title: Patient-ventilator flow dyssynchrony: flow-limited versus pressure-limited breaths.
abstract: OBJECTIVE
  Patient-ventilator flow dyssynchrony occurs when ventilator flow delivery is insufficient to meet patient demands. If sufficiently severe, flow dyssynchrony can produce significant imposed loads on ventilatory muscles. Flow dyssynchrony can be improved by increasing ventilator flow delivery. We hypothesized that the variable flow pressure-limited breath would be a better approach for matching patient flow demands than adjusting a set flow on a conventional volume-cycled breath.
  
  
  METHODS
  Clinical interventional study.
  
  
  METHODS
  Medical intensive care unit.
  
  
  METHODS
  Sixteen stable, mechanically ventilated patients receiving volume-cycled assist-control ventilation.
  
  
  METHODS
  Flow dyssynchrony was produced by reducing the set flow by 50%. Dyssynchrony was quantified by measuring the esophageal pressure time product during the assisted breath. Two strategies were then employed in an attempt to reduce the dyssynchrony. One strategy was to increase flow back to the initial set flow and then further increase flow by an additional 25% (VI strategy). The other strategy was to use a pressure-limited breath feature coupled to a volume assist breath (the P strategy). With the P strategy, the pressure limit was set at 75% and 100% of the static elastic recoil pressure at end-inspiration.
  
  
  RESULTS
  Pressure time product, intrinsic positive end-expiratory pressure, and the ventilatory pattern were measured with each strategy and were analyzed by analysis of variance. Induced baseline flow dyssynchrony, as measured by the pressure time product, was > 5 cm H2O/sec in ten of 16 patients. This dyssynchrony was significantly reduced by both the VI strategy and the P strategy, although the P strategy appeared to be more effective in those patients with the greatest baseline dyssynchrony. Baseline inspiratory time was also shortened by both the VI strategy and the P strategy; the VI strategy shortened baseline inspiratory time more than the P strategy. Baseline tidal volume, frequency, and intrinsic positive end-expiratory pressure were only minimally affected by either strategy.
  
  
  CONCLUSIONS
  The pressure-limited, variable-flow approach to ventilator gas delivery appears to be more responsive to a vigorous patient effort than a fixed-flow approach.
authors.affiliation: Duke University Medical Center, Durham, NC, USA.
authors.forename: N R
authors.initials: NR
authors.lastname: MacIntyre
authors.forename: R
authors.initials: R
authors.lastname: McConnell
authors.forename: K C
authors.initials: KC
authors.lastname: Cheng
authors.forename: A
authors.initials: A
authors.lastname: Sane
mesh_terms: D000328:Adult; D000368:Aged; D000402:Airway Obstruction / Q000503:physiopathology / Q000628:therapy; D000704:Analysis of Variance; D005260:Female; D006801:Humans; D016014:Linear Models; D008297:Male; D008875:Middle Aged; D008991:Monitoring, Physiologic / Q000295:instrumentation; D018467:Positive-Pressure Respiration, Intrinsic / Q000503:physiopathology / Q000628:therapy; D012121:Respiration, Artificial / Q000379:methods* / Q000706:statistics & numerical data; D012128:Respiratory Distress Syndrome / Q000503:physiopathology / Q000628:therapy; D014939:Work of Breathing