Slow to fast shift in inspiratory muscle fibers during heat tachypnea

The firing patterns of single diaphragm fibers and electromyographic moving average (M.A.) of diaphragm and parasternal intercostal muscles (P.I.) were studied in rabbits during control and heat tachypnea at nearly constant tidal volume (VT). During tachypnea the percentage of single diaphragm fibers with relatively high values of peak firing rate (fp) increased. Conversely, fibers with relatively low values of fp at control (likely slow) showed little or no increase of this parameter during tachypnea. These findings, similar to those previously found in P.I., suggest a slow to fast shift in diaphragm fibers during tachypnea. Inspiratory flow at half VT control increased significantly during tachypnea, while the corresponding M.A. of diaphragm did not change. Moreover, during tachypnea mean inspiratory flow increased significantly, while the mean M.A. over inspiratory time did not change either in diaphragm or in P.I. Since during tachypnea the firing rate of most fibers is greater than at control, a lack of increase of M.A. in spite of an increased mechanical power confirms a shift from slow to fast inspiratory muscle fibers under this condition.