Clinical assessment of endothelium dysfunction
Principle
There are both invasive and non-invasive techniques for exploring various aspects of the pathobiology of the endothelium of arteries and veins
The basic principle, however, is similar: Healthy arteries dilate in response to pharmacological and/or physiological stimulation of the endothelium due to release of NO and other vasoactive substances. In disease states, such endothelium-dependent dilatation is reduced or absent
Invasive assessment
A. Coronary Epicardial and Microvascular Function Changes in the epicardial and microvascular
responses to endothelium dependent pharmacological agents are measured during cardiac catheterization using quantitative coronary angiography and the Doppler flow-wire techniques.
Preserved epicardial coronary endothelial function is characterized by vasodilatation in response to acetylcholine which promotes the release of NO
Arteriosclerotic vessels with impaired endothelial function, however, respond with vasoconstriction as a result of a direct vasoconstrictor effect of ACh on the vascular smooth muscle (muscarinic effect) in the absence of NO release
Response of healthy endothelium
Similar induced functional changes in vascular reactivity have been demonstrated with other endothelium dependent and endothelium independent pharmacological substances
Physical measures of endothelium-dependent responses include exercise which induce an increase in coronary blood flow and thus shear stress on the coronary circulation, which leads to flow-mediated endothelium-dependent vasomotion of the epicardial vessels
Similar responses can be seen in response to mental stress
Another “physiological” test to assess epicardial vasoreactivity is the use of the cold pressor test in which the subject puts his or her hand into ice water
The activation of the sympathetic nervous system leads to release of NO and endothelium-derived hyperpolarizing factors via stimulation of endothelial α2-adrenergic receptors and consequently vasodilation in healthy arteries
However, in dysfunctional endothelium, α1-adrenergic–mediated constriction of smooth muscle cells will dominate, closely mirroring the responses to acetylcholine
Coronary flow reserve is the ratio of maximal coronary blood flow during maximal coronary hyperemia with provocative stimuli (such as adenosine infusion, pacing, or exercise) divided by the resting coronary blood flow
This maximal blood flow response (coronary flow reserve) is both endothelium- and non– endothelium-dependent, and a coronary flow reserve <2.0 is considered abnormal
Non-invasive functional tests to assess the coronary microvasculature have been developed, among them positron emission tomography, myocardial perfusion imaging, blood oxygen level–dependent magnetic resonance imaging, and echocardiography
B. Plethysmography of the forearm circulation This semi-invasive (arterial puncture), technique
measures changes in forearm blood flow by venous plethysmography in both arms before and after infusion of vasoactive substances into a cannulated brachial artery
The main advantage is that vasoactive molecules, hormones, or drugs can be infused, thus quantifying endothelium-dependent and endothelium-independent vasodilation in a dose-dependent manner
The dosages required have limited systemic effects, allowing the contralateral limb to serve as an internal control
The results are expressed as the ratio of the changes in flow measured in both arms and are reproducible
C. Determination of venous endothelial dysfunction
As the endothelium is one of the components that regulate the functional capability of the whole circulatory system, it is expected that it regulates the functional status not only in the arteries but also in the venous system
In the veins, the endothelium maintains circulatory homeostasis through its control of vasomotion, coagulation, fibrinolysis and platelet activation
“dorsal hand vein technique” evaluates and quantifies the vascular responsiveness of the pre-constricted dorsal hand vein to different substances
All techniques for direct determination of venous endothelial function are complicated and their reproducibility is limited; therefore, measurement of endothelial function of the peripheral arteries is frequently used as a surrogate to assess venous endothelial function
Invasive assessment in a nutshell
Dorsal hand Dorsal hand Evaluation and quantification vein technique veinof functional responsiveness
of peripheral veins
Non invasive assessmentA. Flow mediated dilation (FMD)
