In vivo human brachial artery elastic mechanics - Effects of smooth musclerelaxation

Background-The effects of smooth muscle relaxation on arterial wall mechani cs are controversial. We used a new, in vivo, noninvasive technique to meas ure brachial artery wall mechanics under baseline conditions and following smooth muscle relaxation with nitroglycerin (NTG). Methods and Results-Eight healthy, normal subjects (6 male, 2 female; age 3 0 +/- 3.1 years) participated in the study. The nondominant brachial artery was imaged through a water-filled blood pressure cuff using an external ul trasound wall-tracking system at baseline and following 0.4 mg sublingual N TG. Simultaneous radial artery pressure waveforms were recorded by tonometr y. Transmural pressure (TP) was reduced by increasing water pressure in the cuff. Brachial artery area, unstressed area, compliance, stress, strain, i ncremental elastic modulus (E-inc), and pulse wave velocity (PWV) were meas ured over a TP range from 0 to 100 mm Hg. Baseline area versus TP curves ge nerated 30 minutes apart were not significantly different. NTG significantl y shifted area versus TP (P < 0.0001) and compliance versus TP (P < 0.001) curves upward, whereas the E-inc versus TP (P < 0.05) and PWV versus TP (P < 0.01) curves were shifted downward. NTG also significantly shifted stress versus strain (P < 0.01) and E-inc versus strain (P < 0.01) curves to the right. Conclusions-We conclude that brachial artery elastic mechanics can be repro ducibly measured over a wide range of TP and smooth muscle tone using a new noninvasive ultrasound technique. Smooth muscle relaxation with NTG increa ses isobaric compliance and decreases isobaric E-inc and PWV in the human b rachial artery.