title: Oxygen cost of increasing tidal volume and diaphragm flattening in obstructive pulmonary disease.
abstract: 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.
authors.affiliation: Pulmonary and Critical Care Medicine Division, University of Texas, Southwestern Medical Center, Dallas 75216.
authors.forename: W D
authors.initials: WD
authors.lastname: Pitcher
authors.forename: H S
authors.initials: HS
authors.lastname: Cunningham
mesh_terms: D000368:Aged; D003964:Diaphragm / Q000503:physiopathology*; D006801:Humans; D006935:Hypercapnia / Q000503:physiopathology; D008173:Lung Diseases, Obstructive / Q000503:physiopathology*; D008297:Male; D008875:Middle Aged; D010101:Oxygen Consumption / Q000502:physiology*; D015656:Respiratory Mechanics / Q000502:physiology; D013990:Tidal Volume / Q000502:physiology*