Interaction between haematocrit and pulmonary blood volume on pulmonary vascular flow resistance and pressure-flow relationships

Objective : Pulmonary vascular flow resistance depends on blood viscosity, mainly due to haematocrit, and on vessel dimensions determining blood volume in this highly compliant vascular bed. We, therefore, evaluated the interaction between haematocrit, blood flow, and transpulmonary vascular pressure gradient under conditions of controlled pulmonary blood volume.

Design: Experimental study in isolated zone-III rabbit lungs perfused with autologous blood.

Setting: Laboratory for experimental studies.

Interventions: Stepwise and independent variation of flow (50, 100, and 200 ml/min) pulmonary blood volume (increments of 2.5 mi and 5 mi imposed by changes of left atrial pressure), and haematocrit (0-50 %) varied by haemodilution (Krebs-Henseleit/albumin) or haemoconcentration (centrifugation). Measurements: Pulmonary arterial, left atrial, and airway pressures as well as reservoir volume (reflecting reciprocal changes of lung blood volume) and lung weight.

Results: Haemodilution from the normal haematocrit (32%) to 10% at constant pulmonary blood volume and flow decreased flow resistance only slightly whereas haemoconcentration (50 %) increased flow resistance up to 130 %. At the same time increments of in pulmonary blood volume of 2.5 and 5 mi (approx, 15 and 30 % of normal pulmonary blood volume) at constant haematocrit significantly shifted downwards pressure-now relationships for all investigated haematocrits (0-50 %).

Conclusions: Because of the multiple interrelationships between haematocrit, blood flow and pulmonary blood volume, haematocrit effects on pulmonary flow resistance and pressure-flow relationships in the pulmonary vasculature should be studied at controlled blood volume, While haemodilution only has minor effects, haemoconcentration changes pressure-flow relationships markedly. Pulmonary blood volume has a major impact on slope and position of pressure-flow relationships for all haematocrits investigated.