The Science Journal of the American Association for Respiratory Care

Original Contributions

August 2002 / Volume 47 / Number 8 / Page 898

The Effects of Tidal Volume Demand on Work of Breathing During Simulated Lung-Protective Ventilation

Richard H Kallet MSc RRT, James A Alonso RRT, Martha Diaz RRT, Andre R Campbell MD, Robert C Mackersie MD, and Jeffrey A Katz MD

BACKGROUND: Lung-protective ventilation (LPV) can result in a ventilator tidal volume (VT) below patient VT demand, which may elevate work of breathing (WOB). Increasing the ventilator inspiratory flow may not sufficiently reduce WOB, because the patient's flow-time requirements may exceed the ventilator's flow-time delivery pattern. We investigated (1) the effects of VT demand on WOB during LPV and (2) which ventilator pattern best reduced WOB while achieving LPV goals. METHODS: A standard WOB lung model simulated assisted breathing. Using 3 ventilators (Hamilton Veolar, Hamilton Galileo, and Dräger Evita 2 dura), we tested volume-control ventilation with a constant flow pattern (VCV-CF), volume-control ventilation with a decelerating flow (VCV-DF), and pressure-control ventilation (PCV). Simulated VT demand was increased from 50-125% of the ventilator-delivered VT (400 mL) as ventilator inspiratory time (TI) was decreased (0.95, 0.80, 0.65, and 0.45 s) relative to simulated TI (0.8 s). WOB was measured with a pulmonary mechanics monitor. RESULTS: During VCV-CF and VCV-DF, a VT demand of > or = 100% drastically increased WOB, attributable to imposed WOB from the inspiratory valve. Increasing inspiratory flow by using the decelerating flow pattern and/or decreasing TI reduced WOB, but generally not to normal levels. "Double-triggered" breaths, with excessive VT delivery, often occurred when ventilator TI was well below simulated TI. PCV was most effective in reducing WOB, but VT delivery exceeded the LPV target unless TI was reduced. CONCLUSIONS: Given our dual goals of reducing both WOB and VT during LPV, VCV-DF with relatively brief TI appeared to be the best option, followed by PCV with a relatively brief TI.
Key words: acute lung injury, assisted mechanical ventilation, imposed work of breathing, inspiratory flow rate, inspiratory flow waveform, lung model, lung-protective ventilation, tidal volume, work of breathing.
[Respir Care 2002;47(8):898–909]

Introduction

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.

The entire text of this article is available in the printed version of the August 2002 RESPIRATORY CARE.

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