantages of a thin PTFE prosthetic graft are the non-wettable electro-negative surface, micropores allowing rapid fibroblast proliferation, and limited neointimal formation ensuring less thrombogenesis and better shunt patency. The material is light, requires no preclotting, and it can be penetrated easily with a 7/0 polypropylene suture needle.13
For neonates, 4- to 5-mm PTFE shunts are usually recommended.3 Ibawi and colleagues7 used 5-mm grafts in most neonates. We used 3.5-mm graft conduits in 13 patients, with one shunt block. There was no statistically significant increase in the incidence of shunt blockage with the use of 3.5-mm grafts. This is contrary to one report in which younger age and smaller shunt size were important factors leading to shunt blockage.10 The more frequent use of 3.5-mm grafts in this study could be because of relatively smaller pulmonary arteries in the neonates in our patient population. It has helped to a great extent in decreasing the possibility of over-shunting. The postoperative course of patients with 3.5-mm shunts may be smoother than those with larger shunts because over-shunting may be reduced in those with small pulmonary arteries, and pulmonary artery distortion can be minimized by the use of a smaller shunt.
Shunt block can be a problem even in a technically well-performed shunt, indicating that other factors may play a role. Three of our patients had shunt block but only one of them had a 3.5-mm shunt, so shunt size alone was not the determinant of shunt block. Although we assumed this to be due to technical factors and performed a shunt revision, the common feature in all of these patients was the occurrence of pulmonary atresia and PDA. Whether this was related to competitive flow between the shunt and the ductus arteriosus, remains conjectural. However, it is essential to make an early diagnosis of shunt block that is usually marked by a drop in oxygen saturation and acidosis. This is attributed to graft thrombosis and extension of the thrombus into the ipsilateral pulmonary artery with a further reduction in pulmonary blood flow.
A word of caution is that a drop in oxygen saturation with metabolic acidosis could also indicate over-shunting. This is particularly common in patients with pulmonary atresia and PDA. Concomitant patency of the ductus and the shunt has potentially harmful effects on coronary blood flow, particularly in patients with pulmonary atresia and intact ventricular septum where sinusoidal communi-cations exist between the right ventricle and the coronary arteries. This is also a frequent cause of postoperative myocardial failure in such patients.14 Preoperative cardiac catheterization could identify this subset of patients in whom the ductus may need to be ligated. The appearance of pulmonary edema on chest radiographs, the presence of a shunt murmur, and echocardiographic correlation help to correctly diagnose over-shunting.
Twenty patients in this series had single-ventricle physiology; an adequate-sized ASD is essential in this subgroup for right-to-left shunting to maintain cardiac output. Those with restrictive ASD have decreased systemic flow. Probably, a shunt procedure adds to the volume load on the heart and increases the left atrial pressure, thereby further decreasing flow through the ASD, which is already limited, leading to systemic hypoperfusion. All 5 patients who died had a shunt procedure in the situation of a physiologically single-ventricle heart. Of these, 4 had a restrictive ASD (p < 0.05); the correlation seems to be direct. The only patient who had a nonrestrictive ASD in this group was in a poor preoperative state requiring prostaglandin E1 infusion and respiratory assistance. Postoperative septostomy in one patient who was already in a poor clinical state, did not help. Thus, an adequate preoperative evaluation of patients with single-ventricle morphology could avoid significant morbidity and mortality. In such a situation, it would be better to perform a septostomy before the patient is given a Blalock-Taussig shunt. Preoperative cardiac catheter-ization could give adequate information on the morphology of the pulmonary arteries, which may not always be detected on echocardiography, particularly in malforma-tions with pulmonary atresia and alternative sources of blood flow, well known to be frequently associated with abnormal distribution of the pulmonary arteries.
Neonates requiring preoperative intervention constitute a high-risk group. Over-shunting due to either PDA or the shunt itself, carries a high risk and needs to be detected and managed early by ligation of the PDA or by clipping the shunt. Judicious use of heparin in the postoperative period in patients with small pulmonary arteries or those with a high hematocrit may help to reduce the incidence of shunt blockage. It was concluded that the modified Blalock-Taussig shunt could be a lifesaving palliative procedure for cyanotic neonates with decreased pulmonary blood flow. Single-ventricle physiology and a restrictive ASD were the major determinants of outcome. Early detection of shunt block and over-shunting is essential to improve results.
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References
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Abstract
Introduction
