Factors affecting the pulmonary capillary blood flow pulse in man.

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Abstract
Instantaneous pulmonary capillary blood flow was measured in man using a modified body plethysmograph with a flowmeter in its wall. This opening permits the plethysmograph pressure to remain approximately atmospheric during the study. The flowmeter measures the airflow in and out of the plethysmograph required to keep the pressure atmospheric. The flow is recorded during exhalations following single breaths of 80% N2O-20% O2 or air. The difference in flow during these exhalations represents the rate of N2O absorption from which blood flow is calculated. Factors which theoretically could affect the pulmonary capillary blood flow pulse were studied with an analog computer model of the pulmonary circulation. The analog shows that the amplitude of this pulse should be reduced when the time constant of the precapillary vasculature is increased, if right ventricular flow is unchanged. The amplitude of the pulmonary capillary flow pulse in man is approximated by the relationship peak flow/stroke volume (Qcmax/SV) and peak flow/ mean flow (Qcmax/Qc). These ratios are reduced at high lung volume. Physiological increases in heart rate result in no significant change in Qcmax/QC. When heart rate is increased with atropine, Qcmax/Qc is reduced but Qcmax/SV is unchanged. Isoproterenol does not alter Qcmax/Qc and Qc max/SV although stroke volume is significantly increased. Isometric exercise increases mean and peak pulmonary capillary blood flow in normal subjects.