Cerebral Blood Flow under Pressure: Investigating Cerebrovascular Compliance with Phase-contrast Magnetic Resonance Imaging during Induced Hypertension.

BACKGROUND

Induced hypertension is used clinically to increase cerebral blood flow (CBF) in conditions such as vasospasm after subarachnoid hemorrhage. However, increased blood pressure also raises pulsatile force. Cerebrovascular compliance plays a key role in buffering flow dynamics and protecting the microcirculation, but whether it adapts to elevated pressure remains unclear. This study assessed the response of compliant cerebral arteries to induced hypertension in healthy adults using phase-contrast magnetic resonance imaging (PCMRI) and two compliance models: a two-element Windkessel (compliance estimated using the Windkessel model, C WK ) and a simplified model (compliance calculated as the ratio of pulsatile volume to pressure, C VP ), representing the extremes of pulsatility transmission at the capillary level.

METHODS

Eighteen healthy adults (median age, 34 yr; nine women) underwent PCMRI at baseline and after increasing mean arterial pressure by 20% using norepinephrine infusion. PCMRI quantified CBF and cardiac output, while cerebrovascular resistance and systemic vascular resistance were derived. Flow waveforms were combined with blood pressure to assess C WK and C VP in CBF, ascending/descending aorta, and external carotid arteries, while corresponding regions of interest were used to calculate cross-sectional flow areas. Data are reported as median (interquartile range).

RESULTS

Norepinephrine increased cerebrovascular compliance significantly: C WK by 110% (56 to 163%; P = 0.001) and C VP by 11% (-2 to 26%; P = 0.018). C WK increased in the external carotid artery by 12% (1 to 32%; P = 0.037) but did not change in the ascending or descending aorta. C VP decreased in the descending aorta by 5% (-11 to 2%; P = 0.028), with no changes in the ascending aorta or external carotid artery. Cross-sectional area of cerebral arteries contributing to CBF decreased by 5% (-17 to -3%; P = 0.033), while the ascending and descending aorta areas increased by 7% (4 to 11%; P = 0.012) and 8% (6 to 11%; P < 0.001), respectively.

CONCLUSIONS

Cerebral arteries enhanced their compliance during norepinephrine-induced hypertension, unlike systemic arteries, regardless of the assumed degree of pulsatility transmission.