title: Pulmonary shunt is independent of decrease in cardiac output during unsupported spontaneous breathing in the pig.
abstract: BACKGROUND
  During mechanical ventilation (MV), pulmonary shunt is cardiac output (CO) dependent; however, whether this relationship is valid during unsupported spontaneous breathing (SB) is unknown. The CO dependency of the calculated venous admixture was investigated, with both minor and major shunt, during unsupported SB, MV, and SB with continuous positive airway pressure (CPAP).
  
  
  METHODS
  In seven anesthetized supine piglets breathing 100% oxygen, unsupported SB, MV (with tidal volume and respiratory rate corresponding to SB), and 8 cm H2O CPAP (airway pressure corresponding to MV) were applied at random. Venous return and CO were reduced by partial balloon occlusion of the inferior vena cava. Measurements were repeated with the left main bronchus blocked, creating a nonrecruitable pulmonary shunt.
  
  
  RESULTS
  CO decreased from 4.2 l/min (95% CI, 3.9-4.5) to 2.5 l/min (95% CI, 2.2-2.7) with partially occluded venous return. Irrespective of whether shunt was minor or major, during unsupported SB, venous admixture was independent of CO (slope: minor shunt, 0.5; major shunt, 1.1% · min(-1) · l(-1)) and mixed venous oxygen tension. During both MV and CPAP, venous admixture was dependent on CO (slope MV: minor shunt, 1.9; major shunt, 3.5; CPAP: minor shunt, 1.3; major shunt, 2.9% · min(-1) · l(-1)) and mixed-venous oxygen tension (coefficient of determination 0.61-0.86 for all regressions).
  
  
  CONCLUSIONS
  In contrast to MV and CPAP, venous admixture was independent of CO during unsupported SB, and was unaffected by mixed-venous oxygen tension, casting doubt on the role of hypoxic pulmonary vasoconstriction in pulmonary blood flow redistribution during unsupported SB.
authors: Department of Surgical Sciences, Anesthesiology and Critical Care, Uppsala University, Uppsala, Sweden. laszlo.vimlati@surgsci.uu.se
authors: Laszlo
authors: L
authors: Vimlati
authors: Anders
authors: A
authors: Larsson
authors: Goran
authors: G
authors: Hedenstierna
authors: Michael
authors: M
authors: Lichtwarck-Aschoff
mesh_terms: D000818:Animals; D021721:Balloon Occlusion; D001980:Bronchi / Q000503:physiopathology; D002245:Carbon Dioxide / Q000097:blood*; D002302:Cardiac Output; D045422:Continuous Positive Airway Pressure / Q000379:methods; D008168:Lung / Q000098:blood supply* / Q000503:physiopathology*; D012119:Respiration; D012121:Respiration, Artificial / Q000379:methods; D013552:Swine; D013990:Tidal Volume; D014682:Vena Cava, Inferior / Q000503:physiopathology
chemical_list: D002245:Carbon Dioxide