A 55-year-old woman is admitted to the hospital in obvious respiratory distress.Her increased rate and depth of breathing have raised her minute ventilation from 10 to 15 L/min.Arterial blood gases show a normal P_aCO₂ of 40 mm Hg.It seems odd that this woman, with a minute ventilation this great, has a normal P_aCO₂.What is the explanation for this?

Question

Quizplus · Accepted Answer

A normal P_aCO₂ associated with high minute ventilation indicates that much of this patient's ventilation is not in contact with blood flow. "Dead space" is the term used to describe alveoli that have normal ventilation but no blood flow (perfusion) through their capillaries. Any factor that decreases perfusion increases alveolar dead space. Increased alveolar dead space decreases alveolar ventilation if minute ventilation stays the same. Pulmonary embolism (obstruction of pulmonary vessels by blood clots) and shock (decreased cardiac output and low perfusion) are conditions that cause increased alveolar dead space. The VD/VT ratio increases as a larger percentage of the VT becomes dead space. In conditions producing dead space, the body tries to maintain a normal P_aCO₂ by increasing the VE. This increased minute ventilation does not necessarily mean alveolar ventilation increases because much of the minute ventilation is directed to dead space units. Thus a physiological consequence of increased dead space is the increased work of breathing required to maintain a normal P_aCO₂.

A 55-Year-Old Woman Is Admitted to the Hospital in Obvious