Why measure endothelial function?

Endothelial dysfunction is the earliest clinical measurement of cardiovascular disease, and is a major physiopathological mechanism leading to coronary artery disease and other atherosclerotic diseases*. It predicts obvious vascular pathology by many years to decades#. Endothelial dysfunction is associated with poor vasomotor control, reduced endothelial anticoagulant properties, increased vascular adhesion molecule expression, increased chemokine and cytokine release, and increased reactive oxygen species production by the endothelium. These lead to inflammation, myofibroblast migration and proliferation inside the vessel wall, which all lead to the development of atherosclerosis. Endothelial dysfunction is an independent predictor of stroke and myocardial infarction@.

*Flammer AJ, Anderson T, Celermajer DS, Creager MA, Deanfield J, Ganz P, Hamburg NM, Lüscher TF, Shechter M, Taddei S, Vita JA, Lerman A (Aug 2012). "The assessment of endothelial function: from research into clinical practice". Circulation 126 (6): 753–67.

#Münzel T1, Sinning C, Post F, Warnholtz A, Schulz E; Sinning; Post; Warnholtz; Schulz (2008). "Pathophysiology, diagnosis and prognostic implications of endothelial dysfunction". Annals of Medicine 40 (18382884): 180–196.

@Gokce N (Jul 2011). "Clinical assessment of endothelial function: ready for prime time?". Circ Cardiovasc Imaging 4 (4): 348–50.


Why measure augmentation index?

Augmentation index is a measure of the blood vessel wall stiffness. Nitric oxide from a healthy endothelium maintains low vessel tone and high compliance at rest. A reduction in age-dependent arterial compliance is a marker for endothelial dysfunction that is associated with functional and structural changes in the microcirculation that are predictive of subsequent morbid events.

Gilani M, Kaiser DR, Bratteli CW, Alinder C, Rajala Bank AJ, Cohn JN (2007). "Role of nitric oxide deficiency and its detection as a risk factor in pre-hypertension". JASH 1: 45–56.

Duprez DA, Jacobs DR, Lutsey PL, Bluemke FA, Brumback LC, Polak JF, Peralta CA, Greenland P, Kronmal RA (2011). "Association of small artery elasticity with incident cardiovascular disease in older adults: the multiethnic study of atherosclerosis". Am J Epidemiol 174: 528–36.


Why is nitric oxide important?

Nitric oxide has a number of beneficial effects that contribute to the inhibition of atherosclerosis:

- shear stress on the endothelium results in NO release, vasodilation and therefore decreased damaging shear stress

- hypoxia increases NO release, leading to vasodilation and increased blood flow

- NO decreases LDL oxidation, a major inflammatory driver

- NO reduces platelet aggregation on the endothelium, decreasing clotting

- NO inhibits smooth muscle proliferation, reducing arteriosclerosis

- NO prevents leukocyte adhesion and infiltration into the vessel wall#.

# Davignon J, Ganz P. "Role of endothelial dysfunction in atherosclerosis. Circulation. 2004 Jun;109(23 Suppl 1):III27-32.


Can endothelial dysfunction be predicted by risk factors?

Endothelial dysfunction cannot be predicted by typical risk factors for atherosclerosis (e.g. obesity, cholesterol, smoking) and hormones.

Reis SE, Holubkov R, Smith AJC, Kelsey SF, Sharaf BL, Reichek N, Rogers WJ, Merz NB, Sopko G, Pepine CJ, "Coronary microvascular dysfunction is highly prevalent in women with chest pain in the absence of coronary artery disease: Results from the NHLBI WISE Study," Am Heart J, V. 141, No. 5 (May 2001), pp. 735-741.


How reproducible are EndoPAT results?

The EndoPAT test is both operator and interpreter-independent.

Selamet et al* tested prospectively the reproducibility and feasibility of the EndoPAT. EndoPAT tests were performed on two different days separated by no more than seven days in 30 healthy fasting adolescents, ages 13 to 19 years. The authors concluded, “In healthy adolescents, Endo-PAT is feasible and has excellent reproducibility.” Moreover, the authors stated, “This technology may provide an easy and reliable means of assessing endothelial function in the pediatric population.”

