Contributions of Vascular Tone and Structure to Elastic Properties of a Medium-Sized Artery

Abstract Isobaric compliance and distensibility of the radial artery were recently reported to be normal or slightly increased in untreated hypertensive patients. However, these findings provide no information on the intrinsic mechanical properties of the wall material. To address this question, we determined intima-media wall thickness, wall-to-lumen ratio, and incremental elastic modulus in the radial artery of 25 untreated hypertensive patients with blood pressure of 150±14/103±6 mm Hg (mean±SD) and 25 matched control subjects with blood pressure of 118±9/79±6 mm Hg. High-resolution echo-tracking for assessment of internal diameter and intima-media wall thickness was combined with measurements of blood flow velocity by Doppler and blood pressure by photoplethysmography. In addition, isobaric compliance and distensibility and incremental elastic modulus were measured at peak diameter during reactive hyperemia after a 5-minute brachial occlusion. No significant difference was found between the two groups for isobaric compliance or distensibility at baseline or during hyperemia. However, incremental elastic modulus at 100 mm Hg tended to be lower in hypertensive patients than control subjects (1.9±1.1 versus 2.5±1.2 mm Hg·10 4 , P =.1) in resting conditions. Hypertensive patients and control subjects had similar internal diameters (2.47±0.32 versus 2.41±0.35 μm), but intima-media wall thickness and wall-to-lumen ratio were significantly increased in hypertensive patients compared with control subjects (0.268±0.032 versus 0.236±0.025 mm [ P ≤.01] and 0.220±0.038 versus 0.195±0.028 [ P ≤.05], respectively). Peak hyperemic blood flow response (hypertensive patients versus control subjects: 349% versus 360% increase from baseline) and reactive hyperemic dilation (7.2% versus 7.9%) were similar in amplitude and duration in the two groups. These results suggest that wall thickening is an adaptive process that reduces wall tension in hypertensive patients while preserving a normal mechanical behavior of the radial artery. This is most likely accomplished by modification of the incremental elastic modulus of wall components rather than by a change in vascular tone.