Mechanical ventilation with overly long inspiratory times and inadequate expiratory times leads to the development of intrinsic positive end expiratory pressure (PEEPi) in patients with obstructive lung disease. Facile means of defining appropriate inspiratory and expiratory times have, however, not been defined or investigated. We have created a method for defining ventilator therapy which avoids the development of hyperinflation and PEEPi.
The development of PEEPi at various inspiratory to expiratory (I:E) ratios with different airway resistances (Raw) was modeled using a series of artificial airway resistances in line with a three-liter test lung utilizing a Puritan Bennett 7200A ventilator. With this model was developed a family of curves of PEEPi vs. duty cycle at various resistances, from which was derived an equation which predicts PEEPi from Raw and duty cycle. The level of PEEPi thus predicted was compared with that which had been measured in thirteen ventilator dependent patients.
In the 13 patients tested, PEEPi predicted by our equation fell within 1cm H2O of the observed values of PEEPi in all cases. Invariably, use of the ramp flow profile more than doubled inspiratory time for any peak flow rate.CONCLUSIONS: The equation developed here seems useful in the adjustment of ventilator settings to avoid the production of PEEPi. With a measured Raw greater than 20 cm H2O/L/sec. duty cycles of 0.5 or greater will result in substantial levels of PEEPi.CLINICAL IMPLICATION: Use of the equation developed here can facilitate safer ventilator management for patients with severe airway obstruction. The ramp flow profile should be used with caution when Raw is greater than 20 cm H2O/L/sec since short inspiratory times are clearly needed in this setting, and the ramp profile more than doubles the inspiratory time for any given peak flow rate.
W.D. Marino, None.