Coronary flow reserve (CFR) and microvascular resistance reserve (MRR) can, in principle, be derived by any method assessing coronary flow.
The aim of this study was to compare CFR and MRR as derived by continuous (CFR _cont and MRR _cont ) and bolus thermodilution (CFR _bolus and MRR _bolus ).
A total of 175 patients with chest pain and nonobstructive coronary artery disease were studied. Bolus and continuous thermodilution measurements were performed in the left anterior descending coronary artery. MRR was calculated as the ratio of CFR to fractional flow reserve and corrected for changes in systemic pressure. In 102 patients, bolus and continuous thermodilution measurements were performed in duplicate to assess test-retest reliability.
Mean CFR _bolus was higher than CFR _cont (3.47 ± 1.42 and 2.67 ± 0.81 [P < 0.001], mean difference 0.80, upper limit of agreement 3.92, lower limit of agreement -2.32). Mean MRR _bolus was also higher than MRR _cont (4.40 ± 1.99 and 3.22 ± 1.02 [P < 0.001], mean difference 1.2, upper limit of agreement 5.08, lower limit of agreement -2.71). The correlation between CFR and MRR values obtained using both methods was significant but weak (CFR, r = 0.28 [95% CI: 0.14-0.41]; MRR, r = 0.26 [95% CI: 0.16-0.39]; P < 0.001 for both). The precision of both CFR and MRR was higher when assessed using continuous thermodilution compared with bolus thermodilution (repeatability coefficients of 0.89 and 2.79 for CFR _cont and CFR _bolus , respectively, and 1.01 and 3.05 for MRR _cont and MRR _bolus , respectively).
Compared with bolus thermodilution, continuous thermodilution yields lower values of CFR and MRR accompanied by an almost 3-fold reduction of the variability in the measured results.