Clinical importance of measuring coronary graft flows in the revascularized heart. Ultrasonic or electromagnetic?

Background: In the past, routine coronary graft flow measurement at the end of coronary artery bypass grafting (CABG) was not universally adopted by cardiac surgeons due to the lack of reliable flow measurement techniques. The purpose of this study was to investigate the efficacy of ultrasonic and electromagnetic techniques in coronary graft flow measurements and to determine the relationship, if any, between intraoperative ultrasonic or electromagnetic coronary graft flows and postoperative early clinical outcome.

Methods: We studied 66 consecutive patients who underwent elective CABG using internal thoracic artery (ITA) and reversed saphenous vein graft (SVG) conduits. All patients were males with the mean age of 65 +/- 1 yrs (range = 45 to 80 yrs). Coronary bypass graft flows (both ITA and SVG) were determined by the use of both ultrasonic and electromagnetic flowmeters. In addition, the flow waveform pattern was continuously recorded and analyzed with the ultrasonic technique. In this prospective non-randomized study, the following variables were considered in the forward stepwise multivariate regression analysis of the data: age, weight, body surface area, ejection fraction, perfusion and ischemia times, number of grafts, amount of allogenic banked blood, platelets, fresh frozen plasma transfusions, cardiac output/index, ultrasonic (USF) and electromagnetic flows (EMF), length of intensive care unit (ICU) and hospital stays, and early (30-day) mortality.

Results: Based on their location, 226 grafts were divided into four groups: (I) ITA to left anterior descending (LAD) (n = 66) 34 +/- 2.5 ml/min USF and 45 +/- 4.4 ml/min EMF; (II) SVG to circumflex (CX) (n = 62) 33 +/- 2.4 m/min USF and 58 +/- 4.9 ml/min EMF; (III) SVG to diagonal (DIAG) (n = 37); 30 +/- 3.5 ml/min USF and 50 +/- 6.0 ml/min EMF; (IV) SVG to right coronary artery (RCA) (n = 61); 36 +/- 3.1 ml/min USF and 56 +/- 5.3 ml/min EMF. Electromagnetic flow measurements were higher than USF values in all locations (p < 0.05). Difficulties in obtaining proper contact with the vessel wall and finding suitable size probes were major drawbacks in measurement of ITA graft flow by the use of electromagnetic technique. All flow measurements were done within 10 minutes or less. There was no demonstrable correlation between the length of stay (ICU and hospital), and coronary graft flows at the ITA to LAD, SVG to DIAG, or SVG to CX locations. However, ultrasonic coronary graft flows at the SVG to RCA location had a significant inverse correlation with the length of ICU and hospital stays (r = -0.45, p < 0.0005 for both). Early mortality was unaffected by the intraoperative coronary graft flow values (p = NS).

Conclusions: The ultrasonic flowmeter is well-suited for intraoperative assessment of arterial and venous coronary graft flows at the completion of CABG. There is a real potential for using intraoperative graft flow values to predict early outcome after coronary bypass.