Chest
Cardiac Output during Exercise in Patients with COPD
Section snippets
PATIENTS
Twenty patients with COPD were investigated. They had been referred to the clinical laboratory for characterization of their effort intolerance. All patients had evidence of expiratory airflow limitation in that two or more expiratory flow variables were more than 1.65 standard deviations below the value predicted for age, sex, race, and anthropometric habitus.8, 9 Patients with evidence of pulmonary restriction, as gauged by reduced total lung capacity, and/or high recoil (measured from the
METHODS
Lung mechanics were measured from maximal expiratory flow-volume loops using a wedge spirometer. For residual volume measurement, for the estimation of total lung capacity, the single breath nitrogen washout method was used as modified by Bründler and Lewis11 to compensate for underestimation of lung volume due to poor gas mixing. The single-breath CO diffusion capacity (Dco) was measured in 15 subjects using a Morgan Transfer-test Model C linked to a microprocessor. The Dco value was
RESULTS
The mean age of the patients in this study was 54.7±9.3 years. The mean FEV1 was 1.68±0.531, FEV1/FVC 54.8±14.6 percent and the forced expiratory flow at 50 percent of FVC (FEF50) 31.9± 18.4 percent of predicted.8 The FRC/TLC ratio was 111.5 ±29.3 percent of predicted while TLC was 103 ±16.6 percent of predicted.8, 9 The slope of phase III of the single breath nitrogen washout curve varied from 0.30 to 21.95 percent/L (mean value, 6.52±5.66 percent/L). The normalized CO transfer factor (TL/VA)
DISCUSSION
In this study, the patients could be clearly divided into a group whose exercise cardiac output was low and a group in whom it was normal. In the low cardiac output group, there was more severe lung hyperinflation and expiratory airflow limitation at rest, but on exercise, both groups utilized a similar proportion of their ventilatory capacity. Although the mean pulmonary artery pressures were similar both at rest and on exercise, the pulmonary vascular resistance values were very much higher
CONCLUSIONS
Despite the linear relationship between arterial oxygenation (SaO2 and pulmonary artery pressure both at rest and on exercise in patients with COPD, the findings of this study suggest that lung hyperinflation and severe expiratory airflow limitation may be important determinants of the severely elevated pulmonary vascular resistance and the low cardiac output observed in eight of the 20 subjects included in this study.
ACKNOWLEDGMENTS
The authors thank the technologists of the Pulmonary Function Laboratory, Tygerberg Hospital, for their assistance.
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Manuscript received May 20; revision accepted July 26.