EFFECTS OF AMINOPHYLLINE ON RESPIRATORY MUSCLE INTERACTION

We studied the effects of aminophylline on respiratory muslce interaction during quiet breathing by measuring (1) changes in rib cage and abdominal cross-sectional area, (2) tidal volume, (3) abdominal and esophageal pressure, (4) diaphragm and parasternal intercostal electromyogram (EMG) and parasternal intercostal intramuscular pressure, and (5) triangularis sterni and transversus abdominis EMG, in 14 supine anesthetized dogs. Measurements were done before and 5 and 10 min after administration of progressively increasing doses of aminophylline, reaching a total dose of 5, 10, 20, 40, and 80 mg/kg. In 12 of 14 dogs after aminophylline administration, quiet inspiration became biphasic, or if inspiration was already biphasic under control conditions, the first phase was clearly enhanced after aminophylline administration. Biphasic inspiration was defined as an inspiratory pattern in which rib cage and abdominal expansion showed two distinct phases. First, the onset of inspiration was characterized by a sudden increase in rib cage cross-sectional area often associated with a decrease in abdominal dimensions. During this initial part the parasternals and the diaphragm were electrically silent, confirming that it was achieved by relaxation of expiratory muscles. Further inspiration occurred predominantly through expansion of the abdomen. In the first phase, a pressure drop in the parasternal intercostals was present, presumably due to passive shortening of these muscles caused by expiratory muscle relaxation. On the average, 42 +/- 28% of the rib cage expansion was due to expiratory muscle relaxation at a dose of 80 mg/kg versus 17 +/- 21% under control conditions (p < 0.05). Expiratory muscle recruitment was further confirmed by an increase in triangularis sterni EMG (319 +/- 300% control at a dose of 80 mg/kg, p < 0.01), while transversus abdominis EMG tended to increase (1,533 3,380% control at a dose of 80 mg/kg; NS) and by significant increases in baseline abdominal and pleural pressure.