The Effect of Patient Circuit Compensation for Different Ventilator Modes

Background:

A key element of lung-protective ventilation is to control tidal volume within 4–8 mL/kg of predicted body weight. In volume-targeted modes, a portion of tidal volume is lost due to compression in the patient circuit. The automatic compensation for compressible volume by the ventilator is important. Many critical care ventilators provide patient circuit compensation to counteract the loss of volume in ventilator circuits based on circuit compliance (C_circuit) and airway pressure (P_aw). The purpose of the study was to assess how inspiratory effort affects volume delivery with or without circuit compensation.

Methods:

This study was conducted using the IngMar Active Servo Lung 5000 (ASL 5000), programmed to simulate an adult patient with moderate ARDS (compliance = 40 mL/cm H₂O and resistance = 10 cm H₂O/L/s) and different inspiratory efforts. The effort models were set to maximum muscle pressure, Pmax = 0, 5, 10, 15 cm H₂O. The experiment was performed with a heated humidifier in the patient circuit, filled with water, but not turned on. Two modes were used on a Servo-U ventilator: volume-controlled continuous mandatory ventilation with set-point targeting scheme (VC-CMVs) and pressure-controlled continuous mandatory ventilation with adaptive targeting scheme (PC-CMVa). The outcome variable was the difference between tidal volume with and without circuit compensation for passive and active breathing (ΔV_T). ΔV_T = (P_aw-PEEP)/C_circuit. We recorded the mean of inspired tidal volume as measured and analyzed by the ASL 5000 for 10 breaths after stabilization.

Results:

Data are shown in the graph below. ΔV_{T decreases as Pmax increases.}

Conclusions:

The effects of circuit compensation are affected by the simulated inspiratory effort, as expected. For volume-targeted modes, increasing effort decreases P_aw. Decreasing P_aw decreases ΔV_T.

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