Effect of aminophylline on respiratory muscle strength after upper abdominal surgery: a double blind study.

BACKGROUND The effect of aminophylline on maximum respiratory muscle strength in patients undergoing upper abdominal surgery was investigated. METHODS An open pilot study was performed in which aminophylline was administered continuously for 48 hours after surgery (protocol I). In a second group of subjects aminophylline was given for 24 hours after cholecystectomy in a double blind placebo controlled trial (protocol II). Twelve patients participated in the pilot study (group A) and 25 in protocol II of which 14 received aminophylline (group B) and 11 placebo (control, group C). Respiratory muscle strength was assessed by measuring mouth pressures during maximum static inspiratory and expiratory efforts. Forced expiratory volume in one second (FEV1), forced vital capacity (FVC), vital capacity (VC), inspiratory maximum pressures (PImax), expiratory maximum pressures (PEmax) were measured 24 hours preoperatively, PImax and serum theophylline 24 hours postoperatively, and FEV1, FVC, VC, PImax, PEmax, and serum theophylline 48 hours after surgery. RESULTS FEV1, FVC, and VC decreased in all groups of patients at +48 hours. PImax fell at +24 hours and +48 hours but this decrease was significantly smaller in the two groups who received aminophylline than in the control group. PEmax showed a decrease at +48 hours but this reduction was similar in all three groups studied, independent of the treatment given. These data suggest that either aminophylline had a protective effect only on the inspiratory muscles or, most probably, that the effect of aminophylline was central, reducing the phrenic nerve inhibition induced by cholecystectomy and thus improving diaphragmatic function. CONCLUSIONS Upper abdominal surgery decreases inspiratory and expiratory muscle strength and aminophylline has a protective effect only on inspiratory muscle function. This may have important clinical applications in minimising pulmonary complications after cholecystectomy.