Relation of pulse features between intracranial pressure and carotid blood flow waveforms

Background: Measurement of intracranial pressure (ICP) is an invasive procedure prohibitive for routine measurement in all but extreme cases. ICP is pulsatile at the frequency of the cardiac cycle. The aim of this study was to investigate the relationship between elements of the ICP and cardiovascular waveforms. Method: Sprague-Dawley rats (18–21 weeks of age, 3 male, 3 female) were anaesthetised (1.3 g/kg urethane) and instrumented to measure and record aortic blood pressure, carotid blood flow, and ICP waveforms. A pacing electrode was used to change heart rate (HR) from resting rate (approximately 300 bpm) upward to 400 and 500 bpm. Measurements were taken at baseline ICP and a raised ICP (infusion of artificial intracranial fluid into the intracranial space). Aortic mean blood pressure was controlled at 100 mm Hg. The minima (p0) and first and second peaks (p1 and p2) of the ICP and corresponding peaks in the carotid flow waveforms were detected. Peak ratios (R) were calculated as (p2-p0)/(p1-p0) for both ICP (RICP) and carotid flow (Rflow). Results: The relationship to ICPmean, and HR was investigated using mixed effects modelling. Rflow had a significant, positive effect on RICP (β = 0.81 [0.40 1.22], mean [95% CI]; p < 0.001), whereas HR had a negative effect on RICP (β = –0.14 [–0.18 –0.10] (100 bpm)–1, p < 0.001). ICPmean across the range of heart rate did not influence RICP (β = –0.13 [–0.32 0.06] (100 mm Hg)–1, p = 0.184). An Rflow-HR interaction term did not significantly improve the model (p = 0.857). Conclusion: The ratio of consecutive peaks of the ICP waveform was negatively correlated with the ratio of consecutive peaks in the blood flow waveform. Given that the carotid flow waveform can be detected non-invasively, this provides a proof-of-concept for further investigation into use of non-invasively acquired cardiovascular signals for the assessment and quantification of the ICP waveform.