Background and aims Hypoxic-ischaemic encephalopathy (HIE) is an important cause of cerebral damage and long-term neurological sequelae. Over-ventilation in infants with HIE can lead to hypocarbia and consequent cerebral vasoconstriction further increasing the risk of brain injury. Our aim was to assess the incidence of hypocarbia in HIE and identify infants at increased risk of hypocarbia.

Methods Retrospective review of term admissions with HIE to a tertiary neonatal intensive care unit from 2008 to 2013. Hypocarbia was defined as a partial blood pressure of CO₂ (pCO₂) ≤ 4 kPa.

Results 74 infants were reviewed. The median (interquartile range) gestational age was 40 (38–41) weeks. 47 (64%) were actively cooled. Arterial cord pH was 6.99 (6.80–7.10). 40 (54%) infants had hypocarbia on day 1: pCO₂ was 3.35 (2.84–3.73) kPa and duration of hypocarbia was 160 (120–300) minutes. 48 (64.9%) were ventilated: 11 (22.9%) with volute targeted-ventilation, 36 (75.0%) with non-volume-targeted ventilation and 1 (1.4%) with high-frequency oscillation. Lowest CO₂ was not significantly different but duration of hypocarbia was significantly longer (p < 0.05) in infants on non-volume-targeted ventilation [200 (180–390) minutes] compared to infants on volume-targeted ventilation [120 (90–225) minutes]. On day 1, a pCO₂ <4 kPa was recorded in 36 of 48 infants that were mechanically ventilated (75%), compared to 4 of 26 that were spontaneously breathing (15.4%) [Odds ratio: 16.5, Confidence Interval: 4.73–57.76].

Conclusions Hypocarbia is frequently encountered in HIE. Mechanical ventilation of infants with HIE should aim to avoid hypocarbia by applying “neuroprotective” ventilation strategies such as volume-targeted ventilation.