Minute Ventilation in Asthma

To evaluate independently the effects of an inspiratory stimulus (a mouthpiece) and a ventilatory depressant (oxygen) on minute ventilation (E) in asthma, we measured E with magnetometers in 18 asthmatic subjects. When room air was inspired from a loose-fitting mask, E was 10.2 ± 0.6 L/min. When oxygen was substituted for room air, E decreased (8.4 ± 0.6). In contrast, when oxygen was given by mouthpiece, E increased (12.9 ± 0.7). We conclude that in the clinical setting, the mouthpiece artifact may operate either to exaggerate the tidal breath or to obscure the effect of a respiratory depressant (e.g., oxygen) upon the measurement of resting ventilation.