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J Appl Physiol 74: 2750-2756, 1993;
8750-7587/93 $5.00
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Journal of Applied Physiology, Vol 74, Issue 6 2750-2756, Copyright © 1993 by American Physiological Society

ARTICLES

Oxygen cost of increasing tidal volume and diaphragm flattening in obstructive pulmonary disease

W. D. Pitcher and H. S. Cunningham
Pulmonary and Critical Care Medicine Division, University of Texas, Southwestern Medical Center, Dallas 75216.

Hypercapnia is associated with a shallow breathing pattern in patients with severe chronic obstructive pulmonary disease (COPD). We sought to determine the oxygen cost of increasing tidal volume and to relate this to hypercapnia [arterial PCO2 (PaCO2) > or = 45 Torr] and diaphragm flattening. We studied 3 normal subjects and 12 patients with stable but comparably severe COPD (forced expired volume in 1 s 1.01 +/- 0.09 liters) who had baseline PaCO2 ranging from 36 to 56 Torr. Oxygen consumption was measured during the subject's native breathing pattern and then while tidal volume was increased by 20%; minute ventilation was held constant by proportionately slowing frequency. There was a significant oxygen cost of increasing tidal volume for hypercapnic patients (235 +/- 23 to 260 +/- 25 ml O2/min; P = 0.002); no significant oxygen cost was observed in normal or eucapnic patients. This oxygen cost was positively correlated to baseline PaCO2 (r2 = 0.88, P < 0.001) and degree of diaphragm flattening assessed from chest radiographs (r2 = 0.74, P < 0.05). Although others have shown that force generation is preserved during chronic hyperinflation (G. A. Farkas and C. Roussos. J. Appl. Physiol. 54: 1635-1640, 1983; T. Similowski et al. N. Engl. J. Med. 325: 917-923, 1991), we conclude that diaphragm flattening produces mechanical inefficiency that may contribute to limiting the effective operating range of the respiratory muscles during tidal breathing.


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