Low inflating pressures during neonatal tidal volume targeted ventilation: occurrence and significance

Objectives: During volume targeted ventilation, the ventilator’s algorithm adjusts the peak inspiratory pressure (PIP) to maintain the expired tidal volume close to the target. In hyperventilating infants PIP is frequently very low, just above the level of positive end-expiratory pressure (PEEP). The impact of this on ventilator parameters and blood gases was investigated. Methods: Data were collected and analysed computationally from 195 infants over 968 days of mechanical ventilation. The median ventilator inflating pressure (Pinfl, which is PIP minus PEEP) was determined before each blood gas (n=3,371) for periods of different duration between 15 minutes and 24 hours. Ventilator parameters and blood gases were compared between periods when the inflating pressure was <5 mbar and periods when it was higher. Results: 1-hour periods when median Pinfl was <5 mbar occurred in 30% of the babies. These were associated with similar tidal volumes and minutes ventilation as periods with higher Pinfl, despite the lower target tidal volume. Babies triggered more ventilator inflations, had more spontaneous breaths and had lower their oxygen requirement when Pinfl was low. Low inflating pressures were associated with lower blood carbon dioxide levels and did not lead to acidosis. Lactate levels were higher but the group included more babies with perinatal hypoxia. Analyzing data over shorter (15 or 30 minutes) or longer (up to 24 hours) periods before the blood gases gave similar results. Conclusions: Episodes of low inflating pressure occur frequently in babies receiving volume targeted ventilation. This does not lead to exhaustion, hypercapnia or acidosis.