Hypoxemia and hypocarbia following intermittent positive-pressure breathing

The authors determined the effects of short-term, vigorous intermittent positive-pressure breathing (IPPB) on arterial blood O2 tension (Pao2) in 10 unmedicated preoperative adult patients (ASA class I). Arterial blood was analyzed before and 0.5, 5, 15, and 40 minutes after 12 minutes of IPPB with nebulized 0.9% HaCl and room air. Pao2 increased from 89.1 +/- 11 torr (mean +/- SD) to 102.1 +/- 13.9 torr, and arterial CO2 tension decreased from 29.5 +/- 4 torr to 18.4 +/- 3 torr, causing significant respiratory alkalosis (pHa 7.62 +/- 0.05). Five minutes after IPPB, Pao2 decreased to 69.2 +/- 8 torr; alveolar-arterial Po2 difference, breathing 21% O2, increased significantly; and serum K+ concentration decreased significantly. All these values returned to baseline levels within 40 minutes. The authors, acknowledging the value of IPPB when specifically indicated, counsel caution in patients with borderline pulmonary reserve and arterial oxygenation.