Minimal Microvascular Resistance: Agreement Between Continuous and Bolus Thermodilution

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 [¹⁵O]H₂O 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 [¹⁵O]H₂O 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.