Which statement best describes the relationship between arterial pH and PaCO2 in respiratory alkalosis?

• A. pH is decreased and PaCO2 is decreased
• B. pH is decreased and PaCO2 is increased
• C. pH is increased and PaCO2 is decreased
• D. pH is increased and PaCO2 is increased

Answer: (C)

In respiratory alkalosis the cause is increased ventilation that blows off carbon dioxide (CO2) faster than it’s produced. CO2 is a volatile acid, so removing more of it reduces hydrogen ion concentration and raises the pH. So the arterial pH is elevated, while the PaCO2 is reduced. The two values move in opposite directions in this condition. If PaCO2 were high, pH would be low (acidosis), and if pH were low, that would indicate acidosis as well. Over time the body may compensate (kidneys excrete bicarbonate), but the immediate relationship remains higher pH with lower PaCO2.