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Ventilation – how often are we correct?

Intensive Care Unit, Sir Charles Gairdner Hospital, Perth, Western Australia, Australia

Summary

Traditional mechanical ventilation used tidal volumes (Vt) of between 10 to 15 ml/kg of body weight in order to achieve normal values of pH and partial pressure of carbon dioxide (PaCO2). Many clinicians today however, adopt lower volumes as a conservative ‘safe’ ventilation strategy in most mechanically ventilated patients. The method by which this is done varies between facilities, but anecdotally doctors use Vt of 6 to 8 ml/kg, and they commonly estimate these volumes at the bedside. This observational study was undertaken in a 23-bed level 3 intensive care unit at a metropolitan tertiary hospital in order to determine whether or not intensive care clinicians are accurately determining the Vt during mechanical ventilation which they purport to do. The primary outcome measure was the Vt being delivered at the time of observation. Thirty patients were recruited into the study, resulting in 55 observations of synchronised intermittent mandatory ventilation with autoflow mode ventilator settings. Although volumes between 6 to 8 ml/kg were recorded in 33 (60%) observations, more detailed exploration of the individual’s clinical circumstances reflects that the actual dialled volumes were correct in all but two patients. Intensive care unit mortality was 13% (n=2) in those patients receiving higher than anticipated Vts (n=15). This study has demonstrated that while we achieve a protective ventilation strategy by adopting lower Vts in most mechanically ventilated patients, we should be constantly monitoring exactly what volume is being achieved, not just what is dialled up to be delivered.

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