A Bench Study Evaluating Gas Temperature and Absolute Humidity During Pediatric Airway Pressure Release Ventilation

Background:

Heated humidifiers (HH) optimally operate with continuous or near-continuous flow. Standard HH performance requires gas temperatures (GT) 34°C - 41°C and absolute humidity (AH) 33 - 44 mg H₂O/L. APRV incorporates a prolonged inspiratory phase creating no-flow conditions. Adjunct techniques used with APRV including external flow bled-in at 2 L/min or humidification augmentation with maximum humidity compensation may influence GT or AH. We sought to determine if APRV settings would provide acceptable levels of GT and AH using a pediatric ARDS lung model. Our null hypothesis is that during pediatric APRV, there is no difference in GT or AH when comparing PC-CMV with APRV, APRV using external flow (APRV-EF), and APRV with humidity compensation (APRV-HC).

Methods:

A conventional ventilator (Servo-i, Maquet) was equipped with an adult heated circuit (RT280, Fisher & Paykel) and connected to a HH (MR850, Fishery & Paykel). The ventilator was calibrated to the circuit with a pre-use check. A hygro-thermometer (Humidity Alert II, Extech) calibrated to NIST standards was placed at the circuit Y-piece. The ventilator circuit was connected to a test lung (ASL 5000, IngMar Medical) set to simulate a 10-year-old with ARDS (compliance 13 cm H₂O L/s, resistance 30 mL/cm H₂O, 6.0 ETT). Targeted lung protective ventilation settings are as follows: PC-CMV (frequency 20 breaths/min, PC 20 cm H₂O, PEEP 12 cm H₂O) and APRV (Phigh 30 Plow 0 Thigh 4 s Tlow 0.2 s). PC-CMV was initiated with heated humidity for 1 h. After each intervention change (PC-CMV -> APRV -> APRV-EF -> APRV-HC) a 10-min stabilization period preceded temperature and relative humidity measurements (every 5 min for 30 min) and AH was calculated from these measurements. Analysis was performed with a one-way ANOVA and post-hoc Tukey, as appropriate (SPSS, v24, IBM, Armonk, NY). Data was normally distributed for both GT and AH. Alpha (2-tail) set at .05.

Results:

There was a statistically significant difference in GT (P < .001) and AH (P < .001) between all interventions . The magnitude of the effect size was large for GT (w² = .92) and AH (w² = .91). Post-hoc analysis revealed a significant difference between all interventions in GT (P < .001) and AH (P < .001), except APRV-EF and APRV-HC groups in both GT (P = .51) and AH (P = .57).

Conclusions:

During APRV with low lung compliance, heating and humidification of inspired gas may not meet recommended standards for invasive mechanical ventilation regardless of adjunct techniques.

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