* Selamet Tierney ES, Newburger JW, Gauvreau K, Geva J, Coogan E, Colan SD, Ferranti SD. Endothelial Pulse Amplitude Testing: Feasibility and Reproducibility in Adolescents. J Pediatr. 2009; 154(6):901-5.


What is the association between the EndoPAT and BAUS (Brachial Artery Ultrasound)?

BAUS is a common research method for peripheral, noninvasive assessment of endothelial function. It differs from EndoPAT in several ways. While the BAUS assesses a single conduit vessel, EndoPAT measures several vascular beds, composed of small vessels and the microcirculation, giving a more representative result. Furthermore, EndoPAT corrects for systemic changes in blood pressure during the test by a simultaneous measurement from the (un-occluded) contra-lateral arm. With minimal training necessary, EndoPAT is practically operator-independent, while BAUS requires a trained ultrasound technician and is highly user-dependent in both data acquisition and analysis. Also, the response measured with EndoPAT has a much larger dynamic range (up to three-fold) compared to the small changes assessed by BAUS (around 10% for a normal response). Several studies have simultaneously measured Flow-Mediated Dilatation (FMD) with EndoPAT and BAUS. Kuvin et al* at the Tufts Medical Center, Boston, demonstrated a significant correlation between the two methods (r=0.55, p<0.0001) in a group of 89 adult patients suffering from chest pain.

*Kuvin JT, Patel AR, Sliney KA, Pandian NG, Sheffy J, Schnall RP, Karas RH, Udelson JE. Assessment of Peripheral Vascular Endothelial Function with Finger Arterial Pulse Wave Amplitude. AHJ 2003; 146: 168-74.


What is the association between the EndoPAT and NO bioavailability?

Nohria and Gerhard et al* at the Brigham & Women’s Hospital, Boston, demonstrated the central role for nitric oxide (NO) in the post-occlusion vasodilatory response measured by EndoPAT. EndoPAT index (RHI) was measured in a group of nineteen healthy volunteers, before and after intra-arterial infusion of L-NAME (a specific inhibitor of endothelial nitric oxide synthase). Fifteen matched controls were infused with saline or phenylephrine (an endothelium-independent vasoconstrictor). The study reported that L-NAME blocked 46% of the vasodilatory response (p=0.002). These results provide direct confirmation that EndoPAT indeed measures a NO-mediated endothelial response.

*Nohria A, Gerhard-Herman M, Creager MA, Hurley S, MitraD, Ganz P. The Role of Nitric Oxide in the Regulation of Digital Pulse Volume Amplitude in Humans. J Appl Physiol 2006; 101:545-8.


What is the association between the EndoPAT and the acetylcholine angiogram?

The infusion of acetylcholine into a catheterised coronary artery whilst assessing the resulting dilation with a coronary angiogram, is the gold standard in measuring coronary endothelial function. However, due to its invasive nature and complex techniques, it is only used during research.

EndoPAT’s Reactive Hyperaemia Index has been shown to have an 80% sensitivity and 85% specificity in diagnosing coronary artery disease when compared to acetylcholine angiogram*.

*Bonetti PO, Pumper GM, Higano ST, Holmes DR Jr, Kuvin JT, Lerman A (Dec 2004). "Noninvasive identification of patients with early coronary atherosclerosis by assessment of digital reactive hyperemia". J Am Coll Cardiol 44 (11): 2137–41.


Why does the EndoPAT test require using both arms?

Think of the EndoPAT evaluation as a one-person clinical study with you comparing yourself to yourself. While endothelial function is being tested in one arm with the blood pressure cuff and finger monitor, your other arm is being used to monitor changes in systemic blood flow that affects both arms. By measuring both arms, EndoPAT automatically corrects for any systemic changes that may occur during the course of the test and calculates a final score based on information gathered from both finger monitors.


Which arm should be used for occlusion/control?

