Variability of arterial PO2 in hospitalized patients: implications for clinical practice and research

The effects of position and ventilation on variability of PO2 in hospitalized patients were studied using a cross-over design. Arterial blood gases were obtained for 51 patients while they were lying and while they were sitting, and after five deep breaths (lying). The PO2 generally increased after five deep breaths but was not systematically changed by sitting up. Nineteen patients (37%) had changes in PO2 greater than or equal to 10 mm Hg. The patient's mean PO2 and respiratory rate influenced these differences. Among patients with a mean PO2 less than 85 mm Hg whose respiratory rates were greater than 20/min, only 11% had a change in PO2 greater than or equal to 10 mm Hg. In contrast, among patients with a mean PO2 greater than 85 mm Hg and a respiratory rate less than 20/min, 60% had a change in PO2 greater than or equal to 10 mm Hg. Considerable variability in PO2 is introduced by commonly encountered changes in position and ventilation. These conditions must be standardized when serial blood gas results are used to assess changes in a patient's condition.