Introduction A PRIMARY reduction in the physically dissolved fraction of carbon dioxide in blood defines the state of primary respiratory alkalosis (primary hypocapnia). The quantity of physically dissolved CO2 ([H2CO3]) in blood depends on the partial pressure of CO2 (Pco2).1 Arterial blood Pco2, is in equilibrium with pulmonary alveolar air Pco2, which in turn is directly proportional to alveolar air CO2 concentration (F CO2). Increase in alveolar ventilation, which defines the state of hyperventilation, results in reduction of alveolar F CO2 and therefore reduction in alveolar and arterial Pco2. A schematic demonstration of F CO2-Pco2 relationship is seen in Fig 1. Thus, primary hyperventilation results in a reduction in alveolar Pco2 and consequential reduction in arterial Pco2-the state of respiratory alkalosis. Hyperventilation. —The term "hyperventilation" should not be used to describe an accelerated rate of respiration (tachypnea), since such does not necessarily result in an increase in alveolar ventilation. Furthermore,