title: Permissive range of hypercapnia for improved peripheral microcirculation and cardiac output in rabbits.
abstract: OBJECTIVE
  Permissive hypercapnia improves outcomes in patients with respiratory failure, most likely because of a reduction in ventilator-induced lung injury. Because hypercapnia is a potent vasoactive stimulus, adequate tissue perfusion and oxygen delivery to dilated microvessels may be restored. We examined how Paco2 affects microvascular changes, hemodynamics, and cardiac output in rabbits. We evaluated the permissive range of Paco2 required for maintenance of the peripheral circulation.
  
  
  METHODS
  Prospective experimental animal study.
  
  
  METHODS
  Animal research laboratory.
  
  
  METHODS
  A total of 31 Japanese domestic white rabbits.
  
  
  METHODS
  The animals were anesthetized with pentobarbital. An ear chamber was prepared to examine blood vessels by intravital microscopy. The rabbits were mechanically ventilated with air, oxygen, and CO2. The values of Paco2 were adjusted to about 20 (hypocapnia), 40 (normocapnia), 60, 80, 100, 125, 150, and >250 mm Hg (hypercapnia). After stabilization at each Paco2 level, microvascular changes were recorded with a microscope-closed video camera to permit analysis of arteriolar diameter and blood flow.
  
  
  RESULTS
  The pH and heart rate decreased and mean blood pressure increased progressively as the Paco2 was increased. When Paco2 was increased from 20 to 80 mm Hg, vessel diameter, blood-flow velocity, and blood-flow rate increased markedly. Cardiac output increased slightly. When Paco2 exceeded 100 mm Hg, all of these variables decreased. When Paco2 exceeded 150 mm Hg, all variables were significantly lower than the control values (p < .01).
  
  
  CONCLUSIONS
  Intravital microscopic visualization of the rabbit ear microcirculation showed that 150 mm Hg is the permissive upper limit of acute hypercapnia with respect to maintenance of the peripheral microcirculation.
authors.affiliation: Department of Anesthesiology, Medical Center East, Tokyo Women's Medical University, Tokyo, Japan. komorim@nifty.com
authors.forename: Makiko
authors.initials: M
authors.lastname: Komori
authors.forename: Katsumi
authors.initials: K
authors.lastname: Takada
authors.forename: Yasuko
authors.initials: Y
authors.lastname: Tomizawa
authors.forename: Keiko
authors.initials: K
authors.lastname: Nishiyama
authors.forename: Miwako
authors.initials: M
authors.lastname: Kawamata
authors.forename: Makoto
authors.initials: M
authors.lastname: Ozaki
mesh_terms: D000142:Acidosis, Respiratory / Q000503:physiopathology; D000818:Animals; D001783:Blood Flow Velocity; D001794:Blood Pressure; D002245:Carbon Dioxide / Q000032:analysis; D002302:Cardiac Output / Q000502:physiology*; D006339:Heart Rate; D006863:Hydrogen-Ion Concentration; D006935:Hypercapnia / Q000503:physiopathology*; D008833:Microcirculation / Q000502:physiology*; D011446:Prospective Studies; D011817:Rabbits
chemical_list: D002245:Carbon Dioxide