Accuracy of Delivery and Effects on Absolute Humidity of Low Tidal Volume by ICU Ventilators

During extracorporeal membrane oxygenation for ARDS, a range of 1-4 mL/kg predicted body weight tidal volume (V_T) is commonly used. We explored whether such a low V_T could be adequately delivered by ICU ventilators, and whether such low V_Ts prevent the heated humidifier (HH) from reaching the recommended target of 33 mg/L absolute humidity (AH).

We attached a lung model to 5 ICU ventilators set in volume controlled mode and body temperature and pressure saturated. We ran 2 protocols over a 100-280 mL V_T range used with adult or neonatal breathing circuit at a breathing frequency (f) of 15 (f15) or 30 (f30) breaths/min. In the first protocol, with the HH off, V_T was measured with a dedicated data logger and expressed in body temperature and pressure saturated. The relationships of measured V_T to set V_T were studied, and the relative error in V_T was analyzed within its 10% boundaries. In the second protocol, the HH was on, and we measured AH using a psychrometric method. The relationship of AH to set V_T was analyzed with linear regression.

For the 5 ventilators used, the slope (95% CI) between measured V_T versus set V_T averaged 0.93 (0.92-0.93), 0.93 (0.93-0.94), 0.91 (0.90-0.91), and 0.91 (0.90-0.91) mL/mL for adult and neonatal circuits at f15 and f30, respectively (P < .05 vs 1 in each instance), indicating a systematic under-delivery of V_T. The V_T relative error fell within the ±10% accuracy range for only 2 ventilators with adult circuits at f15 and f30. AH increased linearly with V_T. The recommended target of 33 mg/L AH was reached with all of the ventilators for the adult circuit at f30 only. The minimum volume that met the recommended threshold for AH was 100, 150, 190, 160, and 100 mL for the G5, Carestation, PB980, Servo-U, and V500 ventilators, respectively, at f30.

Low V_T was systematically under-delivered by modern ICU ventilators by roughly 7-9%. To meet the recommended target of 33 mg/L AH, adult circuit at f30 should be used.