Using the Expiratory Flow Curve With APRV to Personalize Release Phase to Lung Compliance

Abstract

Background: ARDS remains a significant problem with an unacceptably high mortality. The only treatments are supportive mechanical ventilation (MV) and proning. However, if MV is set inappropriately unintended ventilator-induced lung injury (VILI) can significantly increase mortality. A key mechanism of VILI is repetitive alveolar collapse and expansion (RACE). Conventional MV attempts to minimize RACE by arbitrarily setting PEEP, usually adjusted by oxygenation. Using the APRV mode, PEEP can be set by adjusting release phase time (T_low) generating a time-controlled PEEP (TC-PEEP). We hypothesize that setting the release phase using the time-controlled adaptive ventilation (TCAV) method will personalize release phase time, tidal volume (V_T), and TC-PEEP to changes in respiratory system compliance (C_RS). We tested our hypothesis by varying the compliance settings on the ASL 5000 Lung Solution test lung.

Methods: The ASL 5000 test lung was used to simulate compliance in normal lungs, and then adjusted to progressively lower compliance in order to mimic mild, moderate, and severe ARDS. Ventilation was delivered via the Dräger Evita mechanical ventilator using the APRV mode of ventilation with a P_high of 30 cm H₂O, P_low of 0 cm H₂O, T_high of 5 s and T_low (also known as release phase time) set to achieve the termination of peak expiratory flow (T-PEFR) of 75% (an application of the TCAV method of setting/optimizing T_low). At each of the 4 levels of compliance we measured V_T, C_RS, TC-PEEP, and the angle of the slope (slope_angle) of the line connecting peak expiratory flow (PEF) with the termination of peak expiratory flow (T-PEF) at 75% of PEF. Relationships between variables were measured by Pearson r correlation coefficient.

Results: As C_RS decreased the slope_angle increased from 42° to 79° (coefficient of correlation of r = -0.85), V_T decreased from 680 mL to 372 mL (r = +0.82), release phase time decreased from 0.27 to 0.18 s (r = +0.75), and TC-PEEP decreased from 19 cm H₂O to 9 cm H₂O (r = +0.85).

Conclusions: Using the expiratory flow curve to adjust the release phase time via the TCAV method, V_T and TC-PEEP were personalized to the level of C_RS. When C_RS was low, V_T was low, and TC-PEEP was low. Thus, by using the TCAV method the V_T and TC-PEEP are set and adjusted according to patient lung pathophysiology and guided by the previous breath.