Chapter 39: The Newborn Infant

ADAPTATION TO EXTRAUTERINE LIFE

After delivery, the newborn child must undergo major physiologic changes to adapt to a new environment and way of life. The most vital of these are:

1. The establishment of regular breathing and exchange of gases

2. Circulatory alterations

Respiration

In utero, the fetal lung produces fluid that passes through the tracheobronchial tree until it reaches the oropharynx. There it is swallowed or mixes with the pool in the amniotic cavity. In animals, catecholamines secreted by fetuses during labor decrease lung fluid production. During vaginal delivery, the thorax is compressed as it traverses the birth canal. This expresses some fluid from the upper airways. Most of what remains is absorbed by the pulmonary capillaries and lymphatics.

The mechanical expansion of the lungs at first breath and the rise in alveolar Po₂ lead to a rapid decrease in pulmonary vascular resistance and an increase in pulmonary blood flow. A number of factors are involved in the initial stimulus to respiration. Perhaps the most important is the fall in Po₂ and the rise in Pco₂ that follow the cessation of umbilical circulation. Tactile, thermal, and proprioceptive inputs also play significant roles.

Circulation

The changes in the circulation include:

1. Absolute and relative pressure changes

2. Closure of fetal channels

Changes in Pressure

Fetal circulation is characterized by relatively high right ventricular and pulmonary artery pressures. These are maintained by elevated pulmonary arteriolar resistance and by the presence of a large ductus arteriosus. The ductus equalizes pressures between the pulmonary artery and aorta and directs most of the right ventricular output into the systemic circulation. Systemic pressures are decreased by the presence of the umbilical circulation, which acts as a low-pressure shunt.

As outlined earlier, with the first breath, there is a drop in pulmonary vascular resistance and consequently in right ventricular pressure. Clamping of the umbilical cord leads to a sudden rise in systemic vascular resistance. Left ventricular pressure is now elevated above that of the right ventricle.

Closure of Fetal Vascular Channels

1. Foramen ovale: The increase in pulmonary venous return leads to a rise in left atrial pressure. This compresses the valve of the foramen ovale and produces functional closure of the interatrial septum. Anatomic closure takes place over a period of months or years

2. Ductus arteriosus: The ductus arteriosus closes functionally over the first 24 to 72 hours of life. This process is related to the rise in arterial oxygen saturation and is mediated by prostaglandins

IMMEDIATE CARE OF THE INFANT IN THE DELIVERY ROOM

Of all newborn infants, 10 percent will need some assistance to initiate and establish breathing, and around 1 percent ...