RELATIONSHIP OF PULMONARY ARTERIAL AND VENOUS PRESSURE TO DIFFUSING CAPACITY

Pulsatile uptake of CO in the human lung

The instantaneous uptake of CO in the lungs was measured with a water-filled body plethysmograph in normal man. First, control measurements of plethysmograph pressure were made while the subject held his breath for 7 sec after breathing gas mixtures prepared to bring his alveolar P(O2) and P(CO2) close to mixed venous levels. Then, CO uptake measurements were made while he held his breath after inhaling the same gas mixtures with added CO (2.0%). The change in lung volume on CO minus the change in lung volume during the control measurement was a measure of the CO uptake in the lungs. Cardiopneumatic changes in lung gas volume were subtracted electrically. All of five subjects showed pulsatile CO uptake. The mean CO uptake was 103 ml/min. A peak uptake of 2.0 (range 1.6-2.3) times the mean uptake occurred 0.3-0.4 sec after the R wave of the EKG and a minimum uptake of 0.4 (range 0.2-0.5) times the mean uptake occurred during the tenth of a second before the R wave of the EKG. These results suggest that pulmonary capillary blood volume is pulsatile during the cardiac cycle.

Longitudinal distribution of vascular resistance in the pulmonary arteries, capillaries, and veins

A new method has been described for measuring the pressure and resistance to blood flow in the pulmonary arteries, capillaries, and veins. Studies were performed in dog isolated lung lobes perfused at constant flow with blood from a donor dog. Pulmonary artery and vein volume and total lobar blood volume were measured by the ether plethysmograph and dyedilution techniques. The longitudinal distribution of vascular resistance was determined by analyzing the decrease in perfusion pressure caused by a bolus of low viscosity liquid introduced into the vascular inflow of the lobe. The pulmonary arteries were responsible for 46% of total lobar vascular resistance, whereas the pulmonary capillaries and veins accounted for 34 and 20% of total lobar vascular resistance respectively. Vascular resistance was 322 dynes .sec.cm(-5)/ml of vessel in the lobar pulmonary arteries, 112 dynes.sec.cm(-5)/ml in the pulmonary capillaries, and 115 dynes.sec.cm(-5)/ml in the lobar pulmonary veins. Peak vascular resistivity (resistance per milliliter of volume) was in an area 2 ml proximal to the capillary bed, but resistivity was high throughout the pulmonary arterial tree. The pulmonary arteries accounted for approximately 50% of vascular resistance upstream from the sluice point when alveolar pressure exceeded venous pressure. The method described provides the first measurements of pulmonary capillary pressure. Mid-capillary pressure averaged 13.3 cm H(2)O, pulmonary artery pressure averaged 20.4 cm H(2)O, and pulmonary vein pressure averaged 9.2 cm H(2)O. These techniques also provide a way of analyzing arterial, capillary, and venous responses to various pharmacologic and physiologic stimuli.

Regulation of pulmonary capillary blood volume by pulmonary arterial and left atrial pressures

We used single-breath CO diffusing capacity to study the effect of changes in pulmonary arterial and left atrial pressures on pulmonary capillary blood volume in 15 supine dogs whose lungs were perfused with a nonpulsatile pump. Effect of pulmonary arterial pressure: When left atrial pressure measured relative to the bottom of the lungs ≤ alveolar pressure, increasing pulmonary arterial pressure increased diffusing capacity markedly (mean, 0.71 ml/min x mm Hg per mm Hg change in pulmonary arterial pressure). When left atrial pressure ≥ alveolar pressure plus the height of the lungs, increasing pulmonary arterial pressure had less effect on diffusing capacity, although blood flow was increased through a wide range (mean, 0.22 ml/min x mm Hg CO per mm Hg increase in pulmonary arterial pressure). Effect of left atrial pressure: At zero flow, increasing left atrial pressure increased diffusing capacity markedly; when pulmonary arterial pressure was high, increasing left atrial pressure had no significant effect on diffusing capacity. Increasing bronchial arterial pressure from 0 to 150 mm Hg or inhaling 10.5% CO 2 had no effect on diffusing capacity. Injection into the pulmonary artery of glass microspheres decreased diffusing capacity only when left atrial pressure was low; their effect was exaggerated when pulmonary arterial pressure was high in the control state.

Physiologic studies of the pulmonary capillary bed after barium sulfate embolization

22 anesthetized dogs were given a barium sulfate suspension intravenously in a dose sufficient to double mean pulmonary artery pressure. 10 sec breath-holding carbon monoxide diffusing capacity (D(LCO10)) was measured before and after this standard embolization in each dog. No post-embolic decrease in D(LCO10) was observed. In the study of this apparent paradox, it was found that the potential for further increase in D(LCO10) during exercise remained after embolization. During rest prolongation of breath holding to 60 sec decreased CO absorption significantly more in the embolized than in the nonembolized dogs. While D(LCO10) was not affected by standard barium embolization, oxygen diffusing capacity was clearly decreased. The bronchial collateral circulation did not participate in preventing a D(LCO10) decrease after embolization since surgical interruption of the bronchial circulation did not alter the response to barium. Microscopic examination of lung sections taken after standard embolization showed plugging of precapillary vessels in the 40-50 mu range. These studies suggest that acute precapillary embolic obstruction of vessels of this size interferes remarkably little with CO absorption over short periods of time, probably because of continued CO absorption in portions of the capillary net distal to the sites of impaction. The remarkable anastomotic nature of this capillary network with multiple sources of access possibly provides the anatomic basis for this observation. This study demonstrates a clear dissociation between acute changes in pulmonary vascular resistance and D(LCO10)-both during rest and exercise.

Forward and backward transmission of pressure waves in the pulmonary vascular bed of the dog

Pressures were measured in lobar pulmonary arteries, veins, and left atria of anesthetized open-chest dogs. Observations were made before and after snaring the lobar vessels and before and after infusing dextran to raise the mean pressure. Pressure waves were subjected to harmonic analysis. Forward (artery to vein) and backward (vein to artery) transmission (output/input pressure) and transmission ratios (forward/backward transmission) were calculated for each harmonic.

At low frequencies, forward transmission was increased by dextran, but backward transmission was unaffected. At low frequencies and high mean pressures the bed was nonsymmetrical, forward transmission being several times greater than backward at the same mean pressure (transmission ratio >1).

The findings were analyzed with the aid of simple models. Nonsymmetry at low frequencies and high mean pressures appears to be due to arterial exceeding venous compliance. The findings and resulting theory are pertinent to the understanding of the shape and amplitude of arterial "wedge"-pressure traces.