Quantification of pulmonary vascular occlusion in dogs by use of the diffusing capacity

Abstract

The purpose of these experiments was to quantify stagnant intrapulmonary blood caused by a pulmonary arterial occlusion (PAO). The hypothesis was that the diffusing capacity of the lung for CO (DLCO) would be altered little by PAO when measured with the usual inspired concentrations (0.3%) of CO, since stagnant blood distal to the occlusion takes up CO for 20 s or more before significant CO backpressure would develop. However, higher levels of CO (i.e., greater than or equal to 3%) would equilibrate faster with capillary blood (within 5-10 s), and DLCO measured 10-20 s subsequent to the high CO exposure would reflect only the DLCO in the unoccluded regions. Thus the fractional reduction in DLCO measured with 3% CO, with respect to that measured with 0.3% CO, should be related to the fractional occlusion of the pulmonary artery in a predictable way. We occluded the right pulmonary artery (RPAO), the left pulmonary artery (LPAO), or the left lower lobar artery (LLPAO) and found that DLCO measured during rebreathing a 0.3% CO mixture was 80, 87, and 94%, respectively, of the preocclusion value, whereas the DLCO measured during rebreathing a 3.3% CO mixture was 59, 73, and 87% of the preocclusion value. A computer model was developed to predict the reduction in DLCO at different levels of CO exposure that would be caused by varying fractions of PAO. Our data indicated that RPAO corresponded to a 42% vascular occlusion, LPAO a 35% occlusion, and LLPAO a 20% occlusion. Measurement of DLCO using low and high concentrations of CO might be useful in assessing the fraction of vascular bed occluded and in following noninvasively the course of vascular occlusion in a variety of pulmonary diseases.

Pulmonary Artery Occlusion Increases the Ratio of Diffusing Capacity for Nitric Oxide to Carbon Monoxide in Prone Sheep

“…Other investigators 1,2,16 -18 have noted a drop in Dlco after PAO. In the PAO experiments by Chappell et al, 1 the range of the drop in Dlco (6 to 20%) is similar to the range we saw for the balloon occlusion (15 to 30%) experiments, and corresponds to approximately a 35 to 40% vascular occlusion (right or left main pulmonary artery occlusion). The drop in Dlco was much higher during PAO with autologous clots, but this was a much more severe obstruction, resulting in an estimated 64% obstruction of the pulmonary vascular bed.…”

“…Chappell et al 1 demonstrated in a canine model that the ratio of Dlco from sequential measurements done at two concentrations of CO (0.3% and 3.3%) could be used to estimate the fraction of occluded pulmonary capillary bed. They theorized that, in capillaries with stagnant blood, the rate decline in alveolar CO concentration would be slower at higher inspired CO concentration due to saturation of hemoglobin, and thus Dlco would be lower.…”

“…[2][3][4] Because this effect is magnified when the test gas contains higher concentrations of CO, the relative decrease in Dlco caused by increasing CO concentration is a function only of the fraction of the vascular bed with stagnant blood. 1 This approach for quantifying pulmonary vascular obstruction, however, has practical limitations. It requires two serial measurements of Dlco with a washout period in between of 100% O 2 breathing to reduce the hemoglobin-bound CO accumulated during the first measurement.…”

Recruitment of Lung Diffusing Capacity

“…Erythrocyte distribution is difficult to quantify in vivo, although gross redistribution of perfusion, as in acute unilateral pulmonary artery occlusion, reduces Dl in dogs. 30 Simulation shows that Dmco should be highest when capillary erythrocytes are evenly spaced, 31 and lowest when the same erythrocytes are tightly clustered, because adjacent erythrocyte surfaces become less effective in carbon monoxide uptake; random erythrocyte distribution yields intermediate values. Similarly, Dmco should be enhanced when erythrocytes are distributed uniformly among several capillary segments.…”

Acute effects of a single open sea air dive and post-dive posture on cardiac output and pulmonary gas exchange in recreational divers

“…One may relate a decrease in TLCO to a parallel build up of alveolar dead space after a dive. However, in dogs, using 0.3% of CO in inspired gas (as in a standard single-breath TLCO measurement), Chappel et al 19 have shown that the CO uptake in the stagnant blood pool distal to the occlusion is not stopped, but only slightly slowed down (due to a build up of the back pressure and partial collapse of the occluded vessels). Bearing this in mind and the small part of the lung occluded, it is unlikely that the increased dead space accounted for the observed decrease in TLCO.…”