The Impact on Driving Pressure Using the Time Controlled Adaptive Ventilation versus the ARDSnet ARMA Method to Adjust Mechanical Ventilation in Simulated ARDS

Abstract

Background:

ARDS remains a significant problem with high mortality. The only treatments are supportive mechanical ventilation (MV) and proning. However, an unintended ventilator-induced lung injury (VILI) can increase mortality if MV is set inappropriately. Key mechanisms of VILI are overdistension (OD) and repetitive alveolar collapse and expansion (RACE). Conventional MV attempts to minimize OD using a one-size-fits all lower tidal volumes (V_T), typically 6 mL/kg IBW. Higher PEEP, adjusted by oxygenation utilizing the ARDSnet PEEP/FIO₂ protocol, is used to minimize RACE. Driving pressure (ΔP) reflects characteristics of respiratory system compliance (C_RS). It has been shown that high ΔP impacts risk of VILI and ARDS-related mortality independent of V_T, PEEP, or plateau pressure (Pplat). We hypothesize that the personalized TCAV method to set and adjust the airway pressure release ventilation (APRV) mode is superior to the one-size-fits-all low V_T ARMA method at maintaining a low ΔP with reduced C_RS in a test lung.

Methods:

Two groups were studied using a test lung (ASL 5000) and a Dräger Evita ventilator at the following C_RS settings: normal lungs, mild, moderate, and severe ARDS. TCAV group: P_HIGH of 30 cm H₂O, P_LOW of 0 cm H₂O, T_HIGH of 5 s. and T_LOW set to achieve the termination of peak expiratory flow (T-PEF) of 75%. ARMA group: used the Volume Control - Assist Control mode with a V_T of 400 mL (simulating 6 mL/kg IBW), RR of 12 breaths/min, and inspiratory time of 1 s. PEEP of 5 cm H₂O was set for normal C_RS, and increased at each level of worsening C_RS, consistent with predictable settings for mild (8 cm H₂O), moderate (10 cm H₂O), and severe (14 cm H₂O), ARDS. At each compliance setting, we measured peak inspiratory pressure (PIP), Pplat, and driving ΔP (V_T/C_RS). Relationships between variables were measured by Pearson r correlation coefficient; groups were compared using a one-tailed unpaired student t-test. Alpha was set at 0.01.

Results:

ΔP was significantly lower in severe ARDS with TCAV method compared to ARMA method (P = 0.0063). TCAV: as C_RS decreased V_T decreased (from 680 mL to 372 mL; r = +0.82) and ΔP decreased from 25.4 to 10.5; r = -0.93). ARMA: as C_RS decreased ΔP increased (from 7 to 32 cm H₂O; r = -0.92) and PIP increased (from 15-46 cm H₂O; r = -0.95).

Conclusions:

The TCAV method is personalized and adaptive to changes in C_RS using the slope of the expiratory flow curve and thus will reduce V_T in response to a fall in C_RS, keeping the ΔP in the safe range.

Table 1. TCAV Group

 	NormalLungs	MildARDS	ModerateARDS	Severe ARDS
ΔP	10.5	10.6	12.7	25.4
PHIGH	30	30	30	30
VT	680	637	551	372

Table 2. ARMA Group

 	NormalLungs	MildARDS	ModerateARDS	SevereARDS
ΔP	7.0	9.0	14.0	32.0
PIP	15	19	26	46
VT	400	400	400	400