Effects of haemoconcentration and haemodilution on acute hypoxia-induced pulmonary hypertension and changes in vascular compliance of isolated rabbit lungs

Objective: Erythrocytes influence the magnitude of hypoxia-induced pulmonary artery pressure increase. It is, however, unknown to what extent haemoconcentration and haemodilution affect this response and whether intrapulmonary blood volume (and thus vessel dimensions) alters the magnitude of pressure increase. Furthermore, it is unclear whether the haemodilution/haemoconcentration-dependent pressure increase is flow-related, via flow-dependent changes in vasomotor tone or rheologic effects, or can also be observed under no-flow conditions. Design: Experimental study in isolated rabbit lungs (n = 12) perfused with autologous blood at constant flow (100 ml/min) and ventilated with 5 % carbon dioxide in air. Setting: Laboratory for experimental studies. Interventions: Haemoconcentration (centrifugation) and haemodilution (Krebs-Henseleit/albumin) were carried out, resulting in haematocrits between 50 % and 0 %. During hypoxic ventilation, inspiratory oxygen fraction was reduced from 0.20 to 0.03. Measurements and results: Under constant flow conditions, haemodilution (from a Hct of 34–36 % to 0–1 %) decreased hypoxic pulmonary artery pressure response to one-third (from 10.8 ± 2.3 cmH2O to 3.1 ± 1.0 cmH2O, P < 0.05), while haemoconcentration did not affect the magnitude of hypoxic response (10.5 ± 2.0 cmH2O). For all haematocrit values an increase in pulmonary blood volume (by 5 ml) decreased the magnitude of pressure response. Hypoxia-induced changes in static vascular filling pressure (double occlusion pressure) and vascular compliance were used to assess the strength of hypoxic vasoconstriction under static conditions. Neither haemoconcentration nor haemodilution altered hypoxia-induced changes in either variable. Conclusions: The magnitude of the acute hypoxic pressure response is not altered by haemoconcentration, but significantly reduced by haemodilution. In contrast, neither haemoconcentration nor haemodilution influenced hypoxia-induced changes in static vascular filling pressure and compliance. This suggests that the degree of hypoxic pulmonary vasoconstriction is not affected under static conditions and that the red blood cell-dependence of the magnitude of hypoxic pressure response is based on flow-related mechanisms.