Ultrasound Assessment of Endothelial Function: A Technical Guideline of the Flow-mediated Dilation Test

The overall goal of this noninvasive technique is to assess endothelial function in humans. The flow-mediated dilation, or FMD test, can be used to answer key questions in the cardiovascular field, with the potential for standardized clinical monitoring of vascular function in patients. The main advantage of this technique is that it is a noninvasive assessment for evaluating vascular health in humans, with the potential to predict future cardiovascular disease and events.

Generally, individuals will struggle with this technique, because they cannot obtain a good brachial artery image, requiring an adequate level of skill and practice for reliable results. Demonstrating the procedure will be Amy Mangeri, a registered cardiovascular stenographer in my laboratory. Upon arrival, confirm that the participant has abstained for at least 12 hours from exercising, smoking, and high-fat meals, and for at least 72 hours from vitamin supplementation and aspirins, and at least 24 hours from non-steroidal anti-inflammatory agents.

Next, attach a three lead ECG in the standard limb lead II position, and then extend the subject's arm laterally to about 80 degrees of shoulder abduction. Place the forearm cuff immediately distal to the medial epicondyle. Secure the distal forearm in a vacuum-packed pillow to maintain the arm in the appropriate position during the measurement.

Take care that nothing is touching the cuff, including the table. The subject should then rest in the supine position in a quiet, temperature-controlled, 22 to 24 degree Celsius room for approximately 20 minutes to achieve a hemodynamic steady state. Within the b mode, identify the brachial artery and collateral vessels.

To map the brachial artery with the ultrasound, position the probe cross-sectionally, and begin scanning the inner side of the upper arm, starting at the insertion of the bicep and proceeding proximally. Use the color flow mode to confirm the location of the artery, considering the direction of the transducer when interpreting the color and pulsatility to ensure the assessment of the artery and not the vein. After identifying the brachial artery, rotate the probe 90 degrees to scan the arm longitudinally, and obtain an image between two to 10 centimeters above the anticubital fossa.

Identify anatomical landmarks such as veins and fascial planes to complete multiple assessments in the same subject. Next, secure the probe in a stereotactic probe holder, verifying that the instrument is appropriately fixed to avoid any excessive movements. Confirm that the image is as good as the image obtained without the holder, and use the time gain controls to achieve the clearest image possible from the anterior and posterior intimal interfaces between the lumen and the vessel wall.

When the image has been optimized, have the technician manually adjust the gain, focal points, dynamic range, and harmonics to achieve clear and defined images of the near and far walls of the endothelium. Following the b mode acquisition, rock the transducer up on one end more than the other to adjust the image, keeping an insonation angle of 60 degrees. Then, obtain a satisfactory b mode image that identifies the endothelial layers with clear intima intimal walls of the artery, confirming that the Doppler signal emits a sharp and clear sound with no muffles.

To measure the baseline, vascular occlusion, and reactive hyperemia, first freeze and unfreeze the image to reset the ultrasound cine loop. Then, press F1 to begin recording the baseline data in the image software for at least 30 seconds. Next, rapidly inflate the forearm occlusion cuff using compressed air to super-systolic pressures for five minutes to induce the arterial occlusion.

After four minutes and 30 seconds of occlusion, begin acquiring data. Deflate the cuff after five minutes of occlusion and continue to record the data for two more minutes. Then, stop and save the recordings.

Finally, use edge detection and wall tracking software to evaluate the vessel diameter according to the manufacturer's instructions. The FMD test was conducted in an apparently healthy cohort group in the Laboratory of Integrative Vascular and Exercise Physiology. The most representative variables of FMD are shown in this table.

Here, the diameter and velocity response observed during a representative FMD test demonstrate how, during the beginning of the reactive hyperemia, a typical peak velocity was elicited, followed by an increase in the brachial artery diameter. The FMD technique has paved the way for researchers to explore endothelial function in humans, a tool that helps further our understanding of the etiology of multiple cardiocentric pathologies. Once mastered, this technique can be performed in approximately 25 minutes.

While attending this procedure, it's extremely important to remember to acquire the best possible image of the brachial artery. After watching this video, you should have a good understanding of the recommended methodology for minimizing the physiological and technical issues of performing the FMD test, which will improve the validity, reproducibility, and interpretation of the assessment.