title: Spontaneous breathing improves shunt fraction and oxygenation in comparison with controlled ventilation at a similar amount of lung collapse.
abstract: BACKGROUND
  Spontaneous breathing (SB), when allowed during mechanical ventilation (MV), improves oxygenation in different models of acute lung injury. However, it is not known whether oxygenation is improved during mechanically unsupported SB. Therefore, we compared SB without any support with controlled MV at identical tidal volume (VT) and respiratory rate (RR) without positive end-expiratory pressure in a porcine lung collapse model.
  
  
  METHODS
  In 25 anesthetized piglets, stable lung collapse was induced by application of negative pressure, and animals were randomized to either resume SB or to be kept on MV at identical VT (5 mL/kg; 95% confidence interval: 3.8 to 6.4) and RR (65 per minute [57 to 73]) as had been measured during an initial SB period. Oxygenation was assessed by blood gas analysis (n=15) completed by multiple inert gas elimination technique (n=8 of the 15) for shunt measurement. In addition, possible lung recruitment was studied with computed tomography of the chest (n=10).
  
  
  RESULTS
  After induction of lung collapse, PaO2/FIO2 decreased to 90 mm Hg (76 to 103). With SB, PaO2/FIO2 increased to 235 mm Hg (177 to 293) within 15 minutes, whereas MV at identical Vt and RR did not cause any improvement in oxygenation. Intrapulmonary shunt by 45 minutes after induction of lung collapse was lower during SB (SB: 27% [24 to 30] versus MV: 41% [28 to 55]; P=0.017). Neither SB nor MV reduced collapsed lung areas on computed tomography.
  
  
  CONCLUSIONS
  SB without any support improves oxygenation and reduces shunt in comparison with MV at identical settings. This seems to be achieved without any major signs of recruitment of collapsed lung regions.
authors.affiliation: Department of Surgical Sciences, Anesthesiology and Critical Care Medicine, Uppsala University; S-751 85 Uppsala, Sweden. laszlo.vimlati@surgsci.uu.se
authors.forename: Laszlo
authors.initials: L
authors.lastname: Vimlati
authors.forename: Rafael
authors.initials: R
authors.lastname: Kawati
authors.forename: Göran
authors.initials: G
authors.lastname: Hedenstierna
authors.forename: Anders
authors.initials: A
authors.lastname: Larsson
authors.forename: Michael
authors.initials: M
authors.lastname: Lichtwarck-Aschoff
mesh_terms: D000758:Anesthesia; D000818:Animals; D006439:Hemodynamics / Q000502:physiology; D008297:Male; D016343:Monitoring, Intraoperative; D010100:Oxygen / Q000097:blood*; D001261:Pulmonary Atelectasis / Q000503:physiopathology / Q000628:therapy*; D011659:Pulmonary Gas Exchange / Q000502:physiology; D012119:Respiration; D012121:Respiration, Artificial / Q000379:methods*; D013552:Swine; D014057:Tomography, X-Ray Computed
chemical_list: D010100:Oxygen