Aminophylline and fatigue of the sternomastoid muscle

We examined the influence of intravenously infused aminophylline on the contractility of the sternomastoid muscle in 8 subjects. The muscle was stimulated directly using bipolar surface electrodes at 1, 10, 20, and 50 pulses per second (pps), and the force response was recorded. The ratio of force produced at 20 pps and 50 pps (20/50 ratio) was calculated. The effect of aminophylline on the 20/50 ratio was assessed at 2 dosages producing different blood serum levels (10.7 mg/L and 15.3 mg/L). At the lower dosage of aminophylline the potential prophylactic and curative influence of aminophylline was tested using 3 different experimental paradigms performed on separate days: (1) Baseline force frequency responses were determined before and 15 and 35 min after a headlifting maneuver; headlifting resulted in a decrease in the 20/50 ratio (low frequency fatigue). (2) The same sequence as in (1), with the addition of a continuous infusion of aminophylline started after the baseline force frequency responses. (3) The same sequence as in (1) except that the aminophylline infusion was begun only after the headlifting maneuver. After the fatiguing maneuvers, there were comparable and significant falls in the 20/50 ratio on all experimental days. The infusion of aminophylline did not significantly affect the baseline 20/50 ratio. In addition, aminophylline did not prevent the fatigue-related decrease in the 20/50 ratio or enhance its recovery. Experimental paradigms 1 and 2 were repeated at the higher aminophylline levels. Again, aminophylline did not prevent or reverse sternomastoid fatigue. We conclude that aminophylline in therapeutic dosages does not prevent or reverse low frequency fatigue of the sternomastoid muscle.