title: Spontaneous breathing during general anesthesia prevents the ventral redistribution of ventilation as detected by electrical impedance tomography: a randomized trial.
abstract: BACKGROUND
  Positive-pressure ventilation causes a ventral redistribution of ventilation. Spontaneous breathing during general anesthesia with a laryngeal mask airway could prevent this redistribution of ventilation. We hypothesize that, compared with pressure-controlled ventilation, spontaneous breathing and pressure support ventilation reduce the extent of the redistribution of ventilation as detected by electrical impedance tomography.
  
  
  METHODS
  The study was a randomized, three-armed, observational, clinical trial without blinding. With approval from the local ethics committee, we enrolled 30 nonobese patients without severe cardiac or pulmonary comorbidities who were scheduled for elective orthopedic surgery. All of the procedures were performed under general anesthesia with a laryngeal mask airway and a standardized anesthetic regimen. The center of ventilation (primary outcome) was calculated before the induction of anesthesia (AWAKE), after the placement of the laryngeal mask airway (BEGIN), before the end of anesthesia (END), and after arrival in the postanesthesia care unit (PACU).
  
  
  RESULTS
  The center of ventilation during anesthesia (BEGIN) was higher than baseline (AWAKE) in both the pressure-controlled and pressure support ventilation groups (pressure control: 55.0 vs. 48.3, pressure support: 54.7 vs. 48.8, respectively; multivariate analysis of covariance, P < 0.01), whereas the values in the spontaneous breathing group remained at baseline levels (47.9 vs. 48.5). In the postanesthesia care unit, the center of ventilation had returned to the baseline values in all groups. No adverse events were recorded.
  
  
  CONCLUSIONS
  Both pressure-controlled ventilation and pressure support ventilation induce a redistribution of ventilation toward the ventral region, as detected by electrical impedance tomography. Spontaneous breathing prevents this redistribution.
authors.affiliation: San Francisco General Hospital, Department of Anesthesia & Perioperative Care, University of California San Francisco, San Francisco, CA 94110, USA. oradke@pcat.de
authors.forename: Oliver C
authors.initials: OC
authors.lastname: Radke
authors.forename: Thomas
authors.initials: T
authors.lastname: Schneider
authors.forename: Axel R
authors.initials: AR
authors.lastname: Heller
authors.forename: Thea
authors.initials: T
authors.lastname: Koch
mesh_terms: D000328:Adult; D000368:Aged; D000704:Analysis of Variance; D000768:Anesthesia, General; D003422:Critical Care; D017097:Electric Impedance; D005260:Female; D006801:Humans; D007091:Image Processing, Computer-Assisted; D007362:Intensive Care Units; D017214:Laryngeal Masks; D008168:Lung / Q000033:anatomy & histology; D008297:Male; D008875:Middle Aged; D019637:Orthopedic Procedures; D011175:Positive-Pressure Respiration; D012119:Respiration; D012121:Respiration, Artificial; D012129:Respiratory Function Tests; D014054:Tomography; D016896:Treatment Outcome