25. PAEDIATRIC ANATOMY PHYSIOLOGY &  
 PHARMACOLOGY

Children, especially neonates and children weighing less than about 15 kg, differ markedly from adults. There are differences in size, anatomy, physiology, pharmacology and psychology.

Respiratory Anatomy and Physiology

Babies have a relatively larger head with a prominent occiput. The head needs to be stabilised for intubation. The neck is short and the tongue large. The airway is prone to obstruction. The relatively large head with little hair leads to greater heat loss. The head should be covered.

Infants and neonates breathe mainly though their noses. Their nostrils are small and easily obstructed.

The larynx is more anterior and is situated at a higher level relative to the cervical vertebrae (C3 to C4 at birth) compared to an adult (C6). The epiglottis is relatively longer, leaflike and U shaped. The inexperienced anaesthetist may find the baby more difficult to intubate.

The trachea is short and the right main bronchus is angled less than the left. Right main bronchus intubations are more likely. With most infants, if the 10 cm mark on the endotracheal tube is at the gums, the tip of the tube will be just above the carina. In older children the length of the endotracheal tube may be estimated by (age/2) + 12 cm. Always listen to both lungs to check that the endotracheal tube is not in one lung. Because the length of the trachea is short, a small movement of the tube may move it to the wrong position. The tube should be secured to the maxilla rather than the mandible, which is mobile.

The narrowest part of the upper airway is the cricoid ring in the pre-pubertal child. After puberty, the narrowest part of the airway is at the level of the vocal cords. One of the most serious complications of endotracheal intubation is mucosal oedema and post extubation stridor due to pressure from the external surface of the endotracheal tube. The diameter of the trachea in the newborn is 4 to 5 mm. Just 1 mm of oedema can cause serious harm. Children before puberty should have an uncuffed tube and there should be a slight air leak with positive pressure ventilation. It is important to select the correct size endotracheal tube.

Paediatric endotracheal size and age

Their ribs are more horizontal and any increase in the volume of the thorax is due to downward movement of the diaphagm. A distended abdomen or surgical retraction can easily reduce ventilation.

Oxygen consumption in neonates may be greater than 6 ml/kg/min, i.e twice the oxygen consumption of adults. In infancy a gradual change towards the adult rate (3.5 ml/kg) occurs. A higher oxygen consumption means that neonates and infants will rapidly consume their oxygen reserves and become cyanotic if they are apnoeic. The anaesthetist must be skilled at maintaining a clear airway and intubation. Attempts at intubation must not exceed 30 seconds. Higher oxygen consumption leads to a higher carbon dioxide production, which requires increased ventilation to remove it. The increased ventilation is mainly achieved by a higher respiratory rate (newborn 35 to 40 breaths/minute). The tidal volume/kg is similar for adults and children.

Peripheral airways are narrower and airway resistance is relatively higher in babies. In the newborn or the pre-term baby the brain control of respiration is immature. Pre- mature and ex-premature babies up to 52 weeks post conceptual age are at risk of apnoea after general anaesthesia. They must be very closely observed for at least 24 hours.

Cardiovascular Anatomy and Physiology

Cardiac output in the neonate might be 200 to 400 ml/kg/min compared to 70 to 80 ml/kg/min in the adult because of the higher metabolic rate and oxygen requirement in the neonate. Stroke volume is relatively fixed in the newborn due to the poorly compliant ventricular muscle. Stroke volume in the newborn is 5 to 7 ml/kg compared to 1 to 2 ml/kg in adults. Therefore, an increase in cardiac output is achieved by an increase in heart rate. The newborn’s resting heart rate is much higher than that of the adult (130 to 140/min in the neonate, 70/min in the adult) and it is not until about the age of ten that it reaches adult rates.

Blood pressure is lower in children than adults because of low peripheral resistance.

Blood volume in the neonate is about 80 ml/kg compared to 70 ml/kg in the adult.

The sympathetic nervous system is not well developed. Infants can easily become bradycardic. Atropine premedication will reduce the incidence of bradycardia and reduce secretions. (Intravenous or intramuscular dose is 0.01 to 0.02 mg/kg). Maximum dose should be less than 0.06 mg/kg.

