The Science Journal of the American Association for Respiratory Care

2008 OPEN FORUM Abstracts


Robert Chatburn1, Eduardo Eduardo Mireles-Cabodevila1

Background: Studies support the use of small tidal volumes (6-8 mL/kg) during mechanical ventilation to reduce ventilator induced lung injury. The extreme of this philosophy is high frequency ventilation (HFV), which requires specialized ventilators. Conventional adult ventilation, even with low volumes, is applied with relatively low frequencies (<35/min). No studies have described a method of determining an optimum frequency using conventional ventilators. The purpose of this study was to develop a mathematical model that predicts a patient-dependent frequency that minimizes tidal volume while maximizing alveolar minute ventilation (AMV) during conventional pressure controlled continuous mandatory ventilation (PC-CMV).

Methods: We modified the Marini et al model of PC-CMV (J Appl Physiol 1989, 67:1081-92) to include variable dead space fraction (DSF). The new model allows input of patient data including height, compliance, resistance, DSF, inspiratory pressure (IP), PEEP, and duty cycle (%I) and outputs AMV, tidal volume, mean airway, and autoPEEP as functions of frequency. If IP is adjusted until peak AMV equals physiologically required AMV then optimum frequency and volume are those that produce peak AMV. The model simulated ARDS using resistance = 10 cm H2O/L/s and compliance = 0.025 L/H2O, DSF = 0.45, height = 170 cm. We varied model parameters to identify the effects of lung mechanics and DSF on output variables.

Results: At nominal ARDS parameters the optimum frequency was 45/min delivering 4.3 mL/kg with autoPEEP = 1 cm H2O (see Figure). Changing resistance and compliance to double the time constant yielded optimum values at 28/min, 5.4 mL/kg. Increasing DSF to 0.60 yielded 46/min and 5.9 mL/kg. The lowest optimum tidal volume was always at %I = 50%; other values for %I yielded higher lower frequencies and higher tidal volumes.

Conclusion: Measured lung mechanics and dead space can predict optimum settings with this model. PC-CMV optimized for minimum tidal volume and maximum alveolar ventilation results in lower tidal volumes and higher ventilatory frequencies than conventionally used but below that requiring specialized ventilators.