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Patient-ventilator flow dyssynchrony: Flow-limited versus pressure-limited breaths

MacIntyre, Neil R. MD; McConnell, Robert RRT; Cheng, Kuo-Chen G. MD; Sane, Aneysa MD

Clinical Investigations

Objectives  Patient-ventilator flow dyssynchrony occurs when ventilator flow delivery is insufficient to meet patient demands. If sufficiently severe, flow dyssynchrony can produce signlficant 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.

Design  Clinical interventional study.

Setting  Medical Intensive care unit.

Patients  Sixteen stable, mechanically ventilated patients receiving volume-cycled assist-control ventilation.

Interventions  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.

Measurements and Main 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 H2 O/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.

Conclusion  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. (Crit Care Med 1997;25:1671-1677)

From Duke University Medical Center (Drs. Maclntyre and McConnell), Durham, NC; the Section of Respiratory Medicine (Dr. Cheng), Chi Mei Foundation Hospital, Tainan, Taiwan; and Bowman Gray School of Medicine (Dr. Sane), Winston-Salem, NC.

Supported, in part, by a grant from BiCore, Irvine, CA (now Allied Health Products, Riverside, CA).

© Williams & Wilkins 1997. All Rights Reserved.