Effects of volume and frequency of mechanical ventilation on respiratory activity in humans

Abstract

This study evaluated the interaction between respiratory chemical drive and non-chemical factors related to the frequency and level of thoracic displacement during mechanical ventilation in shaping respiratory activity. Ten normal subjects were artificially hyperventilated with a positive-pressure mechanical respirator to a baseline end-tidal P CO 2 of approximately 30 Torr. Thereafter, in separate trials, the end-tidal P CO 2 was increased by (a) progressively raising the concentration of CO 2 in the inspired gas (F I CO 2 ) while holding tidal volume (V T) and breathing frequency (f) constant, (b) lowering f while holding V T and F I CO 2 constant, and (c) lowering V T while maintaining a constant f and F I CO 2 . Initially, as the P CO 2 rose above baseline levels with increases in F I CO 2 , there was no change in inspiratory muscle activity, as measured by the peak inspiratory airway pressure, until the P CO 2 reached 40 Torr. This P CO 2 threshold for a change in respiratory activity was significantly reduced when the tidal volume or frequency of mechanical ventilation was lowered. These results that non-chemical drives related to the frequency and level of thoracic displacement interact with chemical stimuli in shaping respiratory activity.