How Changes in Adaptive Support Ventilation Settings Affect Displayed Patient Effort and Work of Breathing Parameters: A Bench Study

Abstract

Background:

Adaptive support ventilation (ASV) has been proposed as a mode to generate a respiratory rate and tidal volume that will result in the minimum amount of work of breathing (WOB) for the patient. However, it is still unclear as to what degree manipulating the ASV settings will affect patient effort and WOB. Previously published work has suggested that manipulating ASV support can reduce patient WOB. However, when faced with a fixed patient effort, the displayed WOB and patient effort parameters can be difficult to interpret.

Methods:

Conducting a bench study using a lung simulator, 3 inspiratory muscle pressures (P_mus) were used as patient effort controls (-10, -20, and -30 cm H₂O). A frequency of 18 breaths/min, lung compliance of 50 mL/cm H₂O, and airway resistance of 10 cm H₂O/L/s were used for all stages of testing. A single patient profile was used. Three ASV %MinVol settings were assessed (60, 100, 140%). Occlusion pressure (P_0.1) and pressure time product (PTP) were measured when adjusting Ramp (0, 100, and 200 ms) and pressure trigger (-1, -2, and -3 cm H₂O) at each %MinVol and P_mus setting.

Results:

There was no observable difference in P_0.1 within P_mus groups (P_mus -10 cm H₂O: P_0.1 (mean SD ± ) -1.98 [cm H₂O], ± 0.08; P_mus -20 cm H₂O: P_0.1 -3.65, ± 0.09; P_mus -30 cm H₂O: P_0.1 -5.26, ± 0.15). However, there was a statistically significant difference in P_0.1 when P_mus -10 was compared with P_mus -30 ( -1.98 ± 0.08 vs -5.26 ± 0.15; P < .001) (Image 1). PTP values demonstrated an incremental rise that was similar within P_mus groups (P_mus -10 cm H₂O: 0.1 – 1.07 [cm H₂O-s/min]; P_mus -20 cm H₂O: 0.16 – 1.33; P_mus -30 cm H₂O: 0.2- 2.0). PTP values increased with changes in Ramp from 0-200 ms and pressure trigger from -1 to -3 cm H₂O (Image 2).

Conclusions:

Changes in ASV %MinVol setting did not affect P_0.1 within P_mus groups. P_0.1 values did increase with increasing inspiratory P_mus (-1.98, -3.65, and -5.26 cm H₂O) indicating an expected increased patient effort. The P_0.1 values at inspiratory P_mus of -20 and -30 cm H₂O did accurately represent excessive patient effort. The increase in PTP values secondary to changes in Ramp and pressure trigger settings validates that PTP does not accurately represent total WOB by the patient. In our model, there was no effort or work of breathing change within P_mus groups. Consequently, this data more accurately represents how well the ventilator settings (Ramp and trigger) are adapted to the patient rather than an actual change in work of breathing by the patient.