Assessment of endothelium-mediated vasodilation of the peripheral circulation by transcutaneous ultrasonography and venous occlusion plethysmography.

Transcutaneous ultrasonography is a non-invasive technique with the ability to measure the volumetric blood flow of the peripheral circulation. Peripheral blood flow can be determined by high-resolution imaging of vessel diameter coupled with Doppler assessment of flow velocity. This method, however, has not been validated in vivo. Accordingly, brachial artery flow in response to intraarterial infusion of vasodilators was assessed by ultrasonography in 16 healthy subjects and compared to values obtained simultaneously by venous occlusion plethysmography. Blood flow calculated from ultrasound-derived vessel diameter and flow velocity was found to highly correlate with plethysmographic flow, with r values ranging from 0.83 to 0.99. Using this ultrasound technique combined with plethysmography, the response of conduit and resistance vessels to endothelium-mediated vasodilation was characterized. Doppler velocity rose dramatically with endothelium-dependent acetylcholine (970%), but only modestly with endothelium-independent vasodilators, nitroglycerin (292%) and nitroprusside (340%). Despite eliciting the greatest overall forearm flow response, acetylcholine resulted in a smaller increase in conduit diameter (15.4%) than nitroglycerin (21.8%), and only a comparable change than nitroprusside (14.6%). Taken together, these results suggest that acetylcholine acts predominantly on resistance vessels, whereas nitrovasodilators affect mainly conduit vessels. In summary, transcutaneous ultrasonography can be used reliably to assess flow changes in the peripheral circulation. Combined with plethysmography, this technique is useful for determining the relative contribution of conduit and resistance vessels to peripheral flow, particularly in the assessment of endothelium-mediated vasodilation.