The non-dominant arm is recommended as the tested (occluded) arm due to a lower mass of muscle which leads to easier arterial occlusion.


Does it matter which fingers are used for the test?

Any finger but the thumb may be used. Placement of the sensors on the index fingers is recommended. Due to variance between fingers in blood supply, symmetrically-paired fingers on both arms should be used (i.e. either both index fingers or both middle fingers). The thumb should be avoided.


Why does the occlusion last 5 minutes?

Five minutes is the optimal time needed to generate the forced stimulus response of the blood rushing through the vessels after release of an occlusion. Faizi et al. (4) tested the effects of varying occlusion durations as well as the effects of occlusion location in 30 apparently healthy adult volunteers. When comparing different occlusion times (1.5, 3, 5 and 8 minutes) with the cuff placed on the forearm, they saw that the effective maximal response was reached at 5 minutes. The occlusions shorter than 5 minutes had significantly lower responses. The response to a 5 minute occlusion did not differ significantly from 8 minutes, but caused less discomfort.


Does having the blood pressure cuff inflated on the upper arm for five minutes cause any discomfort?

The five minute blood pressure cuff inflation is an accepted standard test to cause reactive hyperemia (the increase of blood flow after a temporary restriction in blood supply) for the assessment of endothelial function. While the occlusion may cause some minor discomfort and tingling in the fingers, the test is absolutely harmless.


Where should we place the blood pressure cuff for the occlusion?

The optimal location is on the upper arm. Faizi et al* tested twenty individuals with the cuff placed on their upper arm, occluding the brachial artery for 5 minutes. These results were compared to their 5 minute forearm occlusion test, showing an RHI of 1.88 (±0.55) for the forearm occlusion and 2.07 (±0.69) for the upper-arm occlusion (p=0.097). Forearm occlusion was reported to cause less discomfort than the upper arm.

*Faizi AK, Kornmo DW, Agewall S. Evaluation of Endothelial Function Using Finger Plethysmography. Clin Physiol Funct Imaging 2009 Jun 22.


What are the clinical setting requirements for the EndoPAT test?

The room selected for the study should be in a quiet area. Thermo-neutral room temperature should be maintain 21oC-24oC, and dimmed lights are preferred when performing the study. The test can be performed on a comfortable bed, exam table or armchair which will enable placing both hands at heart level in a rested position.


What is the PAT signal?

The PAT (Peripheral Arterial Tonometry) signal is a proprietary technology used for non-invasively measuring arterial tone changes in peripheral arterial beds. The PAT signal used in the EndoPAT is measured from the fingertip by recording finger arterial pulse volume changes. Results of the 15-minute test are automatically calculated and an RHI is generated, which indicates the present state of endothelial health.


Abnormal EndoPAT result. Low Reactive Hyperemia Index quantifies the limited dilatation of the vascular bed in the occluded finger once the occlusion was released (lower tracing), compared to the pre-occlusion dilatation. The upper tracing of the non-occluded arm acts as a control for systemic changes in blood flow during the testing interval.


Can the test be performed on children?

Although most of the testing on EndoPAT has been done on adults, there are many published studies with the EndoPAT on children and adolescents. There is no special design of the biosensors for children. The length of the EndoPAT biosensor is 5cm, thus the length of the finger of the child must be at least this long.

Selamet et al* tested prospectively the reproducibility and feasibility of the EndoPAT in adolescents. EndoPAT tests were performed on two different days separated by no more than seven days in 30 healthy fasting adolescents, ages 13 to 19 years. The authors concluded, “In healthy adolescents, Endo-PAT is feasible and has excellent reproducibility.” Moreover, the authors stated, “This technology may provide an easy and reliable means of assessing endothelial function in the pediatric population.”

* Selamet Tierney ES, Newburger JW, Gauvreau K, Geva J, Coogan E, Colan SD, Ferranti SD. Endothelial Pulse Amplitude Testing: Feasibility and Reproducibility in Adolescents. J Pediatr. 2009; 154(6):901-5.