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Cerebral Doppler Resistance Index in Hypoxic-Ischaemic Encephalopathy: Is it still valid in the cooling era?

Presented at the Neonatal Society 2010 Autumn Meeting.

Elstad M1,2, Whitelaw A1, Thoresen M1

1 School of Clinical Sciences, University of Bristol, Bristol, UK
2 Institute of Basic Medical Sciences, University of Oslo, Oslo, Norway

Background: Hypoxic-ischaemic encephalopathy (HIE), if severe, may involve vasoparalysis with reduced cerebral vascular resistance. When measured at 24 hours or later, a resistance index (RI) ≤0.55 measured by ultrasound Doppler has a positive predictive value (PPV) of about 80% for death or disability (1). All the published studies have measured RI at normal body temperature. As temperature may affect the circulation and hypothermia has now become standard therapy, our aim was to determine if RI had the same predictive power in cooled infants. This study was approved by the local research ethics committees.

Methods: 125 infants (gestational age≥36 weeks) who received therapeutic hypothermia in Bristol for HIE grade II or III with outcome data and at least one measurement of RI at 24 hours or later were identified retrospectively and included in the analysis. Poor outcome was defined as death or severe disability or, if under 18 months, MRI changes predictive of disability as in the TOBY trial (2,3).

Results: RI≤0.55 at or after 24 hours had a sensitivity of 63% (95% CI, 49% to 75%) and specificity of 76% (65%, 84%) for predicting outcome after HIE. The PPV was 60% (45%, 74%), while the negative predictive value was 78% (67%, 86%). The positive likelihood ratio was 2.62 (1.67 to 4.11) and the negative likelihood ratio was 0.49 (0.33 to 0.72). The diagnostic odds ratio was 5.39 (2.44, 11.87).

Conclusion: Three out of four cooled infants with RI>0.55 will have good outcome. On the other hand, only 6 out of 10 cooled infants with RI≤0.55 will have poor outcome, compared to 8 out of 10 at normothermia. This could be because cooling changes the cerebral circulation. Alternatively, cooling may have no effect on cerebral vasculature but protects the brain at a molecular and cellular level, thus changing the predictive power of RI. Doppler RI cannot be used on its own for prediction but is a quick, easy and safe measurement which adds to the prognostic jigsaw.

Corresponding author: marianne.thoresen@bristol.ac.uk

References
1. Archer LN, Levene MI, Evans DH. Lancet 1986; 2(8516):1116-1118.
2. Azzopardi DV, et al. N Engl J Med 2009; 361(14):1349-1358.
3. Rutherford M, et al Lancet Neurol. 2010;9:39-45.

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