Inspiratory muscle forces and endurance in maximum resistive loading

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Abstract
The ability of the respiratory muscles to sustain ventilation against increasing inspiratory resistive loads was measured in 10 normal subjects. All subjects reached a maximum rating of perceived respiratory effort, and, at maximum resistance, showed signs of respiratory failure (CO2 retention, O2 desaturation and rib cage and abdominal paradox). The maximum resistance achieved varied widely (range 73-660 cmH2O .cntdot. 1-1 .cntdot. s). The increase in O2 uptake (.DELTA..ovrhdot.VO2) associated with loading was linearly related to the integrated mouth pressure (IMP): .DELTA..ovrhdot.VO2 = 0.028 .times. IMP + 19 ml/min (r=0.88, P < 0.001). Maximum .DELTA..ovrhdot.VO2 was 142 ml/min .+-. SD 68 ml/min. There were significant (P < 0.05) relationships between the maximum voluntary inspiratory pressure against an occluded airway (MIP) and both maximum IMP (r = 0.80) and maximum .DELTA..ovrhdot.VO2 (r = 0.76). In 5 subjects, 3 imposed breathing patterns were used to examine the effect of different patterns of respiratory muscle force deployment. Increasing inspiratory duration (TI) from 1.5-3.0 and 6.0 s, at the same frequency of breathing (5.5 breaths/min), reduced peak inspiratory pressure and increased the maximum resistance tolerated (190, 269 and 366 cmH2O .cntdot. 1-1 .cntdot. s, respectively) and maximum IMP (2043, 2473 and 2913 cmH2O .cntdot. s .cntdot. min-1), but the effect on maximum .DELTA..ovrhdot.VO2 was less consistent (166, 237 and 180 ml/min). The ventilatory endurance capacity and the maximum O2 uptake of the respiratory muscles are related to the strength of the inspiratory muscles, but are also modified through the pattern of force deployment.