Minimal Microvascular Resistance: Agreement Between Continuous and Bolus Thermodilution

Background Continuous thermodilution quantifies absolute microvascular resistance (R μ , Wood units), a key metric of microvascular function. R μ is minimal during hyperemia (R μ,hyper ) with increased R μ,hyper suggestive of microvascular dysfunction. Bolus thermodilution measures the index of microcirculatory resistance (IMR), a dimensionless surrogate of R μ,hyper . Aims We compared R μ,hyper measured by continuous thermodilution (invasive R μ,hyper ) with the gold standard [ 15 O]H 2 O positron emission tomography (PET R μ,hyper ), and assessed the correlation between invasive R μ ,hyper and IMR. Methods First, the accuracy of invasive R μ,hyper was assessed in a cohort of 24 patients in which both invasive R μ,hyper and PET R μ,hyper were measured in the left anterior descending (LAD) and circumflex (LCX) arteries, corresponding to 46 measurements of R μ,hyper in total (LAD = 24, LCX = 22). Next, agreement between invasive R μ,hyper and IMR was evaluated in the LAD in a cohort of 250 patients with angina and non‐obstructive coronary arteries. Results Invasive R μ,hyper exhibited a strong correlation with PET R μ,hyper (r = 0.86 [95% CI 0.76–0.92], p < 0.001), with good absolute agreement (ICC 0.82 [95% CI 0.70–0.90], p < 0.001). Passing‐Bablok regression analysis found no significant systematic (intercept A: 54.53 [95% CI ‐18.95 to 120.96]) or proportional (slope B: 0.90 [95% CI 0.71–1.15]) bias between invasive R μ,hyper and PET R μ,hyper . However, invasive R μ,hyper exhibited no significant correlation with IMR (r = 0.11 [95% CI ‐0.01 to 0.23], p = 0.08). Conclusion Invasive R μ,hyper derived from continuous thermodilution exhibited excellent agreement with noninvasive R μ,hyper measured by [ 15 O]H 2 O PET, the current non‐invasive standard of reference. In contrast, IMR exhibited no significant correlation with invasive R μ,hyper in patients with angina and non‐obstructive coronary arteries.