Tidal volume measurement error in pressure control modes of mechanical ventilation: A model study.

UNLABELLED: Tidal volume (VT) measurement during pressure control (PC) ventilation with preset inspiratory time may produce errors due to patient inspiratory effort. We evaluated VT error in 3 common ICU ventilators.

METHODS: Simulated patient: 60kg adult with ARDS using IngMar Medical ASL 5000 having moderate inspiratory effort. Ventilators evaluated: Covidien PB 840, Maquet Servo-i, and Dräger Evita XL, PC breaths at preset inspiratory time (TI) 0.6-1.4s. VT error was defined as ventilator displayed VT minus the simulator displayed VT (mL/kg or % of true).

RESULTS: Relaxation of inspiratory effort caused flow reversal (exhalation) during TI, which led to VT error. For the PB 840, VT error was proportional to TI (maximum -2.0mL/kg, -19%). For the Servo-i, VT error was not related to TI (maximum error -0.2mL/kg or -1.2%). Volume error for Evita XL was not related to TI (maximum error was -0.7mL/kg or -6%).

CONCLUSIONS: Calculation of VT as the integral of flow over the preset inspiratory time rather than the period between zero crossings of flow may result in underestimation of both inhaled and exhaled volumes. The size of VT error can be large enough to potentially affect patient outcomes on some ventilators.