title: Effects of Constant Flow vs. Constant Pressure Perfusion on Fluid Filtration in Severe Hypothermic Isolated Blood-Perfused Rat Lungs.
abstract: BACKGROUND
  Victims of severe accidental hypothermia are prone to fluid extravasation but rarely develop lung edema. We hypothesize that combined hypothermia-induced increase in pulmonary vascular resistance (PVR) and a concomitant fall in cardiac output protect the lungs against edema development. Our aim was to explore in hypothermic-isolated blood-perfused rat lungs whether perfusion at constant pressure influences fluid filtration differently from perfusion at constant flow.
  
  
  METHODS
  Isolated blood-perfused rat lungs were hanging freely in a weight transducer for measuring weight changes (ΔW). Fluid filtration coefficient (Kfc), was determined by transiently elevating left atrial pressure (Pla) by 5.8 mmHg two times each during normothermia (37°C) and during hypothermia (15°C). The lung preparations were randomized to two groups. One group was perfused with constant flow (Constant flow group) and the other group with constant pulmonary artery pressure (Constant PPA group). Microvascular pressure (Pmv) was determined before and during elevation of Pla (ΔPmv) by means of the double occlusion technique. Kfc was calculated with the formula Kfc = ΔW/ΔPmv/min. All Kfc values were normalized to predicted lung weight (PLW), which was based on body weight (BW) according to the formula: PLW= 0.0053 BW - 0.48 and presented as KfcPLWin mg/min/mmHg/g. At cessation, bronchoalveolar lavage (BAL) fluid/perfusate protein concentration (B/P) ratio was determined photometrically. Data were analyzed with parametric or non-parametric tests as appropriate.p< 0.05 considered as significant.
  
  
  RESULTS
  Perfusate flow remained constant in the Constant flow group, but was more than halved during hypothermia in the Constant PPA group concomitant with a more fold increase in PVR. In the Constant flow group, KfcPLWand B/P ratio increased significantly by more than 10-fold during hypothermia concerted by visible signs of edema in the trachea. Hemoglobin and hematocrit increased within the Constant flow group and between the groups at cessation of the experiments.
  
  
  CONCLUSIONS
  In hypothermic rat lungs perfused at constant flow, fluid filtration coefficient per gram PLWand B/P ratio increased more than 10-fold concerted by increased hemoconcentration, but the changes were less in hypothermic lungs perfused at constant PPA.
authors.affiliation: Anesthesia and Critical Care Research Group, Faculty of Health Sciences, Department of Clinical Medicine, University of Tromsø, The Arctic University of Norway , Tromsø , Norway.
authors.forename: Kathrine
authors.initials: K
authors.lastname: Halsøy
authors.affiliation: Anesthesia and Critical Care Research Group, Faculty of Health Sciences, Department of Clinical Medicine, University of Tromsø, The Arctic University of Norway , Tromsø , Norway.
authors.forename: Timofey
authors.initials: T
authors.lastname: Kondratiev
authors.affiliation: Anesthesia and Critical Care Research Group, Faculty of Health Sciences, Department of Clinical Medicine, University of Tromsø, The Arctic University of Norway , Tromsø , Norway.
authors.forename: Torkjel
authors.initials: T
authors.lastname: Tveita
authors.affiliation: Anesthesia and Critical Care Research Group, Faculty of Health Sciences, Department of Clinical Medicine, University of Tromsø, The Arctic University of Norway , Tromsø , Norway.
authors.forename: Lars J
authors.initials: LJ
authors.lastname: Bjertnaes
keywords: BAL/plasma protein concentration ratio; constant flow perfusion; constant pressure perfusion; fluid filtration coefficient; hypothermia; isolated rat lungs; lung edema