Name: ultrasound assessment of endothelial-dependent flow-mediated vasodilation of the brachial artery in clinical research
Uploaded: 2014-10-22T15:00:00
Description: Watch this Scientific Journal Video about Ultrasound Assessment of Endothelial-Dependent Flow-Mediated Vasodilation of the Brachial Artery in Clinical Research at JoVE.com

From the Vascular Integrated Physiology and Experimental Therapeutics (VIPERx) Laboratory, this work was supported by funds from the Department of Surgery, University of California, San Francisco and the Northern California Institute for Research and Education. The project described was supported by Award Number KL2RR024130 from the National Center for Research Resources. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Center for Research Resources or the National Institutes of Health.

The following procedure, developed as part of an investigator-initiated study, was reviewed and approved by the University of California, San Francisco (UCSF) Committee on Human Research (CHR) and all participants gave informed consent.
1. Equipment
<ol>
	<li>Use an EKG gated image capture system to record and analyze the FMD. Connect a Philips HD11 ultrasound to a desktop PC.</li>
	<li>Connect a video signal from the ultrasound with a special frame-grabber card on the PC.</li>
	<li>Relay an...

Endothelial dysfunction is an imbalance in the chemical mediators affecting vessel tone and an early step in the development of atherosclerosis. Measuring the reactivity of an artery is a way to assess the state of these chemical pathways. Both direct and indirect methods of assessing reactivity exist for the different vascular beds, ranging from direct infusion of an endothelium agonist in the coronary circulation to non-invasive, pulse waveform analysis in the index finger38.
Brac...

The human vascular endothelium provides structural and functional roles within the body. In histological sections, the endothelium appears small, comprising a thin layer of cells 1-2 microns thick sitting atop a layer of smooth muscle cells (the media) and a thick layer of connective tissue (the adventitia). Taken as a whole, the endothelium provides a wide area for the exchange of information between the blood and vascular smooth muscle tissue. By one estimate, a cross sectional area of 700 m2 and a mass of 1,000-1,500 grams in a 70 kg man, is comparable in mass to the liver1. A healthy endothelium allows for mechanical to chemical signal transduction to maintain homeostasis of the blood vessel. Endothelial dysfunction is an imbalance of these mediators and the first step in vascular disease, present prior to histological evidence of atherosclerosis. A non-invasive, in vivo method for quantifying the vasodilatory function of human artery exists. This method, endothelium-dependent, flow-mediated vasodilation (FMD) is widely used in clinical trials.
The endothelium acts as a structural component of the vasculature and manufactures components of the extracellular matrix such as glycosaminoglycans and fibronectin2. Long term changes in blood flow and acute injury to the artery may lead to structural changes. Functionally, the vascular endothelial cells participate in regulation of vessel tone, inflammatory processes, antithrombosis, and anticoagulation. Endothelial cells affect vasoconstriction through endothelin while vasodilation is mediated by nitric oxide (NO), prostacyclin, and endothelial derived hyperpolarizing factor (EDHF)3-6.
Endothelial dysfunction is an impairment of any of these mediators and the first step in atherosclerosis. Not surprisingly, as a mechanism of disease, it is associated with a number of clinically important conditions such as coronary artery disease, hypertension and diabetes mellitus7-11. Importantly, endothelial dysfunction can be observed in individuals without diagnosed cardiovascular disease and is predictive of future cardiovascular events7,12,13. One measure of endothelial dysfunction, in combination with the Framingham score, can provide additional prognostic information above either measure alone14.
Measures of endothelial dysfunction may involve the direct infusion of a pharmacological agent. Intercoronary infusion of acetylcholine, for example, combined with quantitative angiography demonstrates vasodilation in subjects with an intact endothelium. However, individuals with endothelial damage experience paradoxical vasoconstriction.15 In peripheral arteries, infusion of a pharmacological agent with measurement of flow by gauge-strain plethysmography is possible16.
Agents that directly affect the endothelium and elicit a chemical signal are termed endothelium-dependent vasodilators. Acetylcholine, for example, acts on muscarinic receptors on endothelial cells, leading to increased intra-cellular calcium concentration, activation of nitric oxide synthase and vasodilation. Agents that affect vasodilation without involvement of the endothelium are called endothelium-independent agents. Nitroglycerin, for example, activates soluble guanyl cyclase and cyclic guanosine-3&#8217;,-5&#8217;-monophasphate (cGMP) which mediates vasodilation in the vessel wall through protein kinases regulating intracellular calcium concentrations17.
There is a non-invasive, in vivo method for quantifying endothelial dysfunction introduced by Celermajer and associates called &#8220;flow-mediated, endothelium-dependent vasodilation&#8221; (FMD)18. Briefly, changes to arterial blood flow open shear stress sensitive ion channels in the endothelium. The signal is tranduced via a second-messenger cascade and activates endothelial nitric oxide synthase (eNOS), generating NO. This species diffuses across the cell membrane to neighboring smooth muscle cells (SMC). Within the SMC, the signal is transduced, lowering intracellular calcium concentration and affecting vasorelaxation19. The diameter of the artery lumen increases, leading to an increase in blood flow consistent with the Hagen-Poiseullie equation. The effect of FMD may be abolished with administration of an NO synthase inhibitor such as mono-methylarginine (L-NMMA)20.
Celermajer et al.&#8217;s innovative work has allowed the use of high resolution B-mode ultrasound to assess the change in artery diameter during the reactive hyperemia that follows ischemia. In this technique, a human subject rests supine and the diameter of the brachial artery is measured in a longitudinal plane. A blood-pressure cuff is used to produce ischemia in the limb. Following release of the blood pressure cuff the diameter of the artery is measured again. The rapid change in shear stress is the stimulus for NO mediated vasodilation. A simple equation describes the change in the diameter relative to the baseline diameter (Equation 1). A full discussion of the parameters of this equation, hyperemia and baseline diameter, can be found in the Protocol and Results sections.

