The Annals of thoracic surgery 2018 03 29() pii S0003-4975(18)30407-7
Guidelines advocate Transit time flowmetry (TTFM) for intraoperative graft patency verification during coronary artery by-pass grafting (CABG), but studies on accuracy and precision of the TTFM technique are few. In an observational study of CABG patients, we analysed co-variation of left internal mammary artery (LIMA) blood flow with TTFM and free flow measurements.
Co-variation of TTFM and free blood flows was evaluated in 60 patients undergoing CABG using LIMA as one of the conduits. With LIMA flow measurements routinely performed before and after vasodilation results are based on 120 paired intra-operative measurements.
As demonstrated by a combined approach of regression and Bland-Altman analysis for the two flow situations, TTFM was higher than free flow in 64% of measurements, with an overestimation by TTFM of 7.1±16.3% in the overall cohort (pre-vasodilation), statistically carried by measurements with 4 mm probes (overestimation by 13.3±15.4%)(both p<0.01). In a multi-regression analysis, oversizing of the TTFM probe (OR=9.56 (95% CI 2.03-45.10), p=0.004) and high flows (OR=1.02 (95% CI 1.01-1.04), p<0.001) were independent determinants of flow overestimation by TTFM, though in the Bland-Altman analysis no systematic overestimation was seen in the post-vasodilation situation. In a ROC analysis, optimal cut-off value as determined from Youden´s index for assuming flow overestimation was 68 ml/min. CONCLUSIONS
Overall, with slight overall overestimation of 7.1%, TTFM is an accurate indicator of LIMA blood flow during CABG with a clinically acceptable precision. Overestimation may be expected with flows >68 ml/min, but most importantly in situations with oversized TTFM probes.