Blood gas measurements.

pH and blood gas measurements are used to detect and monitor ventilation, oxygenation, and acid-base disturbances. The blood sample must be drawn anaerobically and transported in ice water to the laboratory. Partial pressure of carbon dioxide in arterial blood (PaCO2) reflects alveolar ventilation; partial pressure of oxygen in arterial blood (PaO2) reflects oxygen loading. The alveolar-arterial PO2 gradient (PA-aO2) distinguishes hypoxemia due to hypoventilation from that due to inefficent pulmonary gas exchange. The demand status of the cardiovascular system and the hemoglobin value reflect oxygen delivery to tissues. The relationship among pH, PaCO2, and bicarbonate concentration, when interpreted in the light of clinical findings, specifies the type and duration of acid-base disturbance.