Most commonly used method mainly because of its sensitivity and noninvasive nature
It is based on the endothelial release of NO and other endothelium-derived relaxing factors in response to an increase in shear stress
The ischemia is achieved through a pneumatic cuff, placed on the forearm, distal to the ultrasound image site, and inflated to suprasystolic pressure for 5 min
On deflation of the cuff, the increased flow results in shear stress, which activates endothelial NO synthase to release NO via the L-arginine pathway
The NO diffuses to the smooth muscle cells, causing them to relax, resulting in vasodilation
FMD is measured as the percentage change in brachial artery diameter from baseline to the maximum increase in diameter
A larger baseline diameter yields a smaller percentage of change, and smaller arteries appear to dilate relatively more than larger arteries
Flow-mediated dilation of the brachial artery and the popliteal artery decreases with age, which may be attributable in part to decreased NO release and to diminished smooth muscle cell responsiveness in older subjects
Technical Considerations in Flow-Mediated Dilation
Measurements
1. Subject preparation Fasting state (>6 h) No smoking or any tobacco consumption at least 6 h before study No exercise or food/beverages that contain alcohol or caffeine or are
rich in polyphenols (cocoa, tea, fruit juices) for >12 h No vitamins for at least 72 h Vasoactive medications withheld on the morning of the study if
possible with careful noting of the use and timing of any drugs No exercise >12 h before test Quiet, temperature-controlled room In female patients, repetitive studies should be made at the same
time of the menstrual cycle (ideally on days 1–7 of the menstrual cycle)
Rest for at least 10 min before measurements Supine position Arm resting comfortable with cradle support with the imaged artery at
the heart level Test should be performed at the same time of the day (especially if
multiple tests are performed)
2. Sphygmomanometer probe position and cuff occlusion time
Placement of the cuff 1–2 cm distal to the elbow crease
Other sites are discouraged because proximal cuff positioning affects the magnitude of the peak vasodilatory response
Occlusion time, 5 min (shorter inflation attenuates FMD response)
Cuff inflation to at least 50 mm Hg above systolic pressure
3. Site selection Brachial artery with a minimum diameter (usually 2 mm); small arteries
are difficult to measure, and changes in absolute diameter correspond to big relative changes
If repetitive measurements are planned, site has to been replicated; anatomic landmarks should be used
4. Image acquisition Longitudinal images obtained by high-resolution ultrasound (7.5–12
MHz)
A clear interface between the near and far arterial wall should be achieved
Diameter measurements are obtained in end diastole or averaged over the heart cycle
Stereotactic adjustable prop holding is essential to ensure image quality
Recording of the baseline diameter for at least 1 min
Simultaneous acquisition of pulse-wave Doppler velocity signals for quantification of shear stress (stimulus) if feasible; insonation angle should be <60⁰
5. Measurement
Automated edge detection should be used
Reported as maximal percentage change from baseline diameter (most reproducible)
Baseline diameter and absolute change reported also
Characterization of the hyperemic stimulus (ideally the flow-velocity time integral)
A, Setting for flow-mediated dilation testing. Note the probe position in relation to cuff and the stereotactic apparatus
B, Ultrasound image of brachialartery used to measure both the diameter changes and flow velocity
Although FMD provides crucial information about the ability of the endothelium to respond to a specific stimulus (reactive hyperemia), it is not a measure of the resting production of vasoactive substances
In this respect, vasoconstriction of the brachial artery after inflation of a wrist cuff to suprasystolic pressure was first reported years ago and is mediated mainly through vasoconstrictor substances such as endothelin
This concept has garnered new attention, and the term low-flow–mediated constriction (L-FMC) was introduced
In principle, low-flow– mediated constriction detects the change in brachial artery diameter in response to a decrease in blood flow and shear stress after occlusion of the artery by a distally placed cuff
Low-flow-mediated constriction provides information concerning the control of arterial tone at rest, thus complementing and not overlapping the information provided by FMD
