Anatomical and physiological differences in a neonate:

Airway
All the above differences make the airway more precarious and difficult to maintain in an obtunded or anesthetized neonate. Intubation is slightly more difficult as the larynx is more "anterior" and requires the use of a different laryngoscope blade than in adults. Once intubated, the tube must be very securely fixed to prevent movement as a difference of a cm may mean either endobronchial intubation or accidental extubation.

Respiratory system

The chest wall is very compliant with the ribs being more horizontal; therefore the infant is mechanically disadvantaged. The muscles are still immature & fatigue easily. Breathing is predominantly diaphragmatic in the neonate. The respiratory centre is immature & is easily depressed by the effects of sedatives, anesthetic agents and opoids.

The airway diameters are small & small amounts of secretions or edema will increase the resistance of the airway. It is very important to keep airway clear of secretions, especially for airway surgery.

Cardiovascular system

The myocardium of the neonate is immature and less compliant. It is less able to increase its cardiac output by an increase in contractility. It does so by an increase in H.R. therefore in peri-operative situations where there is hypovolemia, the first response will be to mount tachycardia to increase cardiac output. In this way hypotension is prevented until almost more than 40% of blood volume is lost, at which point compensatory mechanisms fail and a precipitous fall in BP can occur.

The neonate has a high vagal tone that is mostly manifested by bradycardia. The ECG of the infant will show RVH patterns and RAD until about 6 months of age when it will slowly revert to the normal adult pattern.

Hematology

At birth, 75% of the circulating Hb is HbF & it is not until 6 months that HbA haemopoiesis is fully established. The p50 of HbF is 18 mmHg as compared to 26 mmHg of HbA. As the production of HbF falls off & that of HbA picks up there will be a point in time where there is a relative "anemia", the so-called physiological anemia which occurs at about 3 months of age when the Hb falls to about 10 g/dl.

Renal

The renal function of a preterm is "abnormal" due to immaturity. The creatinine clearance of a 28 weeker is only 25% of a full term neonate. By the age of 1 yr, the (GFR) will be equivalent to an adult. Due to tubular immaturity, the premature neonate is less able to reabsorb glucose & bicarbonate that leads to metabolic acidosis and dehydration. It is also less able to concentrate urine.

Temperature regulation

Neonates are less able to maintain body temperature. They have a very large surface area to volume ratio, little subcutaneous fat and also a relatively large head. Heat loss from the head accounts for upto 25% of total heat loss even in an adult, & it is proportionately greater in the neonate. Mechanisms by which the neonate can generate heat are by moving, crying and brown fat. Infants less than 3 months of age seldom shiver.