Regulation of ventilatory capacity during exercise in
asthmatics.
Johnson, Bruce D., Paul D. Scanlon, and Kenneth C. Beck.
Divisions of Pulmonary and Critical Care Medicine and
Cardiovascular Diseases, of the Department of Internal Medicine, Mayo
Clinic and Foundation, Rochester MN 55905
APStracts 2:0213A, 1995.
In asthmatic and non-asthmatic control subjects, we examined the
changes in ventilatory capacity (V.ECAP), end expiratory lung volume
(EELV) and degree of flow limitation during three types of exercise:
1) incremental, 2) constant load (50% of maximal exercise capacity,
36 min) and 3) interval (alternating between 60% & 40% of maximal
exercise capacity, 6 min work loads for 36 min). The V.ECAP at rest
and during the various stages of exercise was estimated by aligning
the tidal breathing flow-volume (f-v) loops within the maximal
expiratory f-v (MEFV) envelope according to measured EELV's. Using
the maximal expiratory airflows (MEF's) in the range of the tidal
breath an estimated maximal breathing frequency (fb,max) was obtained
and the V.ECAP was determined as a product of fb,max and the tidal
volume (VT). The V.ECAP therefore depended on the existing
bronchomotor tone, the lung volume at which subjects breathed (i.e.,
EELV) and the VT. Prior to exercise, asthmatics had reduced forced
expiratory flows at 50% and 75% of the vital capacity (76% and 70%
predicted, respectively) and thus reduced baseline measurements of
V.ECAP relative to control subjects (109 vs 156 L/min). With unloaded
cycling, V.ECAP decreased in both groups due to a decline in EELV
below pre-exercise values. With progressive increases in exercise
intensity to maximum, asthmatic and control subjects increased V.ECAP
(13% in controls vs 44% in asthmatics) due largely to increases in VT
(allowing subjects to take advantage of greater MEF at the higher
lung volumes); however, in asthmatics the increase was also due to
increases in MEF's (_28%) and EELV (_400 ml). At peak exercise, V.E
reached 68% of V.ECAP in normals and 77% of V.ECAP in asthmatics.
During exercise of variable intensity, asthmatics, but not controls,
demonstrated declines in V.ECAP (_21%) that were proportional to the
declines in ventilatory demand (_24%) due to decreases in MEF's of
_30% each time exercise intensity was reduced. The declines in MEF's
and V.ECAP were reversible upon returning to higher work rates. V.E
remained 70% to 80% of V.ECAP at both 60% and 40% of maximal exercise
capacity in asthmatics. Despite the attempt by asthmatic subjects to
optimize V.ECAP relative to ventilatory demand, throughout most
exercise conditions, asthmatics experienced greater flow limitation
and decreased ventilatory reserve relative to controls.
Received 9 May 1994; accepted in final form 27 April 1995.
APS Manuscript Number A436-4.
Article publication pending Journal of Applied Physiology.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 30 May 1995.