Introduction

Since its first successful application in 1953, advances in cardiopulmonary bypass (CPB) in conjunction with developments in operative technique and postoperative care have resulted in increasingly complex cardiac surgery being successfully undertaken on younger and smaller children. With mortality rates improving, the focus is now changing to reducing the morbidity associated with paediatric surgery and CPB. The effects of hypothermia, altered perfusion, haemodilution, acid-base management, embolization and the systemic inflammatory response still pose significant problems. Infants and neonates present additional challenges due to their small size, immature organ systems and altered physiology.

Neonatal Physiology

The normal changes in physiology in the first weeks of life, including the reactivity of the pulmonary vasculature, influence both technique and outcome of surgical repair. Pulmonary hypertension, present at birth, decreases to approximately 50 per cent of systemic values within the first 24 to 48 hours of life but takes several weeks to completely resolve. The neonatal myocardium is relatively resistant to ischaemia but has limited functional reserve, being susceptible to increased afterload (systemic and pulmonary vascular resistance). Neonates have a higher metabolic rate requiring higher pump flow rates per kilogram compared to an adult and an impaired thermoregulatory response with greater dependence upon environmental temperature. Variability in heparin and protamine pharmacokinetics and immaturity of the liver may result in a coagulopathy that will be worsened by the presence of cyanosis. Renal perfusion and glomerular filtration are decreased due to increased renal vascular resistance, resulting in impaired sodium, water and acid-base regulation. While humoral and cellular immune factors are present, the immune response is impaired and subnormal. The underlying cardiac pathology or chronic cyanosis may result in the presence of large intra- and extra-cardiac shunts that may need controlling during CPB in order to maintain adequate systemic perfusion and enable visualization of intra-cardiac defects.

The majority of paediatric CPB utilizes haemodilution with moderate hypothermia. In small patients, limited access for surgery and CPB cannulation together with the need to undertake complex repairs with good visibility necessitates periods of deep hypothermic circulatory arrest.

Prime and Haematocrit

Haemodilution during CPB theoretically counterbalances the increase in blood viscosity caused by induced hypothermia, resulting in improved microcirculatory perfusion. In addition, it is associated with less red blood cell aggregation and increased cerebral blood flow velocity. It does, however, reduce clotting factors and also plasma oncotic pressure, enabling fluid to move from the intravascular space into the intracellular space and causing tissue oedema and impaired organ function. The accumulation of fluid is more common in neonates secondary to increased capillary permeability.

The extremes of haemodilution (~10 per cent haematocrit) are associated with impaired oxygen delivery and delayed cerebral recovery following deep hypothermic circulatory arrest (DHCA). Acceptable levels of haematocrit are not yet clearly defined, and they vary between 20 and 30 per cent for neonatal deep hypothermic bypass. Recent work does suggest that higher haematocrits of 30 per cent are probably beneficial to postoperative long-term psychomotor development.

The blood volume of a neonate is around 85 mL/kg, with a 3-kg child having a circulatory volume of approximately 255 mL. A CPB prime volume of 650 to 800 mL will result in a prime-to-blood-volume ratio of 2:1 or 3:1. The priming volume should be kept as small as possible by the utilization of appropriately sized equipment and with correct positioning of the CPB apparatus to avoid unnecessary lengths of tubing. Administration of crystalloid fluids during CPB to maintain an adequate working volume will result in further haemodilution. During CPB, the addition of fluid to the circuit should only be to maintain the minimum safe working volume, and crystalloid volume replacement should be avoided.

The composition of the prime is dictated by the size of the child, the preoperative haemoglobin, the maintenance of colloid oncotic pressure, the surgical procedure and personal preference. Typically for a neonate, the prime may consist of Plasmalyte 148 (30 mL/kg), 1 unit of fresh frozen plasma, heparin (2,500 units), methylprednisolone (20 mg/kg), calcium chloride (2.5 mmol), sodium bicarbonate 8.4 per cent (10 mL), mannitol (0.5 g/kg) and irradiated red blood cells.

With attention to these issues and with the development of paediatric CPB components, it is possible to reduce priming volumes to approximately 300 mL for a 3-kg child. Despite this reduction, maintenance of an intra-operative haematocrit of 30 per cent is likely to require the addition of donor blood. Cardiac surgery and CPB without using blood have been successfully undertaken in children weighing less than 10 kg. To date, there has been no systematic assessment of the safety and efficacy of blood versus blood-free cardiac surgery in children.