Low Tidal Volume Ventilation Can Result in High Driving Pressure With Low Compliance

Abstract

Background: ARDS remains a significant problem with high mortality. The only treatments are mechanical ventilation (MV) and proning. However, unintended ventilator-induced lung injury (VILI) can increase mortality if MV is set inappropriately. Key mechanisms of VILI are repetitive alveolar collapse and expansion and overdistension. Conventional MV attempts to minimize both mechanisms using lower tidal volumes (V_T), typically 6 mL/kg IBW, combined with higher PEEP adjusted by oxygenation and directed by the ARDSnet PEEP/FIO₂ protocol. Driving pressure (ΔP) reflects respiratory system compliance (C_RS) characteristics. High ΔP correlates with increased ARDS-associated mortality better than higher V_T, PEEP, and plateau pressure (Pplat). We hypothesize that using a one-size-fits-all V_T (6 mL/kg) will cause an unsafe rise in ΔP with very low C_RS associated with severe ARDS. We tested our hypothesis by varying compliance settings on the ASL 5000 test lung and measuring ΔP while maintaining a constant low V_T (simulated 6 mL/kg).

Methods: The ASL 5000 test lung was first set at C_RS of normal lungs, and then lowered to mimic mild, moderate, and severe ARDS. Ventilation was delivered via the Dräger Evita ventilator using the volume control-assist control mode with a V_T of 400 mL (reflecting current standard of care by simulating 6 mL/kg IBW), RR = 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 compliance, consistent with typical settings for mild (8 cm H₂O), moderate (10 cm H₂O), and severe (14 cm H₂O) ARDS. We measured peak inspiratory pressure (PIP) and Pplat and calculated C_RS (volume/pressure) and ΔP (V_T/C_RS). Relationships between variables were measured by Pearson r correlation coefficient.

Results: With a one-size-fits-all V_T (400 mL), as C_RS decreased, PIP increased from15 to 46 cm H₂O (coefficient of correlation of r = -0.95), and ΔP increased from 7 to 32 cm H₂O (r = -0.92). This suggests that the one-size-fits all V_T of 6 mL/kg may cause VILI in patients with very low C_RS. Thus, a personalized approach with ventilation adjustments based on patient pathophysiology may be required to further reduce ARDS-related mortality.

Conclusions: As expected, we demonstrated that ΔP would decrease at the same V_T as compliance increases (P <0.001). In non-compliant lungs, even low V_T breaths resulted in high PIPs and high ΔPs and are not lung protective in severe ARDS with very low C_RS.