title: Interaction between haematocrit and pulmonary blood volume on pulmonary vascular flow resistance and pressure flow relationships.
abstract: 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.
  
  
  METHODS
  Experimental study in isolated zone-III rabbit lungs perfused with autologous blood.
  
  
  METHODS
  Laboratory for experimental studies.
  
  
  METHODS
  Stepwise and independent variation of flow (50, 100, and 200 ml/min), pulmonary blood volume (increments of 2.5 ml and 5 ml imposed by changes of left atrial pressure), and haematocrit (0-50%) varied by haemodilution (Krebs-Henseleit/albumin) or haemoconcentration (centrifugation).
  
  
  METHODS
  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 ml (approx. 15 and 30% of normal pulmonary blood volume) at constant haematocrit significantly shifted downwards pressure-flow 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.
authors.affiliation: Institute für Experimentelle Anaesthesiologie,  Heinrich-Heine-Universit, Düsseldorf, Germany.
authors.forename: S A
authors.initials: SA
authors.lastname: Loer
authors.forename: T W
authors.initials: TW
authors.lastname: Scheeren
authors.forename: J
authors.initials: J
authors.lastname: Peters
mesh_terms: D000818:Animals; D001794:Blood Pressure; D001810:Blood Volume; D006400:Hematocrit; D006438:Hemodilution; D008168:Lung / Q000502:physiology*; D011652:Pulmonary Circulation; D011817:Rabbits; D014655:Vascular Resistance