The objective of the study was to evaluate the wavelet spectral energy of oscillations in the intracranial pressure (ICP) signal in patients with acute traumatic brain injury (TBI). The wavelet phase coherence and phase shift in the 0.006-2 Hz interval between the ICP and the arterial blood pressure (ABP) signals were also investigated. Patients were separated into normal or impaired cerebrovascular reactivity, based on the pressure reactivity index (PRx). Spectral energy, phase coherence and phase shift in the low frequency and cardiorespiratory intervals were compared for the two groups. Data were prospectively collected and analyzed retrospectively in 22 patients, within the first week after acute TBI. The ICP and ABP signals were continuously recorded for 40 min and the wavelet transform was used to calculate the spectral energy and phase of the signals. The average ICP wavelet energy spectrum showed distinct peaks around 1.0 (cardiac), 0.25 (respiratory) and 0.03 Hz. Patients with normal cerebrovascular reactivity (negative PRx) had 38.6 % (+/- SD 16.7 %) of the mean wavelet energy below the lower limit of the respiratory frequency band (0.14 Hz) compared to only 18.1 % (+/- SD 17.8 %) in patients with altered cerebrovascular reactivity (positive PRx) (difference: p = 0.0057). Wavelet phase coherence between the ABP and ICP signals was statistically significant (p < 0.05) in the 0.006-2 Hz interval. The phase shift between the ABP and ICP signals was around zero in the 0.14-1.0 Hz interval. Seven patients with PRx between -0.4943 and -0.1653 had a phase shift in the interval 0.07-0.14 Hz, whereas 15 patients with PRx between -0.1019 and 0.3881 had a phase shift in the interval 0.006-0.07 Hz. We conclude that the wavelet transform of the ICP signal shows spectral peaks at the cardiac, respiratory and 0.03 Hz frequencies. Normal cerebrovascular reactivity seems to be manifested as increased spectral energy in the frequency interval < 0.14 Hz. A phase shift between the ICP and ABP signals in the interval 0.07-0.14 Hz indicates normal cerebrovascular reactivity, while a phase shift in the interval 0.006-0.07 Hz indicates altered cerebrovascular reactivity.
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