In multiple studies, percent FMD has been found to predict cardiovascular events in patient with established cardiovascular disease21-24. A correlation between brachial artery percent FMD and coronary artery FMD was established by Anderson et al., demonstrating a link between peripheral measurements and the more clinically-relevant ischemic changes to the heart25. FMD does not demonstrate the maximum vasodilation of the vessel. To evaluate this, FMD can be followed by endothelium-dependent, nitroglycerin-mediated vasodilation of the same vessel.
There are technical issues affecting the measurement of percent FMD. Since the introduction of the technique, several studies showed a high degree of within-subject and inter-operator variability26. It has been shown that physiological factors such as cigarette smoking, antihypertensive medications, time of day, and fasting state affect percent FMD. Likewise, technical choices such as the position of the cuff relative to the site of measurement and duration of occlusion have been shown to affect the measurement27,28. Guidelines have been published that describe the current consensus and allow for standardization of technique between laboratories19,29.
Despite the evolving consensus on technique, flow-mediated vasodilation remains heavily operator dependent with a long learning curve. Corretti, for example, recommends the sonographer complete 100 scans under the supervision of an experienced investigator before operating independently. To maintain a level of adequate expertise, it is recommended the technician complete 100 scans annually. For investigators with a small sample population and limited resources, the learning curve presents a barrier to entry. This article will demonstrate a method for flow-mediated vasodilation of the brachial artery in the upper arm and offer technical suggestions to reduce intra-operator variability.

