Physiological response to "pressure-demand" respirator wear.

This investigation determined cardiorespiratory responses of subjects with normal lung function and exercise tolerance and compared them with subjects with moderate impairment of lung function and exercise tolerance. The respirator was an air-line full-face mask (MSA-Ultravue) "pressure-demand" breathing type equipped with an inspiratory resistance of 85 mmH2O at 85 L/min air flow. This resistance was operable in conjunction with the fixed 25 mmH2O inspiratory and expiratory resistance required to pressurize the face piece. Physiologically and subjectively the response of the normal and moderately impaired subjects to respirator wear during rest, 35%, 50% and 80% of their maximal aerobic capacity (VO2max) were not different. However, the pressure swings inside the face piece exceeded 24 cmH2O and resulted in 50% of the subjects being unable to finish 10 minutes of work at 80% VO2max. The greater the ventilatory demand placed upon the respirator due to increasing workload, the more like a "demand" system pressure-flow response the "pressure-demand" system produced. Hence, the concept of increased protection and reduced inspiratory resistance as a result of pressurizing the facepiece during heavy work is seriously questioned.