Relationship between central pulsatile aortic pressure and middle cerebral artery flow waveforms in normal young adults

There is inconsistency in the interpretation of the relationship between pulsatile pressure and flow entering the brain in terms of arterial “Windkessel” compliance and resistive properties, and usually done in the time domain only. Interpretations are at variance with those applied to other vascular beds including the kidney, which also has very high blood flow and low resistance. This subject is important if one is to best treat such disease conditions as stroke, cerebral trauma, and neoplasm, and delay progression of the arterial disease which with aging and hypertension causes dementia. In 9 normal young subjects age 28±3 years, radial artery pressure was recorded non-invasively by applanation tonometry and middle cerebral artery flow (MCAF) with transcranial Doppler under control conditions and physiological challenge (Valsalva). Aortic pressure (AP) waves were generated from radial using SphygmoCor®. Comparisons were made of the flow and pressure waves in the time domain as flow (FAIx) and pressure augmentation index (PAIx), and in the frequency domain as vascular impedance. Mean MCAF was relatively high (63±14 cm/s) and AP 86±6 mm Hg, but within normal limits. FAIx was 84 %, and PAIx 11 %, also within normal limits for age, but with PAIx lower and FAIx higher than in older subjects with suspected and confirmed cerebrovascular disease. Impedance modulus was 1.8x10³ dyne.s.cm⁻³ at zero Hz and fell to an average of 0.3x10³ dyne.s.cm⁻³ at 5 Hz, and phase was negative for all harmonics. This is consistent with a capacitive load but dominated by a low resistance vascular bed downstream. Patterns of a low resistive vascular bed persisted during Valsalva manoeuvre. Contrary to previous reports, pressure and flow waves in arteries supplying the brain, both in time and frequency domain, are similar to those in the renal artery, with resistance dominating vascular load, and compliance relatively low. Properties appear to change with age and disease; measurement is practical and may assist in management of hypertensive patients with potential or actual cerebrovascular disease.1 page(s