Determinants of pulmonary blood volume. Effects of acute changes in airway pressure

To examine the effects of airway pressure (AWP) on pulmonary blood volume (PBV) at various pulmonary vascular pressures and flows, experiments were performed in anaesthetized, open-chest dogs. The AWP was raised by elevating end-expiratory pressure, and PBV was calculated as the product of electromagnetic aortic flow and pulmonary mean transit time for ascorbate (polarographic method). When AWP was raised from 3 to 13 mmHg, changing lung conditions from zone 3 [left atrial pressure (LAP) higher than AWP] to zone 2 (AWP higher than LAP), PBV decreased by 14.5 +/- 6.2%. When LAP was raised above 7 mmHg at constant pulmonary arterial pressure (PAP), PBV increased under zone 2 but not under zone 3 conditions. During blood volume expansion to LAP 15 mmHg, PBV rose by 30-50% and became equal at AWP of 4 and 14 mmHg, whereas the pulmonary vascular resistance remained 40% higher at high AWP. These data suggest that PAP, LAP and AWP regulate PBV by acting on compliant vessels surrounding the alveoli. Under zone 2 conditions with collapsed aveolar capillaries, elevation of LAP results in re-expansion of the alveolar capillaries, and PBV is restored without a rise in PAP. Under zone 3 conditions, a rise in LAP cannot increase PBV without raising PAP, explaining why PBV remains constant when PAP is kept constant.