The vascular endothelium is a monolayer of cells that coats the inside of the closed circulatory system of all vertebrates. Once thought to be an inert lining of the vascular lumen, the endothelium is now widely viewed as an organ in and of itself with a variety of complex structural, signaling, and metabolic functions.1 The normally functioning endothelium is a selective barrier between the circulating blood and every living cell in the body.2 Endothelial cells play a critical role in proper vasomotion, cellular growth, coagulation, and inflammation in local vascular beds, and systemically.3 Their location throughout the body, as well as their inherent ability to act independently makes endothelial cells ideal for these regulatory functions.4
A healthy endothelium maintains normal blood flow through the vascular system by regulating a baseline vasodilatory, antithrombotic, and anti-inflammatory state. The properly functioning endothelium will also limit excessive vascular smooth-muscle growth and coordinate the growth of newly forming vessels.1 Endothelial dysfunction complicates cardiovascular and other life-threatening diseases in humans and can have negative local and systemic effects.5–7 In various disease states, the homeostatic balance normally maintained by the endothelium may be shifted toward vasoconstriction, coagulation, inflammation, and deleterious thickening or convolution of vessels.8 In humans, this can result in a life-threatening disease process when it involves the coronary or cerebral circulation.5,9
Nitric oxide is a critical regulatory molecule involved in many of the endothelial-dependent functions that maintain normal circulatory homeostasis.10,11 Baseline and inducible vasodilation in response to local stimuli, such as tissue hypoxia and increased shear stress from increased blood flow, is an important NO-mediated, endothelial-dependent function.8,12,13 In a paracrine fashion, NO stimulates relaxation of the smooth muscle cells in the surrounding vascular wall.14 In part, endothelial dysfunction results from decreased synthesis of NO or increased oxidation of NO to nitrite and nitrate end products.15 This decrease in NO bioavailability associated with endothelial dysfunction is known to play a role in cardiovascular and other diseases in humans.6,16-19
Depressed endothelial-dependent vasodilation has been observed in dogs with experimental heart failure induced by rapid ventricular pacing.20–22 These studies20–22 used invasive monitoring techniques that are impractical for clinical use. Alternatively, serum nitrite and nitrate, end products of NO metabolism, have been measured in dogs with heart disease but are nonspecific indicators of endothelial function.23,24 A noninvasive technique to specifically assess endothelial function in dogs would be beneficial in the study of endothelial function in cardiovascular and other serious diseases commonly afflicting companion animals. For example, the measurement of endothelial function could help elucidate the pathophysiologic properties of heart failure in dogs, as well as provide an end point for the study of various medical or nutritional treatments for cardiac disease in dogs.
Various procedures to monitor endothelial function exist for humans, but no noninvasive test of endothelial function has been widely accepted for use in veterinary medicine. Flow-mediated vasodilation involves the ultrasonographic measurement of endothelial-dependent vasoreactivity in a peripheral artery of an arm or leg.11,25 An increase in the local blood flow (ie, reactive hyperemia) to the limb is created after release of a 5-minute arterial occlusion, created by inflation of a sphygmomanometric (blood pressure) cuff to suprasystolic pressures. The increased blood flow and resulting shear stress on the luminal endothelial surface are the triggers for NO release and subsequent vasodilation of the conduit arteries of the limb.25,26 This series of events is recorded with ultrasonographic measurements of the change from baseline in arterial diameter and blood flow velocity through these dilated conduit arteries. Flow-mediated vasodilation is impaired in humans with cardiovascular disease, and a decreased FMD response has been correlated with numerous cardiovascular risk factors.8,27-32 Flow-mediated vasodilation improves in human patients receiving certain cardiovascular treatments, including preventative medication, exercise, and dietary and lifestyle changes.3,28,29,33
The purpose of the study reported here was to develop a protocol to noninvasively test endothelial function in dogs on the basis of the FMD procedure used in humans. The specific objective of this study was to determine whether ultrasonography could be used to reproducibly measure the arterial diameter and blood flow velocity at baseline in dogs; compare brachial and femoral imaging sites for measurement of arterial diameter and blood flow velocity in dogs; compare a 1-, 3-, and 5-minute occlusion period in creating a significant change in arterial diameter and blood flow velocity in dogs; and compare times after cuff deflation (15, 30, or 60 seconds) to document a change in the arterial diameter and blood flow velocity measurements in dogs. Once this noninvasive technique to measure endothelial function is developed in healthy dogs, future studies can evaluate whether changes in FMD occur in dogs with cardiovascular diseases, such as dilated cardiomyopathy, chronic valvular disease, or congenital heart diseases. Furthermore, if dogs with cardiovascular disease have altered FMD, then FMD may prove useful as a risk assessment tool or as a technique to document the clinical benefit from drug, nutritional, or lifestyle interventions.
Multistix, Bayer Corp, Elkhart, Ind.
HDI-5000, Philips ATL, Bothell, Wash.
SC 5, Hokanson, Bellevue, Wash.
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