Analysis of phase shift between pulse oscillations of macro- and microvascular cerebral blood flow in patients with traumatic brain injury

After a traumatic brain injury (TBI), monitoring of both macrovascular and microvascular blood circulation can potentially yield a better understanding of pathophysiology of potential secondary brain lesions. We investigated the changes in phase shift (PS) between cardiac-induced oscillations of cerebral blood flow (CBF) measured at macro (ultrasound Doppler) and microvascular (laser Doppler) level. Further we assessed the impact of intracranial pressure (ICP) on PS in TBI patients. A secondary aim was to compare PS to TCD-derived cerebral arterial time constant (τ), a parameter that reflects the circulatory transit time.

TCD blood flow velocities (FV) in the middle cerebral artery, laser Doppler blood microcirculation flux (LDF), arterial blood pressure (ABP), and ICP were monitored in 29 consecutive patients with TBI. Eight patients were excluded because of poor-quality signals. For the remaining 21 patients (median age = 23 (Q1: 20–Q3: 33); men:16,) data were retrospectively analysed. PS between the fundamental harmonics of FV and LDF signals was determined using spectral analysis. τ was estimated as a product of cerebrovascular resistance and compliance, based on the mathematical transformation of FV and ABP, ICP pulse waveforms.

PS was negative (median: −26 (Q1: −38–Q3: −15) degrees) indicating that pulse LDF at a heart rate frequency lagged behind TCD pulse. With rising mean ICP, PS became more negative (R = −0.51, p < 0.019) indicating that delay of LDF pulse increases. There was a significant correlation between PS and cerebrovascular time constant (R = −0.47, p = 0.03).

Pulse divergence between FV and LDF became greater with elevated ICP, likely reflecting prolonged circulatory travel time.