August 2002 / Volume 47 / Number 8 / Page 898
The Effects of Tidal Volume Demand on Work of Breathing During Simulated Lung-Protective Ventilation
During assisted mechanical ventilation (AMV) flow asynchrony increases the work of breathing (WOB) performed by the patient because (it is thought) the ventilator fails to push the inspired gas at the same flow as the inspiratory muscles attempt to pull gas into the lungs. In this "push-pull system," total WOB is thought to remain relatively constant, with only the distribution of work between the patient and the ventilator varying because of patient effort. Yet it is also speculated that an "additional inspiratory load" occurs when patient flow demand exceeds the ventilator flow delivery and "the patient tries to accelerate gas against the inspiratory resistance of the ventilator circuit," which implies that the ventilator creates imposed WOB (WOBimp) based on patient effort. However, this theory has not been confirmed by actually measuring WOBimp during AMV.
Increased WOB from flow asynchrony occurs because patient flow demand approximates the contractile velocity of the inspiratory muscles. By extension, tidal volume (VT) demand reflects global inspiratory muscle shortening. Hence, WOBimp may occur during AMV when ventilator VT delivery does not meet patient VT demand. Patient-ventilator VT mismatching may become a problem when lung-protective ventilation (LPV) is used to treat acute respiratory distress syndrome (ARDS). LPV requires a VT between 4 and 7 mL/kg, which can be less than the spontaneous VT reported in ARDS patients (300-400 mL). Recently, a case of apparent VT mismatching resulting in acute alveolar edema was reported in a patient enrolled into the National Institutes of Health ARDS Network study.
Does a specific manipulation of the ventilator flow waveform and/or inspiratory time (TI) effectively reduce WOB during VT mismatching? Both pressure-control ventilation (PCV) and volume-assured pressure support ventilation are associated with less patient WOB than the constant flow pattern commonly used during volume-control ventilation (VCV). But the difference in efficacy between VCV with a fixed decelerating flow pattern and the variable decelerating flow pattern of PCV and volume-assured pressure support has not been studied. Alternatively, increasing the inspiratory flow during VCV with constant flow pattern (VCV-CF) by shortening ventilator TI causes similar reductions in patient effort compared to pressure-limited ventilation. Yet if the patient's TI exceeds that of the ventilator, "double-triggering" (ie, 2 ventilator breaths for a single breathing effort) may occur and frustrate attempts to achieve LPV.