A complete and very clear description of this entity can be found at Hypoplastic Left Heart Syndrome.

A few services have proposed a heart transplantation as the treatment of choice for this complex malformation instead of a staged approach as proposed by Norwood.

The procedure designed for palliation of the HLHS has been well described. The principle of this operation is to provide an unobstructed systemic outflow tract from the right ventricle by connection of the proximal pulmonary artery to the longitudinally incised ascending aorta and aortic arch [1]. Pulmonary blood flow is provided by a 3-4 mm central PTFE graft connecting the reconstructed aortic arch to the pulmonary arterial confluence. An integral part of the operation includes creation of a large atrial septal defect to ensure relief of obstruction at the atrial level.

Graft size for the central shunt is of paramount importance and the choice varies among different surgical teams. Generally speaking, for patients with normal or near-normal chest films and preoperative saturation greater than 80 percent, shunt size in mm is a gross approximation of the weight in kilograms, (e.g. for a 3.5 kg baby a 3.5 mm shunt is used). The more congested the chest film is, the lower the saturation, and the greater the weight of the baby, the larger is the shunt chosen to construct the anastomosis. Premature babies with body weight lower than 3.0 kg, a 3.0 mm shunt is usually employed.

As it has been extensively discussed, the most challenging aspect of this procedure is the maintenance of an appropriate ratio of pulmonary to systemic blood flow postoperatively. The objective is to maintain a satisfactory arterial blood flow and oxygen saturation and at the same time to avoid pulmonary overflow. The adequate balance between the systemic and pulmonary blood flows is critical to survival. This critical balance has been accomplished by a meticulous postoperative surveillance and management.

Ashburn et cols [2] studied a cohort of 710 neonates submitted to the Norwood procedure, among 985 with critical aortic stenosis or atresia enrolled in a prospective 29-institution study between 1994 and 2000. Overall survivals after the Norwood operation were 72%, 60%, and 54% at 1 month, 1 year, and 5 years, respectively. Risk factors for death occurring before subsequent transition included patient-specific variables (lower birth weight, smaller ascending aorta, older age at Norwood operation), institutional variables (institutions enrolling < or =10 neonates, two institutions enrolling >/=40 neonates), and procedural variables (shunt originating from aorta, longer circulatory arrest time, and management of the ascending aorta). Improved results have been reported with the use of alpha-blockade with phenoxybenzamine (POB) for systemic afterload reduction and selective cerebral perfusion. Some centers have relied upon a special postoperative care team with a large knowledge and experience in the management of these critical patients, named “Norwood team” to spend 24 to 48 hours never leaving the bedside, prepared to intervene with any adversity.

In order to create a more favorable course of actions in the immediate postoperative care of patients with HLHS submitted to the Norwood procedure, Ungerleider et cols [3] employed mechanical ventricular assistance to routinely support their patients. The next few paragraphs constitute a condensation of their important work.

Although some centers have achieved excellent results, overall success with respect to hospital survival and neurologic outcome following the Norwood procedure at most centers remain inconsistent. Hospital mortality is relatively high, especially compared to the outcomes for other types of pediatric intracardiac surgery, even in neonates. Survivors often showed a high incidence of neurologic impairment as manifested by mental retardation, cerebral palsy, and learning disability.

Experience with postoperative mechanical circulatory support following the Norwood procedure has been inconsistent, and some reports were discouraging and suggested that the use of extracorporeal membrane oxygenation (ECMO) was complicated, expensive, and only rarely successful, mostly due to bleeding and the necessity of anticoagulation. However, by leaving the systemic-to-pulmonary shunt open, it is not necessary to place an oxygenator into the circuit because the patient’s lungs serve quite effectively as an oxygenator. Thus, a much lower level of anticoagulation is needed leading to less postoperative bleeding. This strategy has led to excellent results in patients who experience hemodynamic collapse following the Norwood procedure. Survival depends on an adequate cardiac output to perfuse the pulmonary and the systemic circulations. As long as cardiac output is adequate, systemic and pulmonary perfusion is maintained resulting in good hospital survival and neurologic outcome.

