Paediatric Anaesthesia
PHYSIOLOGY IN THE NEONATE
Before the age of 8 years, the calibre of the airways is relatively narrow. Airway resistance is therefore relatively high. Small decreases in the diameter of the airways as a result of oedema or respiratory secretions significantly increase the work of breathing. Elastic tissue in the lungs of small children is poorly developed. As a result of this, compliance is decreased. This has important consequences in that airway closure may occur during normal tidal ventilation, thereby bringing about an increase in alveolar–arterial oxygen tension difference (PA − aO2). This explains why PaO2 is lower in the infant than in the child. The decreased compliance results in ventilatory units with short time constants. Consequently, the infant is able to achieve adequate alveolar ventilation whilst maintaining a high respiratory rate. However, because of the increased resistance and decreased compliance, the work of breathing may represent up to 15% of total oxygen consumption (Table 36.1). The high respiratory rate is necessary because the metabolic rate of the infant is nearly twice that of the adult. The high alveolar minute ventilation explains why induction and emergence from inhalational anaesthesia are relatively rapid in small children. The high metabolic rate also explains why desaturation occurs very rapidly in children.
TABLE 36.1
Lung Mechanics of the Neonate Compared with the Adult
Neonate | Adult | |
Compliance (mL cmH2O−1) | 5 | 100 |
Resistance (cmH2O L−1 s−1) | 30 | 2 |
Time constant (s) | 0.5 | 1.3 |
Respiratory rate (breath min−1) | 32 | 15 |
The ratio of physiological dead space to tidal volume (VD/VT) is similar to that of the adult at about 0.3. However, because the volumes are smaller, modest increases in VD produced by equipment such as humidification filters may have a disproportionately greater effect (Table 36.2).
TABLE 36.2
Respiratory Variables in the Neonate
Tidal volume (V) | 7 mL kg−1 |
Dead space (VD) | (VT) × 0.3 mL |
Respiratory rate | 32 breath min−1 |
Cardiovascular System
The process of growth demands a high metabolic rate. It is, therefore, not surprising that infants and children have a higher cardiac index compared with the adult, so that oxygen and nutrients may be delivered to actively growing tissues. The ventricles of neonates and infants are poorly compliant, so even though the ventricles of infants demonstrate the Frank-Starling mechanism, the main determinant of cardiac output is heart rate. Infants tolerate heart rates of 200 beat min−1 with ease (Table 36.3). Bradycardia may occur readily in response to hypoxaemia and vagal stimulation and it results in a decrease in cardiac output. Immediate cessation of the stimulus, and treatment with oxygen and atropine, are absolutely crucial. A heart rate of 60 beat min−1 in an infant is considered a cardiac arrest and requires cardiac massage. Arrhythmias are rare in the absence of cardiac disease. The usual cardiac arrest scenarios are electromechanical dissociation and asystole, not ventricular fibrillation.
Haemoglobin
At birth, 75–80% of the neonate’s haemoglobin is fetal haemoglobin (HbF). By the age of 6 months, adult haemoglobin (HbA) haematopoiesis is fully established. HbF has a higher affinity for oxygen than HbA. This is demonstrated by the leftward shift of the oxygen haemoglobin dissociation curve (Fig. 36.1). Low tissue PO2 and metabolic acidosis in the tissues result in the avidity of HbF for oxygen being reduced, thereby aiding delivery of oxygen. Alkalosis produced by hyperventilation results in less oxygen being available and it is therefore sensible to maintain normocapnia.
FIGURE 36.1 Effects of fetal haemoglobin (HbF) on the oxygen dissociation curve. HbA, adult haemoglobin; PO2, partial pressure of oxygen.
Renal Function and Fluid Balance
Body fluids constitute a greater proportion of body weight in the infant, particularly the premature infant, compared with the adult (Table 36.4). In an adult, most of the total body water is in the intracellular compartment. In a newborn infant, most of the total body water is in the extracellular compartment. With increasing age, the ratio reverses. Plasma volume remains constant throughout life at about 5% of body weight.
Fluid Therapy
An intravenous infusion delivering maintenance fluids should be in place for all neonates requiring surgery. Maintenance fluid requirements increase over the first few days of life (Tables 36.5, 36.6). The normal infant requires of the order of 3–5 mmol kg−1 of sodium and an equivalent amount of potassium per day to maintain normal serum electrolyte concentrations. The ability of the infant’s kidneys to eliminate excess sodium is limited. Exceeding this amount in the absence of loss results in hypernatraemia and its sequelae. Infants undergoing any procedure more than the briefest should also have their calorific needs addressed. This may be achieved by including glucose-containing fluids in the regimen; failure to do so results in hypoglycaemia and ketosis. This may occur rapidly because of the limited glycogen stores and high metabolic rate of the infant.
TABLE 36.5
Fluid Requirements in the First Week of Life
Day | Rate (mL kg−1 Day−1) |
1 | 0 |
2, 3 | 50 |
4, 5 | 75 |
6 | 100 |
7 | 120 |
TABLE 36.6
Maintenance Fluid Requirements
Weight (kg) | Rate (mL kg−1 Day−1) |
Up to 10 kg | 100 |
10–20 kg | 1000 + 50 × [weight (kg) – 10] mL |
20–30 kg | 1500 + 25 × [weight (kg) – 20] mL |
It is imperative that the anaesthetist recognizes and resuscitates the dehydrated infant appropriately before surgery. Clinical examination of skin turgor, capillary refill, tension of fontanelles, arterial pressure and venous filling may aid estimation of hydration, but electrolyte and haemoglobin concentrations and haematocrit, urine volumes and plasma and urine osmolalities should be monitored if problems of fluid balance exist (Table 36.7).