Haemoglobin at birth is high (18 g/dl) and falls to a low at 3 to 6 months of about 11 g/dl. The change is due to a decrease in foetal haemoglobin. Foetal haemoglobin is not able to deliver oxygen to the tissues as efficiently as adult haemoglobin. A haemoglobin of less than 13 g/dl in the newborn and less than 10 g/dl in the first 6 months of life may be significant.

Paediatric Cardiovascular Parameters:

Renal System and Fluid Balance

Neonates have a greater total body water (70 to 75% of body weight) compared to adults (60% of body weight). There is a larger extracellular compartment (ECF) and smaller intracellular compartment (ICF). By the first year of age the proportions are the same as for adults (ECF 45%, ICF 55% of total body water). The increased metabolic rate of infants results in a faster turnover of extracellular fluid. An interruption of the normal fluid intake can therefore rapidly lead to dehydration and the anaesthetist must take care with fluid management. The anaesthetist must estimate replacement fluid, maintenance fluid and ongoing fluid losses.

Estimating Maintenance Fluid Requirements:

Newborn first 24 hours                       3 ml/kg/h

Newborn day 1 to 7                           5 ml/kg/h

Infant                                                4 ml/kg/h for the first 10kg

adding 2 ml/kg for the second 10 kg and 1 ml/kg/h for each kg over 20 kg.

[for example a 16 kg child needs (10 kg x 4 ml) + (6 kg x 2 ml) = 52 ml/h or 1248 ml/day]

Remember that the maintenance fluid volume will need to be reduced (70% maintenance) in many unwell children: ie children with suspected neurological [meningitis and encephalitis] and respiratory [bronchiolitis and pneumonia] disease.

Well children 0.45% NaCl with 5% glucose (+/- 20 mmol KCl/l)

Unwell children 0.9% NaCl

Remember that most children in hospital should receive oral fluids and nutrition.

Children who are dehydrated preoperatively need fluid replacement before surgery.

The anaesthetist must assess the degree of dehydration.

Children have a relatively small blood volume. A 5kg infant will have a blood volume of only 400 ml. Blood loss of only 40 ml is a 10% decrease in blood volume and 80 ml a 20% loss of blood volume. A soaked swab will contain at least 5 ml and a small pack at least 20 ml of blood. 

Urine output should be at least 0.5 ml/kg/h.

The neonate has decreased glomerular filtration and tubular function. The ability to excrete a fluid load is initially poor but this function rapidly increases in the first month of life. The ability to produce concentrated urine is also initially poor and improves rapidly in the first two months reaching adult levels by two years of age.

Temperature

The newborn is at a greater risk of cooling when exposed to a cold environment because the ratio of body surface area to body weight is double that of older patients. Skin and subcutaneous fat is thinner, providing less insulation and leading to greater heat loss. Heat production is low and the ability to shiver is not well developed. Temperature regulation is immature. The environmental temperature range in which oxygen consumption is minimal (thermoneutral range) is narrow. A decrease in environmental temperature of two degrees Celsius may double the oxygen consumption of a newborn. Infants must be kept warm. The operating theatre should be heated and the infant kept covered. Try to warm intravenous fluids.

Hepatic Physiology

Liver metabolism may be poor in the newborn but develops rapidly in the first few weeks. Drugs such as opioids, benzodiazepines and barbiturates may not be metabolised as rapidly in neonates.

Paediatric Pharmacology

The differences in physiology of the infant will alter the effect of some drugs.

All opioids and central nervous system depressants must be given with caution in neonates unless the patient is being ventilated and closely monitored. Morphine clearance in neonates is one quarter that of adults so that the elimination half time will be four times that of adults. The immature respiratory centre makes the neonate more sensitive to the respiratory depressive effects of morphine.

The proportion of cardiac output going to the brain is greater in the neonate than in older children. The dose of intravenous induction agents should be reduced in neonates.

Decreased renal and liver function results in certain drugs being excreted more slowly. The dosing interval should be increased to avoid toxicity.

Neonates and infants require a greater dose suxamethonium (2 mg/kg) than adults (1 mg/kg).

The MAC of inhalational agents is greater in the young and decreases with increasing age, however neonates require lower concentrations than infants do. There may be nearly a 30% greater anaesthetic requirement for inhalation agents but there is a smaller margin of safety between adequate anaesthesia and cardiovascular and respiratory depression in infants compared with adults. Both induction and recovery from inhalation agents is more rapid in children than adults.