Abstract

The effect of theophylline on diaphragmatic blood flow (di) and oxygen consumption (o2di) was studied in eight lightly anesthetized dogs during quiet breathing and inspiratory resistive loading. di was determined with the radioactive microsphere tracer technique, and o2di was calculated as the product of di and the diaphragmatic arterio-venous oxygen difference. During quiet breathing, theophylline increased minute ventilation (9.3 ± 1.7 versus 5.1 ± 0.4 L/min), mean inspiratory 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 di or o2di. Conversely, di increased significantly during loaded breathing compared with quiet breathing (37 ± 4 versus 27 ± 3 ml/100 g/min) and was further increased by theophylline (45 ± 7 ml/100 g/min). 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 significant increase in o2di (2.66 ± 0.53 versus 1.78 ± 0.26 ml/100 g/min). Similarly, total-body oxygen consumption (o2TB) during inspiratory loading increased significantly after theophylline (24%), but the tension-time index of the inspiratory muscles (TTi), a measure of the total respiratory load, was unchanged. We conclude that theophylline significantly increases o2di and o2TB at the same TTdi and TTi during resistive loading. This enhanced energy expenditure needs consideration in the clinical management of pulmonary disorders that increase the work of breathing.

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