EFFECT OF MAXIMUM VENTILATION ON ABDOMINAL MUSCLE-RELAXATION RATE

Background - When the demand placed on the respiratory system is incre ased, the abdominal muscles become vigorously active to achieve expira tion and facilitate subsequent inspiration, Abdominal muscle function could limit ventilatory capacity and a method to detect abdominal musc le fatigue would be of value. The maximum relaxation rate (MRR) of ske letal muscle has been used as an early index of the onset of the fatig uing process and precedes failure of force generation. The aim of this study was to measure MRR of abdominal muscles and to investigate whet her it slows after maximum isocapnic ventilation (MIV). Methods - Five normal subjects were studied. Each performed short sharp expiratory e fforts against a 3 mm orifice before and immediately after a two minut e MIV. Gastric pressure (PGA) was recorded and MRR (% pressure fall/10 ms) for each PGA trace was determined. Results - Before MIV the mean (SD) maximum PGA MRR for the five subjects was 7.1 (0.8)% peak pressur e fall/10 ms. Following MIV mean PGA MRR was decreased by 30% (range 2 5-35%), returning to control values within 5-10 minutes. Conclusions - The MRR of the abdominal muscles, measured from PGA, is numerically s imilar to that described for the diaphragm and other skeletal muscles. After two minutes of maximal isocapnic ventilation abdominal muscle M RR slows, indicating that these muscles are sufficiently heavily loade d to initiate the fatiguing process.