Pulsation of brain tissue increases in response to caffeine: a pilot healthy volunteer study

<i>Objective.</i>Caffeine is known to induce cerebral vasoconstriction. We used this effect in a pilot ultrasound-based healthy volunteer study to investigate the directionality of response of brain tissue pulsations (BTPs) with changing middle cerebral artery velocity (MCAv) following caffeine ingestion.<i>Approach.</i>BTPs were measured in healthy volunteers using transcranial tissue Doppler (TCTD) ultrasound and MCAv was measured using conventional transcranial Doppler ultrasound. Measurements of blood pressure, heart rate, and end-tidal carbon dioxide (EtCO₂) were also recorded. Data were collected at rest and at multiple timepoints over a 60 min period following ingestion of 250 mg of caffeine.<i>Main results.</i>A multivariate multilevel model identified significant decreases in mean MCAv of -0.17 (-0.21, -0.14) (cm s^-1) min^-1, ΔMCAv of -0.06 (-0.1, -0.04) (cm s^-1) min^-1, and EtCO₂of -0.02 (-0.04, -0.01) mmHg min^-1. Significant increases in mean arterial pressure of 0.21 (0.15, 0.28) mmHg min^-1and bulk BTP amplitude of 0.08 (0.02, 0.14)<i>μ</i>m min^-1were observed. These changes confirm the expected physiological effects of caffeine and provide novel evidence of an inverse relationship between MCAv and BTP amplitude, suggesting that these variables respond in opposite directions following a vasoconstrictive challenge.<i>Significance.</i>We hypothesise that increased bulk BTP amplitude reflects a reduction in intracranial pressure (ICP), driven by caffeine-induced cerebral vasoconstriction, allowing greater brain tissue mobility. This interpretation is supported by magnetic resonance imaging studies, which show increased brain tissue motion with lowered ICP. Measurement of BTPs may provide real-time information on intracranial haemodynamics.