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			<td height="42" style="height:42px;width:216px;">Name of Material/ Equipment</td>
			<td style="width:71px;">Company</td>
			<td style="width:119px;">Catalog Number</td>
			<td style="width:167px;">Comments/Description</td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:216px;">Philips HD 11XE ultrasound</td>
			<td style="width:71px;">Philips Healthcare</td>
			<td style="width:119px;"></td>
			<td></td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:216px;">5-12 MHz linear array transducer</td>
			<td style="width:71px;">Philips Healthcare</td>
			<td style="width:119px;">L12-5</td>
			<td></td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:216px;">Ultrasound gel</td>
			<td style="width:71px;">Parker Laboratories</td>
			<td style="width:119px;"></td>
			<td></td>
		</tr>
		<tr height="84">
			<td height="84" style="height:84px;width:216px;">Vascular Research Tools v.5.0</td>
			<td style="width:71px;">Medical Imaging Applications, LLC</td>
			<td style="width:119px;"></td>
			<td></td>
		</tr>
		<tr height="84">
			<td height="84" style="height:84px;width:216px;">MIA Gating module</td>
			<td style="width:71px;">Medical Imaging Applications, LLC</td>
			<td style="width:119px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Desktop PC</td>
			<td style="width:71px;">Dell, Inc</td>
			<td style="width:119px;"></td>
			<td></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Windows XP</td>
			<td style="width:71px;">Microsoft, Inc</td>
			<td style="width:119px;"></td>
			<td></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">5 cm tourniquet blood pressure cuff</td>
			<td style="width:71px;">Hokanson</td>
			<td style="width:119px;">SC 5</td>
			<td></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Hand-held aneroid manometer</td>
			<td style="width:71px;">Welch Allyn</td>
			<td style="width:119px;">DS66</td>
			<td></td>
		</tr>
	</tbody>
</table>

ultrasound assessment of endothelial-dependent flow-mediated vasodilation of the brachial artery in clinical research

The vascular endothelium is a monolayer of cells that cover the interior of blood vessels and provide both structural and functional roles. The endothelium acts as a barrier, preventing leukocyte adhesion and aggregation, as well as controlling permeability to plasma components. Functionally, the endothelium affects vessel tone.
Endothelial dysfunction is an imbalance between the chemical species which regulate vessel tone, thombroresistance, cellular proliferation and mitosis. It is the first step in atherosclerosis and is associated with coronary artery disease, peripheral artery disease, heart failure, hypertension, and hyperlipidemia.
The first demonstration of endothelial dysfunction involved direct infusion of acetylcholine and quantitative coronary angiography. Acetylcholine binds to muscarinic receptors on the endothelial cell surface, leading to an increase of intracellular calcium and increased nitric oxide (NO) production. In subjects with an intact endothelium, vasodilation was observed while subjects with endothelial damage experienced paradoxical vasoconstriction.
There exists a non-invasive, in vivo method for measuring endothelial function in peripheral arteries using high-resolution B-mode ultrasound. The endothelial function of peripheral arteries is closely related to coronary artery function. This technique measures the percent diameter change in the brachial artery during a period of reactive hyperemia following limb ischemia.
This technique, known as endothelium-dependent, flow-mediated vasodilation (FMD) has value in clinical research settings. However, a number of physiological and technical issues can affect the accuracy of the results and appropriate guidelines for the technique have been published. Despite the guidelines, FMD remains heavily operator dependent and presents a steep learning curve. This article presents a standardized method for measuring FMD in the brachial artery on the upper arm and offers suggestions to reduce intra-operator variability.

The key variables of flow-mediated vasodilation are shown in Table 1.
<table border="0" cellpadding="0" cellspacing="0" fo:keep-together.within-page="always">
	<tbody>
		<tr>
			<td>Variable</td>
			<td>Description</td>
		</tr>
		<tr>
			<td>Mean Velocity (cm/sec)</td>
			<td>The mean arterial velocity of blood in the middle 50% of the lumen during one cardiac cycle estimated from Doppler spectral waveforms, proportional to blood flow and inversely proportional to cross-sectional area (see Fi...

Watch this Scientific Journal Video about Ultrasound Assessment of Endothelial-Dependent Flow-Mediated Vasodilation of the Brachial Artery in Clinical Research at JoVE.com

Ultrasound Assessment of Endothelial-Dependent Flow-Mediated Vasodilation of the Brachial Artery in Clinical Research

Endothelial dysfunction is associated with numerous disease states and is predictive of adverse cardiovascular events in humans. Flow-mediated vasodilation (FMD) is a non-invasive ultrasound method of evaluating endothelial function. Methodological choices and operator experience may affect results. A systematic approach to FMD in human studies is discussed here.

The vascular endothelium is a monolayer of cells that cover the interior of blood vessels and provide both structural and functional roles. The ...

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