NEWBORN HYPERCAPNIA HYPOVENTILATION

Hypercapnia usually is caused by hypoventilation or severe ventilation/perfusion (V/Q) mismatch. Carbon dioxide normally diffuses readily from the blood into the alveoli. Elimination of carbon dioxide from the alveoli is directly proportional to alveolar minute ventilation, which is determined by the product of tidal volume (minus dead space ventilation) and frequency.

Thus, the alveolar minute ventilation is calculated as follows:
Alveolar Minute Ventilation = (Tidal Volume - Dead Space) X Frequency

Tidal volume is the volume of gas inhaled (or exhaled) with each breath. Frequency is the number of breaths per minute. Dead space is the part of the tidal volume not involved in gas exchange, such as the volume of the conducting airways. Dead space is relatively constant.

Thus, increases in either tidal volume or frequency increase alveolar ventilation and decrease partial pressure of carbon dioxide, arterial (PaCO2). Because dead-space ventilation is constant, changes in tidal volume appear more effective at altering carbon dioxide elimination than changes in frequency.

For example, a 50% increase in tidal volume (ie, 6-9 cc/kg) with a constant dead space (ie, 3 cc/kg) doubles alveolar ventilation (3-6 cc/kg X frequency). In contrast, a 50% increase in frequency increases alveolar ventilation by 50%, because an increase in dead space ventilation (dead space X frequency) occurs when frequency is increased.

Even though increases in minute ventilation achieved with large tidal volume increase alveolar ventilation more, the use of relatively small tidal volume and high frequencies usually are preferred as volutrauma can be minimized.