Simultaneous measurement of FMD and L-FMC provides more comprehensive information on the different pathways involved in the control of vascular homeostasis
While the mechanisms underlying FMD are based on the synthesis and release of NO, inhibition of the synthesis of NO does not modify L-FMC, suggesting that NO has a less important role in maintaining the resting tone of arteries
Under resting conditions, other substances such as prostaglandin are secreted by the endothelium of conduit arteries and that this production is decreased when shear stress is reduced
L-FMC might be a result of the common effect of vasodilator release (prostaglandins, endothelium-derived hyperpolarizing factor) and increased endothelin-1 production
B. Pulse wave velocity
Aortic PWV is usually measured between the carotid and femoral artery by synchronically detecting the arrival of the wave at both locations and measuring the distance. This method renders an average velocity value
In response to a reactive hyperaemic stimulus, which increases shear stress and stimulates endothelial NO release, pulse wave velocity (PWV) slows due to the resultant drop in smooth muscle tone
Using either applanation tonometry or photo plethysmography, pressure vs time data can be acquired in different arteries, such as the radial, femoral or carotid arteries
Tonometry of the radial artery provides an accurate, reproducible, non-invasive assessment of the central pulse pressure waveform
Pulse wave analysis using oscillometric data obtained from an arm cuff inflated to suprasystolic pressure has the advantage of having a high level of automation, which provides better reproducibility
Oscillometric device for assessing pulse wave velocity. All the factors that reduce distensibility of the vessel (ie, increase ‘‘stiffness’’ of the wall) lead to a faster PWV
Applanation tonometry is performed by placing a pressure sensor over the radial artery
C. Finger plethysmography With peripheral arterial tonometry, beat-to-beat
plethysmographic recordings of the finger arterial pulse wave amplitude are captured with pneumatic probes
With the device, a counter pressure of 70 mm Hg on the digit is applied to avoid distal venous distention, thus inhibiting venous pooling and venoarteriolar reflex responses
In principle, an increase in arterial blood volume in the finger tip causes an increase in pulsatile arterial column changes, thus increasing the measured signal
Similar to the assessment of endothelial function with the FMD technique, a pressure cuff is placed on the arm, and after baseline blood volume changes are obtained, the blood pressure cuff is inflated above systolic pressure and deflated after 5 minutes to induce reactive hyperemia on 1 arm.
A main advantage of the system is that the contralateral arm serves as its internal control that can be used to correct for any systemic drift in vascular tone during the test, and an index between the 2 arms is calculated to adjust for any such drift. This index is a validated marker for endothelial function.
In 1937 Alrick Hertzman produced a ‘‘photoelectric plethysmograph,’’ which he described as a device that ‘‘takes advantage of the fact that the absorption of light by a transilluminated tissue varies with its blood contents.’’
This is a consequence of the Lambert–Beer law, which relates light absorption to optical density.
Photoplethysmography, once calibrated with a blood pressure measurement, can be used on the finger to provide continuous finger blood pressure monitoring
A modern photoplethysmograph incorporating a light-emittingdiode and sensor within a finger clip. A typical waveform (solid line) is shown, together with a radial pressure waveform (obtained using a tonometer) in the same individual.
Non invasive assessment in nutshell
References Flammer AJ, Anderson T, Celermajer DS, Creager MA, Deanfield J, Ganz
P, et al. The assessment of endothelial function: from research into clinical practice. Circulation. 2012 Aug;126(6):753-67.
Arrebola-Moreno AL, Laclaustra M, Kaski JC. Noninvasive assessment of endothelial function in clinical practice. Rev Esp Cardiol (Engl Ed). 2012 Jan;65(1):80-90.
Elizabeth A. Ellins and Julian P. J. Halcox, “Where Are We Heading with Noninvasive Clinical Vascular Physiology? Why and How Should We Assess Endothelial Function?,” Cardiology Research and Practice, vol. 2011, Article ID 870132, 9 pages, 2011
Poredos P, Jezovnik MK. Testing endothelial function and its clinical relevance. J Atheroscler Thromb. 2013;20(1):1-8.
