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 .
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.
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.
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).
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.