Mohammad W. Akhtar e-mail: mwakhtar3@uh.edu Stanley J. Kleis 1 e-mail: kleis@uh.edu Ralph W. Metcalfe e-mail: metcalfe@uh.edu Department of Mechanical Engineering, University of Houston, Houston, TX 77204 Morteza Naghavi Fairway Medical Technologies, Inc., 710 N. Post Oak Road, Suite 204, Houston, TX 77024 e-mail: mn@vp.org Sensitivity of Digital Thermal Monitoring Parameters to Reactive Hyperemia Both structural and functional evaluations of the endothelium exist in order to diagnose cardiovascular disease (CVD) in its asymptomatic stages. Vascular reactivity, a func- tional evaluation of the endothelium in response to factors such as occlusion, cold, and stress, in addition to plasma markers, is the most widely accepted test and has been found to be a better predictor of the health of the endothelium than structural assessment tools such as coronary calcium scores or carotid intima-media thickness. Among the vascular reactivity assessment techniques available, digital thermal monitoring (DTM) is a non- invasive technique that measures the recovery of fingertip temperature after 2–5 min of brachial occlusion. On release of occlusion, the finger temperature responds to the amount of blood flow rate overshoot referred to as reactive hyperemia (RH), which has been shown to correlate with vascular health. Recent clinical trials have confirmed the potential importance of DTM as an early stage predictor of CVD. Numerical simulations of a finger were carried out to establish the relationship between DTM and RH. The model finger consisted of essential components including bone, tissue, major blood ves- sels (macrovasculature), skin, and microvasculature. The macrovasculature was repre- sented by a pair of arteries and veins, while the microvasculature was represented by a porous medium. The time-dependent Navier–Stokes and energy equations were numeri- cally solved to describe the temperature distribution in and around the finger. The blood flow waveform postocclusion, an input to the numerical model, was modeled as an in- stantaneous overshoot in flow rate (RH) followed by an exponential decay back to base- line flow rate. Simulation results were similar to clinically measured fingertip tempera- ture profiles in terms of basic shape, temperature variations, and time delays at time scales associated with both heat conduction and blood perfusion. The DTM parameters currently in clinical use were evaluated and their sensitivity to RH was established. Among the parameters presented, temperature rebound (TR) was shown to have the best correlation with the level of RH with good sensitivity for the range of flow rates studied. It was shown that both TR and the equilibrium start temperature (representing the base- line flow rate) are necessary to identify the amount of RH and, thus, to establish criteria for predicting the state of specific patient’s cardiovascular health. DOI: 10.1115/1.4001137 Keywords: reactive hyperemia, endothelial dysfunction, vascular reactivity, noninvasive assessment, digital thermal monitoring, cardiovascular disease 1 Introduction According to the World Health Organization, cardiovascular disease CVD is a leading cause of death worldwide, representing nearly 30% of all deaths an estimated 17.5  10 6  in the year 2005 1. CVD includes diseases of the heart, its blood supply, and the arteries supplying blood to the brain and peripheral tissues 2. Nearly 25% of all cardiovascular diseases are diseases of the heart coronary heart disease CHD itself including heart muscle failure cardiomyopathy, heart valve disorders, and congenital and heart rate abnormalities 3. More than 75% of all CVD is comprised of occlusive disorders of the coronary arteries causing coronary artery disease CAD or the cerebral arteries causing stroke, broadly classified as atherosclerosis 3. Atherosclerosis, an inflammatory disease of the intima and surrounding areas of the internal lamina of medium to large sized arteries, has reached epidemic proportions in most industrialized societies 2. In a sig- nificant proportion of patients, coronary atherosclerosis CAD or CHD first manifests itself in the form of sudden cardiac death 4. Patients who survived heart attack and stroke, in particular, those with rheumatic heart disease, generally have prolonged illness and disability. Necropsy studies have demonstrated that atherosclerotic changes often occur at early stages in life, although patients with CAD become symptomatic only after age 40 years 5. CVD di- agnosis in its early stages is important in order to control the development of the disease with effective therapeutic intervention. Unless awareness of CVD risk factors, symptoms, treatment op- tions, and screening tools are improved, an estimated 20  10 6 people will die from cardiovascular diseases in the year 2015 1. The detection and assessment of cardiovascular diseases at the asymptomatic stages in its early development primary preven- tion would provide a powerful tool to address the epidemiologi- cal issues and evaluate the effects of various therapeutic interven- tions before CVD presents itself in the form of a clinical event 6. As a means of primary prevention of CHD and other forms of atherosclerotic diseases, traditional risk score assessments such as the Framingham risk score FRS have been employed in order to identify asymptomatic patients at high risk 7. These risk score assessments are based on major independent risk factors such as 1 Corresponding author. Contributed by the Bioengineering Division for publication In the JOURNAL OF BIOMECHANICAL ENGINEERING. Manuscript received November 17, 2008; final manu- script received December 29, 2009; accepted manuscript posted January 28, 2010; published online March 25, 2010. Editor: Michael Sacks. Journal of Biomechanical Engineering MAY 2010, Vol. 132 / 051005-1 Copyright © 2010 by ASME Downloaded 05 Apr 2010 to 129.7.158.43. Redistribution subject to ASME license or copyright; see http://www.asme.org/terms/Terms_Use.cfm