Normal Neonatal Physiology

Newborn infants have a high metabolic rate and oxygen consumption. This is reflected in a high resting RR and CI (neonate: 300 ml kg^–1 min^–1, adult 70–80 ml kg^–1 min^–1). They have a limited capacity to increase the SV in response to increased filling and the resting HR is near maximal (Table 20.1). Neonates are exquisitely sensitive to negative inotropic or chronotropic agents.

The sarcoplasmic reticulum in neonatal myocytes is poorly developed. Calcium for cardiac contraction is derived from the extracellular fluid and infants do not tolerate ionized hypocalcaemia. There is a relative imbalance of sympathetic and parasympathetic nervous systems at birth and neonates are prone to vagal reflexes.

The infant lung is very compliant, the ribs are also relatively compliant. The lower airways are small and easily obstructed by secretions. Infants are consequently prone to respiratory failure.

Other important factors to consider include immature renal function, temperature regulation, hepatic function and drug, particularly opiate, metabolism.

Transitional Circulation

In utero blood bypasses the foetal lung via two shunts, the foramen ovale and the ductus arteriosus (DA). With the first few breaths, there is a dramatic reduction in the PVR and closure of foetal shunts. Pulmonary vasodilatation continues during the first few weeks of life, due to thinning of smooth muscle in the media of the pulmonary arterioles. The PVR reaches adult levels by a few weeks of age (Figure 20.1). During this time the pulmonary vasculature remains reactive and stimuli such as hypoxia, hypercarbia and acidosis will cause pulmonary vasoconstriction and possibly reopen the DA (see below). Persistent PHT of the newborn may result; hypoxia may become critical and require treatment with inhaled NO or, in extreme cases, ECMO.

Figure 20.1 Perinatal changes in pulmonary haemodynamics.

Reprinted with permission from Rudolph AM, 1996.

Closure of the DA occurs in two phases. Functional closure occurs within 2–4 days in nearly all healthy infants under the influence of increasing PaO₂, falling PaCO₂ and prostaglandins. Anatomical closure of the DA due to fibrosis occurs within the first 3 weeks of life.

Continued ductal patency may occur due to prematurity (inadequate ductal smooth muscle) or in sick infants under the influence of excessive endogenous prostaglandin, released in response to stimuli such as hypoxia (causing relaxation of the ductal smooth muscle). A large duct may result in cardiac failure due to excessive pulmonary blood flow (PBF); the diastolic BP will be low and may be associated with impaired renal or intestinal blood flow (possible renal impairment or necrotizing enterocolitis). Prostaglandin synthetase inhibitors such as indomethacin promote closure of the duct.

Duct-Dependent Circulation

In certain situations, continued ductal patency may be required for survival of the neonate (Table 20.2). In this situation prostaglandin E₂ (PGE₂) infusion will be required and should be continued until definitive surgery. High doses of PGE₂ can result in apnoea, fever and systemic vasodilatation. Where PGE₂ infusion has been continued long term (for instance, a premature neonate awaiting surgery), it should be remembered that prostaglandins are effective pulmonary vasodilators and may have to be weaned gradually after surgery.

Infants with a duct-dependent systemic circulation typically present with cardiac failure or collapse during the first week of life as the duct closes and shunting from the pulmonary to systemic circulation is lost. Treatment comprises resuscitation with inotropes and fluids, and institution of PGE₂ infusion prior to definitive surgery.

Infants with duct-dependent pulmonary circulation become cyanosed after duct closure and the loss of left-to-right shunting across the duct. Cyanosis will be unresponsive to an increased FiO₂ and the PBF must be restored with prostaglandin infusion before a definitive surgical procedure; for example, valvotomy or a systemic to pulmonary shunt is performed.

Infants with duct-dependent ‘mixing’ have two parallel closed-loop circulations and are dependent on mixing between the right and left side. PGE₂ infusion and balloon atrial septostomy will be required where there is inadequate intracardiac mixing.

Balancing Systemic and Pulmonary Circulations in Neonates

An appropriate balance between systemic blood flow and PBF can be crucial, particularly in the neonate when alteration in the direction of shunt blood flow may cause dramatic changes in saturation or CO.

Oxygen is a potent pulmonary vasodilator in neonates, while hypercarbia and acidosis cause pulmonary vasoconstriction. A high FiO₂ and hyperventilation may be beneficial in infants with reduced PBF. Conversely, they may have a detrimental effect in infants with high PBF, or with balanced systemic and pulmonary shunts.

Exposure of neonates with large left-to-right shunts (e.g. a large VSD) to a high FiO₂ will cause pulmonary hyperaemia and worsening cardiac failure. Infants in cardiac failure preoperatively should be given sufficient inspired oxygen to maintain the SaO₂ in the low 90s only. Similarly, neonates with a duct-dependent systemic circulation (see above) or with high-volume, high-pressure shunts (e.g. atrioventricular (A-V) septal defect, truncus arteriosus, a large Blalock–Taussig (B–T) shunt), may have balanced shunts between systemic and pulmonary circulation. Ventilation with 100% oxygen may cause a marked fall in the PVR, excessive left-to-right shunting and a fall in systemic perfusion, leading to hypotension and metabolic acidosis. Strategies should be adopted to improve the CO (e.g. fluids, inotropes) and reduce the PBF. Mechanical ventilation in the operating room should be with air (or for cyanotic lesions, sufficient oxygen to maintain the SaO₂ in the mid 80s), and with moderate hypercarbia. Conversely, a marked fall in the SVR should be avoided as this may result in increased right-to-left shunting and critical cyanosis. Inotropic support may be required to increase the SVR and CO, thus improving the PBF. Similar principles should be followed postoperatively in the ICU, particularly after the first-stage Norwood procedure for hypoplastic left heart syndrome (HLHS).

