Voluntary versus reflex regulation of maximal exercise flow: volume loops.

We determined the efficiency with which maximal exercise ventilatory output could be mimicked voluntarily. Five normal subjects exercised to maximal volitional effort and flow:volume and pressure:volume loops, and end-expiratory lung volume (EELV) and breathing pattern were measured. All subjects increased expiratory flow rate and reduced EELV sufficiently so that the forced vital capacity loop was approximated during at least some portion of expiration, but the generation of pleural pressure remained effective, i.e., equal to or only slightly in excess of that required to produce maximal expiratory flow (Pmax). Subsequently, while at rest, subjects used visual feedback and were able to closely mimic the flow:volume, EELV, and breath-timing achieved in maximal exercise; however: (1) expiratory pressures were excessive and usually exceeded average Pmax; (2) abdominal expiratory muscle activity was increased, as indicated by positive shifts in expiratory gastric pressure; and (3) total ventilatory work was increased 15 to 40% greater than that achieved in maximal exercise. Maximal voluntary efforts (MVV) caused EELV to increase and ventilatory work was increased 20 to 300% greater than during maximal exercise. We conclude that accurate determination of maximal effective ventilatory output available for maximal exercise or precise quantitation of the metabolic cost of producing maximal exercise ventilation requires replication of the pressure:volume, breath-timing, and EELV characteristics achieved in maximal exercise.