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We have devised a method for measuring ventilatory muscle endurance as the sustainable inspiratory pressure (SIP), which is the highest pressure a subject can generate in each breath for 10 min. We used a weighted plunger as an inspiratory valve. This both ensures that a constant pressure is generated with each breath and allows the subject to vary his tidal volume freely. Fifteen normal subjects, ages 5--75 yr, had SIP of 82 +/- 6 (SE) cmH2O or 68 +/- 3% of their maximum inspiratory pressure. The respiratory rate was 13 +/- 1 breaths/min with 52 +/- 4% of the respiratory cycle spent in inspiration; end-tidal CO2 pressure increased by 3.3 +/- 1.0 Torr during runs at SIP. Oxygen consumption measured in two subjects rose with increasing pressure below SIP. There was no further increase in oxygen consumption when these subjects breathed with inspiratory pressures above SIP. Our method allows reproducible measurement of ventilatory muscle endurance without dependence on the subject's flow rates or the resistance chosen.
Nickerson et al. (Mon,) studied this question.