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