THEOPHYLLINE INCREASES OXYGEN-CONSUMPTION DURING INSPIRATORY RESISTIVE LOADING

The effect of theophylline on diaphragmatic blood flow (Qdi) and oxyge n consumption (VO(2)di) was studied in eight lightly anesthetized dogs during quiet breathing and inspiratory resistive loading. Qdi was det ermined with the radioactive microsphere tracer technique, and VO(2)di was calculated as the product of Qdi and the diaphragmatic arterio-ve nous oxygen difference. During quiet breathing, theophylline increased minute ventilation (9.3 +/- 1.7 versus 5.1 +/- 0.4 L/min), mean inspi ratory flow (547 +/- 60 versus 378 +/- 56 ml/s), and duty cycle (0.270 +/- 0.042 versus 0.192 +/- 0.024) but did not significantly alter Qdi or VO(2)di. Conversely, Qdi increased significantly during loaded bre athing compared with quiet breathing (37 +/- 4 versus 27 +/- 3 m1/100 g/min) and was further increased by theophylline (45 +/- 7 m1/100 g/mi n). Theophylline did not alter the tension-time index of the diaphragm (TTdi) during inspiratory resistive loading (0.054 +/- 0.006 versus 0 .056 +/- 0.004, p NS) but resulted in a disproportionate and significa nt increase in VO(2)di(2.66 +/- 0.53 versus 1.78 +/- 0.26 ml/100 g/min ). Similarly, total-body oxygen consumption (VO2TB) during inspiratory loading increased significantly after theophylline (24%), but the ten sion-time index of the inspiratory muscles (TTi), a measure of the tot al respiratory load, was unchanged. We conclude that theophylline sign ificantly increases VO(2)di and VO2TB at the same TTdi and TTi during resistive loading. This enhanced energy expenditure needs consideratio n in the clinical management of pulmonary disorders that increase the work breathing.