Continuous monitoring of cerebrovascular reactivity through pulse transit time and intracranial pressure

Objective: Cerebrovascular reactivity (CR) is a mechanism that maintains stable blood flow supply to the brain. Pressure reactivity index (PRx), the correlation coefficient between slow waves of invasive arterial blood pressure (ABP) and intracranial pressure (ICP) has been validated for CR assessment. However, in clinical ward, not every subarachnoid hemorrhage (SAH) patient has invasive ABP monitoring. Pulse transit time (PTT), the propagation time of a pulse wave travelling from the heart to peripheral arteries, has been suggested as a surrogate measure of ABP. In this study, we proposed to use PTT instead of invasive ABP to monitor CR. Approach: Forty-five SAH patients with simultaneous recordings of invasive ABP, ICP, oxygen saturation level (SpO2) and electrocardiograph (ECG) were included. PTT was calculated as the time from the ECG R-wave peak to the onset of SpO2. PTT based pressure reactivity index (tPRx) was calculated as the correlation coefficient between slow waves of PTT and ICP. Wavelet tPRx (wtRx) was calculated as the cosine of wavelet phase shift between PTT and ICP. Meanwhile, PRx and wPRx were also calculated using invasive ABP and ICP as input. Main results: The result showed a negative relationship between PIT and ABP (r = -0.58,p < 0.001). tPRx negatively correlated with PRx (r = -0.51, p = 0.003). Wavelet method correlated well with correlation method demonstrated through positive relationship between wPRx and PRx (r = 0.82,p < 0.001) as well as wtPRx and tPRx (r = 0.84, p < 0.001). Significance: PTT demonstrates great potential as a useful tool for CR assessment when invasive ABP is unavailable.