Isoproterenol and Aminophylline Improve Contractility of Fatigued Canine Diaphragm1–3

We investigated the effects of aminophylline and isoproterenol on diaphragmatic fatigue produced by phrenic stimulation in dogs. With a cast around the abdomen, the diaphragm contracted quasi-isometrically while the thorax was open and the animal was ventilated. We assessed contractility by measuring transdiaphragmatic pressure during supramaximal stimulation of the phrenic nerves at different frequencies and also during spontaneous inspiratory efforts. At doses of 20,40, and 80 mg/kg, aminophylline significantly improved contractility (p < 0.01) in a dose-dependent manner during low, but not during high frequency stimulation. The maximal improvement (24%) was observed with 80 mg/kg. With intravenously administered doses of 5 and 10 µg/min of isoproterenol, contractility was also significantly enhanced (p < 0.05) during low frequency stimulation. Maximal improvement (12%) occurred with 5 µg/min. Similar results were obtained when spontaneous inspiratory efforts were recorded and transdiaphragmatic pressur...