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

BACKGROUND

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

METHODS

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 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 was measured with a dedicated data logger and expressed in body temperature and pressure saturated. The relationships of measured V to set V were studied, and the relative error in V 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 was analyzed with linear regression.

RESULTS

For the 5 ventilators used, the slope (95% CI) between measured V versus set V 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 ( < .05 vs 1 in each instance), indicating a systematic under-delivery of V. The V relative error fell within the ±10% accuracy range for only 2 ventilators with adult circuits at f15 and f30. AH increased linearly with V. 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.

CONCLUSION

Low V 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.