Friday, January 23, 2009

Low tidal volume ventilation for ALI and ARDS---read the fine print

The care of patients with ALI/ARDS was revolutionized several years ago with publication of the ARDSnet trial of low tidal volume ventilation (LTVV). Recently investigators reporting in Critical Care Medicine looked at an underappreciated complication of LTTV: breath stacking. Breath stacking refers to patient triggering of machine breaths in rapid succession. When two or more breaths occur in sufficiently close proximity they behave as a single breath with higher tidal volume.

The low tidal volumes recommended following publication of the ARDSnet trial are insufficient to satisfy the dyspnea of many mechanically ventilated patients, leading to more frequent inspiratory efforts and, consequently, breath stacking. The resulting high tidal volume (which is easily determined as the area under the curve of the flow-time graph) counters the beneficial effects of the LTTV strategy.

According to the Critical Care Medicine paper breath stacking occurs frequently, is often unrecognized and is more frequent with lower set tidal volumes. Although breath stacking can be mitigated by increasing the tidal volume or via sedation (both of which are allowed for in the ARDSnet protocol) breath stacking in this patient series was frequent despite deep sedation.

An editorial in the same issue of the journal suggested that slavish adherence to the protocol may have served as a substitute for clinical judgment, leading to failure to recognize and address the unintended consequences. There’s a larger lesson to be learned here regarding the application of EBM.


Practical aspects of LTVV via NEJM.

ARDSnet resource page containing the LTVV protocol, determination of dosing body weight for tidal volume, the PEEP scale and more.

Ventilator graphics made easy.

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