Pulsatile uptake of CO in the human lung

Pulmonary capillaries are recruited during pulsatile flow

Capillaries recruit when pulmonary arterial pressure rises. The duration of increased pressure imposed in such experiments is usually on the order of minutes, although recent work shows that the recruitment response can occur in <4 s. In the present study, we investigate whether the brief pressure rise during cardiac systole can also cause recruitment and whether the recruitment is maintained during diastole. To study these basic aspects of pulmonary capillary hemodynamics, isolated dog lungs were pump perfused alternately by steady flow and pulsatile flow with the mean arterial and left atrial pressures held constant. Several direct measurements of capillary recruitment were made with videomicroscopy. The total number and total length of perfused capillaries increased significantly during pulsatile flow by 94 and 105%, respectively. Of the newly recruited capillaries, 92% were perfused by red blood cells throughout the pulsatile cycle. These data provide the first direct account of how the pulmonary capillaries respond to pulsatile flow by showing that capillaries are recruited during the systolic pulse and that, once open, the capillaries remain open throughout the pulsatile cycle.

Fractional changes in lung capillary blood volume and oxygen saturation during the cardiac cycle in rabbits

Changes in local pulmonary capillary blood volume (Vc) and oxygen saturation (S) have been difficult to measure in live animals. By utilizing the differences in absorption of light at two wavelengths (650 and 800 nm), we estimated the fractional change in Vc and S during the course of the cardiac cycle in eight anesthetized, ventilated rabbits at low and high lung volumes. Observations were made of the pattern of diffusely backscattered light, from an approximately 1-cm3 volume of lung illuminated with a point source placed on the pleural surface through a thoracotomy. At low lung volume, the fractional change in Vc was approximately 13%, the change in S was approximately 4.6%, and the mean S was close to 77%. The fluctuations in Vc and S lagged behind peak systemic blood pressure by about one-fifth and three-fifths of a cycle, respectively. At high lung volume, there were no important fluctuations in Vc or S, and the mean S was approximately 82%. These results are consistent with fluctuations in pulmonary capillary pressure and gas exchange over the cardiac cycle, and with decreasing capillary compliance with increasing lung volume.

Pulmonary O2 transfer during pulsatile and non-pulsatile perfusion

The importance of the perfusion pattern for the oxygen transfer has been examined in isolated rabbit lungs perfused with plasma at constant volume inflow. The lungs were ventilated with constant tidal volume and constant end-expiratory pressure. Following a standardized rise in FIO2 the rate of rise in pulmonary venous PO2 (delta PO2/delta t) was measured during alternately pulsatile and non-pulsatile perfusion in normal lungs and in lungs made edematous by elevation of left atrial pressure. In normal lungs there was no difference in delta PO2/delta t when the two modes of perfusion were compared. In edematous lungs delta PO/delta t was statistically higher during pulsatile perfusion, indicating a beneficial effect of flow- and pressure pulsations, e.g. a better distribution of V/Q ratios throughout the lungs. In a separate series of expts. the advancement of a high O2 front through the airways was measured, and the two perfusion patterns compared. Since no difference was found, we suggest that the phenomenon of "cardiogenic gas mixing" in the airways in vivo is a result of a direct action of the heart on the lungs rather than arterial pulsations.

Analysis of the effects of pulsatile capillary blood flow and volume on gas exchange

Blood flow into the pulmonary capillaries and the volume of blood within the capillary bed are both pulsatile with the cardiac cycle. We have developed a quantitative model of diffusional gas exchange in the lung to investigate the effects of coupling between these two time-varying parameters on lung O2 and CO2 exchange. For normal man breathing room air at rest, the computed results agree well with previous predictions for the constant flow and volume case, and for the case of pulsatile flow alone. When coupled time-varying pulmonary capillary blood flow and volume are included, using the best data available in the literature to define these parameters, diffusional O2 exchange is improved over the cases of pulsatile flow or volume alone, and closely approximates that obtained for the hypothetical constant flow and volume case. CO2 exchanges, and O2 exchange during hypoxia, are not affected by pulsatile flow and/or volume. These results suggest that O2 exchange is efficient in the presence of coupled blood flow and blood volume pulsations as they exist in the lung capillaries, and that these conditions may be optimal for gas exchange under certain physiological (or pathological) conditions.

Alterations in distribution of blood flow to the lung's diffusion surfaces during exercise

We measured simultaneously, by single breath methods, pulmonary capillary blood flow (Q(c)), carbon monoxide diffusing capacity (DL(CO)), and isotopic oxygen ((18)O(18)O) diffusing capacity (DL(18) (O2)) in five normal males during conditions of rest and moderate exercise at mixed venous O(2) tensions (PO(2) 33-44 mm Hg). During moderate exercise at a work load of 100 W. pulmonary capillary blood flow increased from 6.9+/-1.5 to 12.9+/-3.4 min(-1) and DL(18) (O2) increased from 25+/-4 to 43+/-3 ml.min(-1).mm Hg(-1), whereas DL(CO) showed no significant change (45+/-5 to 49+/-10 ml.min(-1).mm Hg(-1)). DL(18) (O2) increased proportionally to Q(c) (r = 0.74), where DL(CO) did not (r = 0.08). The greater increase in DL(18) (O2) during exercise can be explained by a more homogeneous diffusion/perfusion (DL(O2)/Q(c)) distribution in the individual respiratory exchange units during exercise. This improved distribution of DL(O2)/Q(c) acts to help prevent an increase in alveolar-arterial O(2) tension difference from developing despite the decrease in pulmonary erythrocyte transit times that occur during exercise. The insignificant rise in DL(CO) with exercise under these hypoxic breathholding conditions may result from pulmonary vasomotor responses to short-term hypoxia or from relative insensitivity of DL(CO) to moderate levels of exercise.

Regulation of the pulmonary circulation

Factors regulating pressure and flow in the lungs are reviewed with particular emphasis on their role in regulating blood flow velocity and distribution within the lung capillaries. The behaviour of the pulmonary arterial, system, alveolar capillaries, and pulmonary venous system are considered individually. The effect of heart disease on lung capillary blood flow is examined.