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A novel index to assess cerebrovascular reactivity in preterm infants

Presented at the Neonatal Society 2011 Autumn Meeting.

Mitra S1, O’Reilly H1, Czosnyka M2, Smielewski P2, Austin T1

1 Neonatal Unit, The Rosie Hospital, Cambridge University Hospitals NHS Foundation Trust, UK
2 Department of Academic Neurosurgery, University of Cambridge, UK

Background: Cerebral autoregulation controls cerebral blood flow over a range of cerebral perfusion pressures and is a key protection for ensuring adequate perfusion and oxygenation of the brain. A number of indices of cerebrovascular reactivity have been developed; near infrared spectroscopy (NIRS) allows the non-invasive assessment of cerebral haemodynamics and oxygenation. Indices of cerebrovascular reactivity using NIRS have shown to correlate with invasive measures (1).
Using NIRS, we aimed to investigate the relationship between novel index of cerebrovascular reactivity, tissue oxygen heart rate reactivity (TOIHRx), and clinical outcome in a cohort of preterm infants in intensive care. Local ethics committee approval was granted for the study and data was collected only after written informed consent was obtained from the parent(s).

Methods: Thirty one preterm infants born at a median (range) gestational age of 26+1weeks (23+5 – 32+4) weeks were studied. The median (range) age at study was 2 (1-18 days of age). All infants had indwelling umbilical arterial catheters.
The cerebral tissue oxygenation index (TOI) was measured using the NIRO 200NX near-infrared spectrophotometer (Hamamatsu Photonics, KK, Japan). Other physiological variables were simultaneously recorded, and all data were collected and analysed using ICM+ software (2). Severity of clinical illness was assessed using the CRIB II score.
TOIHRx was calculated from moving correlation coefficient, using 5-minute time windows between 10-second average values of TOI and HR. An averaged value of TOIHRx for each infant was calculated by averaging over a 2-hour measurement period. Correlation between TOIHRx and CRIB II was assessed using linear regression analysis.

Results: The median (range) of TOIHRx was -0.0169 (-0.4611 – 0.4236). TOIHRx demonstrated a significant correlation with gestational age (R=-0.57, p<0.008), birthweight (R=-0.58, p=0.006) and CRIB II R= 0.55, p<0.0015).

Figure1 : Regression between the average values of TOIHRx and CRIB II for each of the 31 infants. Values of TOIHRx lie between -1 and +1. The more negative the value the more reactive the cerebral circulation is to changes in systemic haemodynamics while the more positive the value the more systemic changes are transmitted to the cerebral circulation.

Conclusion: TOIHRx is a novel indicator of cerebrovascular reactivity, demonstrating how cerebral oxygenation responds to changes in heart rate. Impaired regulation of cerebral vasculature in response to changes in heart rate was seen in infants with a worse clinical risk index score. TOIHRx may be of diagnostic value in identifying those infants with impaired cerebrovascular reactivity and worse clinical outcome.

Corresponding author: subhabrata.mitra@addenbrookes.nhs.uk

1. Zweifel C, Castellani G, Czosnyka M. et al. Noninvasive monitoring of cerebrovascular reactivity with near- infrared spectroscopy in head-injured patients. J Neurotrauma. 2010 Nov;27(11):1951-8
2. Smielewski P, Czosnyka M, Steiner L, et al. ICM+: software for on-line analysis of bedside monitoring data after severe head trauma. Acta Neurochir Suppl. 2005;95:43-9.

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