Expiratory effort enhancement and peak expiratory flow in humans.

Peak expiratory flow (PEF) has previously been considered an effort-dependent, non flow-limited parameter that is constrained by the force-velocity relationship of the respiratory muscles. It has also been assumed that, if the muscles were able to augment the expiratory pressure, the PEF would increase. We tested the validity of this notion in normal volunteers who were able to enhance their expiratory pressure with maneuvers utilizing the stretch-shortening cycle (greater force when contractions were immediately preceded by eccentric contractions). Five healthy volunteers [35 (2) years] performed two successive maximal expiratory flow-volume maneuvers (MEFV) in rapid sequence. MEFV1 was a standard maneuver, whereas MEFV2 included a forceful inspiration to total lung capacity; a strategy designed to augment expiratory pressure via the stretch-shortening cycle. Neither maneuver included a post-inspiratory pause. We measured PEF, esophageal pressure (P(es)), and the electromyographic activity of the abdominal muscles. Compared to MEFV1, MEFV2 produced greater activation of the abdominal muscles during inspiration (eccentric contraction), greater peak expiratory P(es), greater rate of rise of P(es), shorter time to PEF, but similar PEF. Our findings directly demonstrate the inability of the augmented expiratory effort to increase PEF and thus support the notion that PEF is determined by a flow-limiting mechanism and not by the velocity of muscle shortening.