Tidal volume delivery from ICU ventilators at BTPS conditions: a bench study

BACKGROUND: Even though it is not a common practice, an external filter to the expiratory limb of the breathing circuit may protect the expiratory valve from water saturation in case of nebulization, or from the environment in case of lung infection with multi-drug-resistant micro-organisms or H1N1 influenza. METHODS: We added an external filter to the expiratory limb and measured tidal volume (VT) from 6 ICU ventilators: 2 with built-in expiratory filter (Avea, Puritan Bennett 840), and 4 without (Engstrom Carestation, Evita XL, Evita V500, and Servo-i), set in volume controlled mode, at BTPS (body temperature and pressure saturated) condition, with a heated humidifier and a lung model (compliance 16 mL/cm H2O, resistance 20 cm H2O/L/s) placed inside a neonatal incubator. The temperature was targeted at 37°C for both the heated humidifier and the incubator. The setup was run continuously for 24 hours. In the latter 4 ICU ventilators, a Hygrobac or Sterivent S external filter was placed upstream from the expiratory valve for an additional 24-hour period for each. At the end of this period, VT was measured at 4 nominal VT values (300, 400, 500, and 800 mL) with a pneumotachograph. The volume error computed from the ratio of set to measured VT (% set VT) was the primary end point. RESULTS: In these warm and wet conditions, volume error averaged 96 ± 3% for Avea, 100 ± 7% for Puritan Bennett 840, 90 ± 2% for Evita XL, 100 ± 7% for Evita V500, 105 ± 2% for Servo-i, and 108 ± 4% for Engstrom Carestation ( P < .001). With the Hygrobac the values were 93 ± 1% for Evita XL, 94 ± 4% for Evita V500, 110 ± 4% for Servo-i, and 99 ± 2% for Engstrom Carestation ( P < .001). With the Sterivent the corresponding values were 95 ± 2%, 105 ± 2%, 112 ± 5%, and 98 ± 2%, respectively ( P < .001). CONCLUSIONS: In BTPS condition, volume error differed substantially across ICU ventilators for VT delivery, with further significant changes occurring after addition of a filter at the distal expiratory limb.