The SVR should be maintained in infants with large right-to-left shunts, such as tetralogy of Fallot. Excessive vasodilatation on induction of anaesthesia may result in worsening cyanosis; vasoconstrictors may be required. Extreme right-to-left shunting is seen in the ‘spelling Fallot’ due to spasm of the RVOT infundibulum in the presence of excess catecholamines. Measures to overcome infundibular spasm and increase the PBF are required. These include adequate sedation, hyperventilation with 100% oxygen, a fluid bolus, bicarbonate or phenylephrine – the latter to reverse the direction of the shunt across the VSD and improve forward flow to the lungs. Propranolol or esmolol may also be considered to reduce infundibular spasm.

The SVR must be maintained in infants with left (or right) ventricular outflow obstruction. A fall in the SVR may lead to critical hypoperfusion of the hypertrophied ventricle. A reduction in the PVR may be beneficial in infants with RVH.

Cardiopulmonary Interactions

Positive pressure ventilation is generally beneficial to infants in cardiac failure due to poor myocardial function or left-to-right shunts (reduced work of breathing and afterload, improved oxygenation and CO₂ clearance), with attention to the FiO₂, as described above. However, hyperventilation at high airway pressures or lung volumes may increase the PVR by distension of the lungs, disproportionately reducing the cross-sectional area of the pulmonary vasculature.

Spontaneous ventilation – where intra-pleural pressure is negative during inspiration – will augment systemic venous return to the right heart and improve the PBF. This fact is utilized in the postoperative management of patients with cavopulmonary connections.

Surgical Strategy

The timing of surgical repair is dictated by the functional impact of the cardiac lesion. Urgent balloon septostomy may be indicated in neonates with transposition of the great arteries. Duct-dependent lesions require corrective or palliative surgery within days of birth. Similarly, infants with obstructed total anomalous pulmonary venous drainage (TAPVD) may present with extreme cyanosis and cardiac failure, requiring urgent corrective surgery.

Systemic arterial to pulmonary shunts such as the B–T (subclavian artery-to-main PA) shunt are performed in infancy for conditions associated with a low PBF such as severe tetralogy of Fallot or pulmonary atresia. Definitive corrective surgery is performed, usually in the first year, after further growth of the pulmonary arteries.

Conditions involving left-to-right shunts increase the PBF and cause cardiac failure and PHT. Typically, high-volume shunts cause heart failure as the PVR falls to adult levels at a few weeks of age (e.g. a large VSD). A continued high PBF will result in irreversible changes in the pulmonary vasculature and will severely limit treatment options. Early surgery is therefore indicated. PA banding may be performed in infants with a high PBF not suitable for early definitive surgery (e.g. multiple VSDs). Low-volume shunts (e.g. ASDs) may be closed when the child is older to avoid irreversible PHT in adult life.

Palliative surgery to create a univentricular circulation is performed in conditions where there is only a single functional ventricle or great vessel (univentricular A-V connection – double-inlet ventricle, HLHS, tricuspid atresia or severe pulmonary atresia with intact ventricular septum). The PBF may initially be provided with a B–T shunt in the neonatal period while the PVR remains high. Systemic venous shunts are performed after the PVR falls – initially a Glenn (SVC-to-PA) shunt, followed by an IVC-to-PA anastomosis to complete the Fontan circulation (or total cavopulmonary venous connection – see Chapter 21).

Closed Cardiac Surgery

Cardiac operations may be ‘closed’ or ‘open’. Open operations are performed on CPB and are discussed in the next chapter. Closed operations are usually performed on the great vessels and do not require CPB. The four commonest indications for closed operation in neonates and infants are:

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  Ligation of a patent ductus arteriosus (PDA) via left thoracotomy

  
  
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  Repair of coarctation via left thoracotomy

  
  
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  PA banding via sternotomy or thoracotomy; PA banding is a temporary measure to prevent PHT and is reversed at the time of definitive surgery

  
  
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  Systemic to pulmonary shunts, for example, a B–T shunt via sternotomy or thoracotomy

Cyanosis

Children with cyanotic heart disease maximize the tissue oxygen delivery by becoming polycythemic and having a mild metabolic acidosis (causing a right shift of the Hb–O₂ dissociation curve). The progression of cyanosis is reflected in an increase in the haematocrit; venesection and haemodilution may be indicated. There is a risk of thromboembolism, including cerebral infarction, which is exacerbated by dehydration. Prolonged preoperative starvation must therefore be avoided. The prothrombotic tendency is partially compensated for by a mild coagulopathy – clotting factors should be available post CPB.