Abstract SNACC-15

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Non-invasive investigation of cerebral autoregulation and flow metabolism coupling following acute brain injury

1Highton D, 2Elwell C, 1Smith M
1University College London Hospitals, London, , London; 2University College London, London, , London

Impaired cerebrovascular reactivity may potentiate secondary cerebral hypoxia-ischaemia after acute brain injury (ABI). Cerebral autoregulation (CA) maintains cerebral perfusion across a range of perfusion pressure, and various indices of CA have been promoted to monitor the injured brain [1]. However, none directly assesses the adequacy of brain perfusion. Flow metabolism coupling mediates the haemodynamic response to increased metabolic demand, resulting in an increase in cerebral oxy- [HbO2] and reduction in deoxy- [HHb] haemoglobin. Measurement of such haemoglobin changes is the basis of functional MRI (fMRI) and functional near infrared spectroscopy (NIRS), a non-invasive optical technique. Following ischaemia a different pattern of change is seen in response to increase in metabolic demand – a rise in [HbO2] and [HHb] [2]. This study investigates whether impaired CA is associated with this same pattern of disturbed flow metabolism coupling. METHODS: Ventilated, sedated patients with ABI were recruited following ethics approval and representative consent. One hour datasets were gathered from transcranial Doppler (TCD) ultrasound and in-house developed broadband NIRS. Scaled absolute [HbO2] and [HHb] were derived using spatially resolved spectroscopy from 4 detectors at 20mm to 35mm from the light source, placed on the scalp over frontal cortex avoiding the midline. The mean velocity index (Mx) a continuous index of CA, was derived from blood pressure and TCD [1]. The relationship between [HbO2] and [HHb] during spontaneous oscillations was investigated using wavelet semblance (0.1Hz – 0.003Hz) where a measure of phase tending towards 1 indicates in-phase (abnormal response) and -1 out-of-phase (normal response) [3]. The [HbO2]/[HHb] phase relationship was compared to Mx using Pearson correlation. RESULTS: 20 patients were studied during 36 recording episodes. The mean [HbO2]/[HHb] phase relationship was 0.15+/-0.36(sd). Mean Mx correlated with [HbO2]/[HHb] phase (r=0.43 p=<0.01). The figure illustrates the relationship between blood pressure, Mx and phase from every 10 second time point – identifying a pressure “plateau” where both phase and Mx tend towards a normal value. DISCUSSION: We identified abnormal [HbO2]/[HHb] oscillations in ABI that are consistent with fMRI findings in ischaemia [2]. These are associated with impaired CA which might be attributed to the underlying brain injury or an interaction between the two mechanisms. [HbO2]/[HHb] relationships hold considerable promise as a marker of metabolic compromise and can be monitored non-invasively using NIRS alone. Further work is required to elucidate the exact pathophysiology encoded in these optical measurements, and temporal relationships which might identify windows for intervention.
[1] Zweifel,C Medical Engineering & Physics.(2014);36(5), 638–645
[2] Takemura,N.Adv.Exp.Med.Biol.(2014);812,225–231
[3] Cooper,G.R Computers & Geosciences.(2008),34(2), 95–102

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