Minimal coronary microvascular resistance: agreement between continuous and bolus thermodilution

Abstract Background Continuous thermodilution quantifies absolute microvascular resistance (Rµ, Woods units), the quintessential metric of microvascular function. Rµ is minimal during hyperaemia (Rµ,hyper) with increased Rµ,hyper suggestive of microvascular dysfunction. Bolus thermodilution measures the index of microcirculatory resistance (IMR), a dimensionless surrogate of Rµ,hyper. Purpose We compared Rµ,hyper measured by continuous thermodilution (invasive Rµ,hyper) with the gold standard [15O]H2O positron emission tomography (PET Rµ,hyper), and explored the correlation between invasive Rµ,hyper and uncorrected IMR. Methods The accuracy of invasive Rµ,hyper was assessed in a cohort of 24 patients in which both invasive and PET Rµ,hyper were measured (cohort 1). For both techniques, Rµ,hyper was assessed in both the left anterior descending artery (LAD) and the circumflex artery (LCX), corresponding to 46 measurement of Rµ,hyper in total (LAD=24, LCX=22). Agreement between invasive Rµ,hyper and IMR was evaluated in a cohort of 250 patients with angina and non-obstructive coronary arteries (ANOCA) evaluated in 3 European centres (cohort 2). All measurements in this cohort were performed in the LAD, with patients randomly assigned in a 1:1 ratio to undergo either bolus or continuous thermodilution first. Both techniques were performed by the same operator during the same procedure with a strict interval of 5 minutes between techniques. Results In cohort 1, invasive Rµ,hyper exhibited a strong correlation with PET Rµ,hyper (r=0.86 [95% CI 0.76-0.92], p<0.001), with excellent absolute agreement (ICC 0.82 [95% CI 0.70-0.90], p<0.001) (Figure 1). Furthermore, 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 to 1.15]) bias between invasive Rµ,hyper and PET Rµ,hyper. However, in cohort 2, invasive Rµ,hyper exhibited no significant correlation with IMR (r=0.11 [95% CI -0.01 to 0.23], p=0.08) (Figure 2). Conclusion Invasive Rµ,hyper derived from continuous thermodilution exhibited excellent agreement with non-invasive Rµ,hyper measured by [15O]H2O PET, the current standard of reference. However, IMR exhibited no significant correlation with invasive Rµ,hyper in patients with stable coronary syndromes.Figure 1.PET vs invasive Rµ,hyperFigure 2.Invasive Rµ,hyper vs IMR