A healthy, 37-year-old woman loses her job at the auto factory. She picks up her three young children from school and met with a road traffic accident. Her 5-year-old son, sustains a severe head injury. The woman was not hu in the accident, but is hyperventilating as she sits in the waiting room at the hospital. She complains of feeling faint and has blurred vision. Which of the following is decreased in this woman?

The key symptom here is hyperventilation. Hyperventilation results in hypocapnia, alkalosis, increased cerebrovascular resistance, and decreased cerebral blood flow. Carbon dioxide plays an impoant role in the control of cerebral blood flow. An increase in aerial PCO2 dilates blood vessels in the brain and a decrease in PCO2 causes vasoconstriction. The anxious, hyperventilating woman is "blowing off" carbon dioxide, which lowers her aerial PCO2. This decrease in PCO2 has caused the cerebrovascular resistance to increase, thereby decreasing cerebral blood flow. The decrease in cerebral blood flow has caused the woman to feel faint and to have blurred vision. Other symptoms commonly associated with the hyperventilation of anxiety states are feelings of tightness in the chest and a sense of suffocation. Hyperventilation increases the aerial oxygen content and PO2 in a normal person. A decrease in aerial PCO2 causes the aerial pH to increase, i.e. the patient becomes alkalotic.