Am. J. Respir. Crit. Care Med., Vol 151, No. 4, Apr 1995, 1000-1005.
Theophylline increases oxygen consumption during inspiratory resistive loading
S Janssens, E Derom, J Vanhaecke and M Decramer
Respiratory Muscle Research Unit, University of Leuven, Belgium.
The effect of theophylline on diaphragmatic blood flow (Qdi) and oxygen
consumption (VO2di) was studied in eight lightly anesthetized dogs during
quiet breathing and inspiratory resistive loading. Qdi was determined with
the radioactive microsphere tracer technique, and VO2di was calculated as
the product of Qdi 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 Qdi or VO2di. Conversely, Qdi 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 VO2di (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
tension-time index of the inspiratory muscles (TTi), a measure of the total
respiratory load, was unchanged. We conclude that theophylline
significantly increases VO2di and VO2TB 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.
