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Cumulative Index of Exposure to Hypocarbia during Transport in Neonates with Hypoxic Ischaemic Encephalopathy receiving Therapeutic Hypothermia

Presented at the Neonatal Society 2017 Spring Meeting.

Nalliannan R1, Shaik NB1, Le Poidevin A1, Thwaites R2, Johnson M1,3, Gupta N1

1 Department of Neonatal Medicine, University Hospital Southampton NHS Foundation Trust and Southampton Oxford Neonatal Transport Service (SONeT), UK
2 Neonatal Unit, Portsmouth Hospitals NHS Trust, UK
3 NIHR Biomedical research Centre Southampton, University Hospital Southampton NHS Foundation Trust and University of Southampton, UK

Background: Early periods of hypocarbia in hypoxic ischemic encephalopathy (HIE) have been associated with adverse neurodevelopment outcomes. Avoidance of hypocarbia in these neonates during transfer to cooling centres can be challenging. The aim of the study was to determine the cumulative index of exposure (CIE) to hypocarbia during transfer of neonates with HIE requiring therapeutic hypothermia and its correlation to clinical outcome.

Methods: Retrospective study over a 4-year period including neonates with HIE requiring therapeutic hypothermia who were transferred to a cooling centre. Ventilation, blood gas and transport details were collected. Hypocarbia was defined as pCO2 < 4.5kPa and CIE was calculated by multiplying the extent of the hypocarbia (4.5kPa minus the lowest pCO2) by the number of hours until normocarbia was achieved. Regression modelling was carried out to ascertain the relationship between CIE and outcomes.

Results: Fifty infants were included in the study. Hypocarbia occurred in 68% neonates with median pCO2 of 4.1kPa (range 2.1- 7.8). Linear regression modelling with adjustment for cord gas base deficit and ventilation mode demonstrated that an increase in CIE of 1 kPa.hour was associated with an increase in HIE grade of 0.02 (p=0.018). Modelling CIE as a 5-level variable demonstrated that only the highest quartile for CIE (>14.2 kPa.hours) was significantly associated with grade of HIE (p=0.002, see figure). There were no significant associations between CIE with ventilation mode.

Cumulative Index of Exposure to Hypocarbia during Transport in Neonates with Hypoxic Ischaemic Encephalopathy receiving Therapeutic Hypothermia

Conclusion: Increase in CIE to hypocarbia was associated with severity of HIE. Avoidance of hypocarbia poses a challenge not only due to ventilatory support, but also clinical state. Therapeutic interventions to avoid hypocarbia besides ventilatory strategy need to be explored.

Corresponding author: regelle83@yahoo.com

References
1. Azzopardi D et al. Moderate hypothermia to treat perinatal asphyxial encephalopathy. NEJM. 2009; 361; 1349- 1358 2) Pappas A et al. Hypocarbia and Adverse Outcome in Neonatal HIE. J Pediatr. 2011; 158(5): 752-758
2. Pappas A, Shankaran et al. Hypocarbia and Adverse Outcome in Neonatal Hypoxic -Ischaemic Encephalopathy. J Pediatr. 2011; 158(5): 752-758.

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