Eighteen infants underwent Norwood for HLHS or Damus-Kaye-Stansel (with arch augmentation) for variations of HLHS. They were routinely placed on mechanical ventricular assist immediately at the end of their procedure. Operative conduct was similar in all cases with regards to aortic arch augmentation using pulmonary homograft material, atrial septectomy, and placement of a systemic-to-pulmonary shunt between the innominate artery and the right pulmonary artery. All patients were cooled on cardiopulmonary bypass to 18⁰ C. Arch reconstruction was performed during a period of deep circulatory arrest with intermittent perfusion or with continuous selective antegrade cerebral perfusion (n=5). All patients were rewarmed, separated from CPB, and treated with a period of modified ultrafiltration (MUF). Atrial and neoaortic cannulas were then attached to a ventricular assist device (VAD) circuit (roller pump for 16 pts; centrifugal pump in 2 pts) and the flow was slowly increased to 200 ml/kg/min. The circuit used for mechanical support was a modified ECMO circuit without an oxygenator. It would be relatively simple to add an oxygenator to the circuit and convert the patient to ECMO support if necessary. Because of the high flow rates, it is not necessary to anticoagulate the patient (at least initially) and the ability to give protamine while the patient’s cardica output is being supported by VAD helps with hemostasis and maintenance of post repair stability. Protamin sulfate was administered and hemostasis was achieved. Sternuns were left open and covered with Esmarch dressings and the patients were transported to the intensive care unit (ICU). Heparin Intravenous infusion was restarted in the ICU after several hours and once hemostasis was complete and the goal was to achieve an activated clotting time (ACT) between 160 to 180 seconds. The systemic-to-pulmonary shunt was left open in all cases. Patients were ventilated to achieve an arterial PaO2 between 30 and 45 mmHg and PaCO2 between 35 and 45 mmHg. All infants received milrinone intravenous infusion, which was begun during rewarming on CPB and continued during the VAD period (0.5 to 1.0 mcg/kg/min). Once the systemic perfusion became normal, VAD flow rate was weaned to off and cannulas were removed. Chests were closed in the ICU. If chest closure resulted in significant elevation of the CVP, it was delayed for 1 to 2 days after decannulation.

All patients were successfully weaned off from ventricular support. Two patients (both nonsurvivors) had hemodynamic instability and required urgent replacement on mechanical support within a short time of removal from VAD. All the others recovered and were eventually discharged from the hospital. Hospital survival was 16/18 = 89% with confidence interval of 63.9%. Complications related to VAD were minimal. Two patients required mediastinal exploration in the ICU for bleeding, which was controlled.

Because cardiac output is supported by mechanical menas, it is possible to utilize larger shunts (16 patients had 4 mm shunts, including a 1.6 kg premature). This leads to improved pulmonary blood flow, higher oxygen saturations, and increased cerebral oxygen delivery. By maintaining cardiac output with VAD during the early postoperative period, patients achieve their own circulatory balance and they no longer require the same level of vigilant support from the ICU staff used for patients managed in more conventional ways.

Survivors were usually hemodynamically stable the day following surgery, but often were supported with VAD for an additional day or two so that they could be diuresed. When patients were weaned from VAD, it was commonly necessary to augment their hemodynamics with inotropic support for 1 to 2 days. All of this tends to increase the lenght of ICU stay compared to survivors of conventional management. However, the normal neurologic outcome of survivors, the excellent hospital survival, and the stability during postoperative period make a compelling case for this strategy, especially for programas that are currently struggling with their results from Norwood.

The routine use of VAD may help some programas achieve excellent hospital survival with good neurologic outcomes for survivors, but will most likely increase cost as wll as the need for additional support personnel.

Supporting Norwood patients with an ECMO circuit without an oxygenator is not an easy task. However it may contribute to an increased survival and the absence of hemodynamic oscillations commonly observed with the conventional approach. The excellent circulatory support contributes to a better recovery and to a better cerebral oxygenation during the first 24-48 hours after the operation. This innovative approach does not represent an universal indication to support Norwood patients but it may be helpfull for those most demanding patients, for units with a smaller case load, and for teams still not obtaining good results with the management of these difficult patients.

REFERENCES:

1. Rebeyka IM, Coles JG, Williams WG, Trusler GA, Benson LN and Freedom RM. Glossary of congenital cardiac operations. In Freedom RM, Benson LN and Smallhorn JF (eds) Neonatal heart Disease, Springer-Verlag, London, 1992.

2. DA, McCrindle BW, Tchervenkov CI, Jacobs ML, Lofland GK, Bove EL, Spray TL, Williams WG, Blackstone EH. Outcomes after the Norwood operation in neonates with critical aortic stenosis or aortic valve atresia. J Thorac Cardiovasc Surg. 2003 May;125(5):1070-82.

3. Ungerleider RM, Shen I, Yeh TJr, Schultz J et cols. Routine mechanical ventricular assist following the Norwood procedure - Improved neurologic outcome and excellent hospital survival. Ann Thorac Surg 2004;77:18-22.