Use of peripheral arterial tonometry in detection of abnormal coronary flow reserve

We assessed the utility of EndoPAT, a device that measures reactive hyperemia index (RHI) as a clinical screening tool for identifying low coronary flow reserve (CFR). Distinguishing normal from low CFR aids assessment for coronary microvascular dysfunction (CMD) or large vessel coronary artery disease (CAD).

From June 2014-May 2019, in a convenience sample, we measured RHI in adults undergoing clinically indicated cardiac Rubidium-82 positron emission tomography/computed tomography (PET/CT) at a single center. Exclusion criteria were inability to consent, lack of English proficiency, and physical limitation. We defined low RHI as <1.67 and low CFR as <2.5. Distribution of RHI was skewed so we used its natural logarithm (LnRHI) to calculate Pearson correlation and area under the curve (AUC).

Of 265 patients with PET/CT, we enrolled 131, and 100 had adequate data. Patients had a mean age of 61 years (SD = 12), 46% were female, 29% non-white. Thirty-six patients had low RHI, and 60 had depressed CFR. LnRHI did not distinguish patients with low from normal CFR (AUC = 0.53; 95% Cl, 0.41-0.64) and did not correlate with CFR (r = -0.021, p = 0.83). Low RHI did not distinguish patients with traditional CAD risk factors, presence of calcification, or perfusion defect (p > 0.05). Conversely, mean augmentation index, a measure of arterial stiffness, was higher with low RHI (p = 0.005) but not CFR (p = 0.625). RHI was lower in patients we identified as CMD (low CFR, no perfusion defect and calcium score of 0) (1.88 versus 2.21, p = 0.35) although we were underpowered (n = 12) to meet statistical significance.

Peripheral RHI is insufficient as a clinical screening tool for low CFR as measured by cardiac PET/CT. Differences in vascular pathology assessed by each method may explain this finding.