Cardiovascular effects of the breathhold used in determining pulmonary diffusing capacity.

BACKGROUND: The single-breath technique for determination of the diffusion capacity of the lung for CO (DlCO) requires a 10-s breathhold at total lung capacity. The assumption has been that this breathhold does not alter the components of DlCO, i.e., the diffusion capacity of the membrane (Dm) and the pulmonary capillary blood volume (Qc), and therefore measurement of these variables during breathhold represents these variables as they exist during normal breathing. The purpose of this study was to determine the effect of the 10-s breathhold on cardiac output (Q) and Qc while supine and standing. The hypothesis was that the standing posture would have a greater influence on Q and Qc during the breathhold than would the supine posture. METHODS: Twelve male subjects participated. Q, stroke volume (SV), heart rate (HR), BP (MAP), and total peripheral resistance (TPR) were determined before and during the 10-s breathhold determination of DlCO, Qc, and Dm. RESULTS: Results while supine were compared with those while standing. DlCO was reduced on standing, due mainly to a reduction in Qc. SV and Q decreased significantly during the 10-s breathhold in both postures. Both SV and Q decreased more when standing (-53% and -49.5%, respectively) than when supine (-40.5% and -36.5%, respectively). Thus, the 10-s breathhold caused significant reductions in Q and therefore may alter the measurement of DlCO and Qc. CONCLUSIONS: The greater decline in Q during the measurement of DlCO when standing would suggest that the DlCO and Qc values while breathing might be underestimated in the upright posture.