The Science Journal of the American Association for Respiratory Care

2005 OPEN FORUM Abstracts

OTIS LEAST WORK EQUATION: THE EFFECT OF MINUTE VENTILATION ON OLW PROPOSED SETTINGS

Francoise Boubert, BA, David Carney, MD, Joseph DiRocco, MD, Katy Hojnowski, Ryan LaFollette; Nils-Tomas McBride, Jillian Norton, Gary Nieman, BA. Department of Surgery, SUNY Upstate Medical University, Syracuse, NY

Background: The goal of increasing minute ventilation (MV) in patients with acute ventilatory failure is to optimize alveolar ventilation and maintain a neutral pH. An ideal method to determine how changes in tidal volume and breathing rate may best accommodate the demand for increased MV while minimizing ventilation induced lung injury (VILI) does not exist. We postulate that ventilation determined by least work of breathing would promote protective ventilation at any given MV. This study applied the Otis least work of breathing formula (OLW) while varying minute ventilation and compliance to determine if OLW-proposed settings, based on limiting work of breathing, would also be lung protective in the setting of acute lung injury.

Methods: A test lung was ventilated with a Galileo GoldT with the following static settings: IBW = 70 Kg, PEEP 5 cmH20, Ti = 1.2 sec, resistance 5 cmH2O·s/L, Vt 9 cc/kg. Variable settings included minute ventilation (100, 200 and 300 cc/kg/min) and compliance (60, 30 and 15 mL/cmH20). The OLW-proposed settings for Respiratory rate (RR), tidal volume (Vt) and inspiratory time (Ti) were recorded for each combination of minute ventilation and compliance.

Results: Independent of MV, OLW consistently recommended a reduction in Vt and increased RR with decreasing compliance (Table).

Conclusions: The OLW formula consistently selects a low Vt (maximum 8.3 cc/kg) to achieve target minute ventilation and further reduces Vt with decreasing compliance. In the setting of acute lung injury, dynamic changes in assisted ventilation may be best determined by a strategy which limits work of breathing. This method appears to provide appropriate minute ventilation while remaining consistent with the principles of low volume ventilation, which has been shown to reduce VILI.

TABLE

Compliance (mL/cmH20) OLW Proposed RR (min-1) OLW Proposed Vt (cc/kg) OLW Proposed Ti (s)
MV 100 cc/kg/min
60 16 6.4 1.93
30 17 5.9 1.77
15 18 5.5 1.66
MV 200 cc/kg/min
60 26 7.8 1.07
30 29 7.0 1.05
15 31 6.5 0.97
MV 300 cc/kg/min
60 36 8.3 0.62
30 38 7.8 0.69
15 41 7.2 0.72
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