Value of diastolic flow with transit-time flow meters in coronary artery bypass surgery

Abstract

Cardiac surgeons who concentrate on coronary revascularization realize that not every coronary artery is the same. We also know after each anastomosis, whether done on a beating heart or under cross-clamp, if the graft will be successful. For years, we would rely on symptom or event recurrence or repeat cardiac catheterization to determine graft patency. Recently, experience has grown with a variety of Doppler flow signals, the most commonly used being transit-time flow meters. Today, many of us view these flow probes as an adjuvant crutch to give us confirmation of anastomotic quality; yet, parenthetically, there is a paucity of data to demonstrate shortor long-term graft patency with good intra-operative flow assessments. The article by Nordgaard and colleagues [1] describing the value and variance of pulsatility index (PI) highlights another immediate limitation of the flow meter — that being interpreting patterns of values registered by the flow meters and conveying that to quality of the bypass graft. PI is one of the more-valued measurements, and is regarded as an indicator of flow resistance, or more appropriately, as a potential stricture at the anastomotic site. The PI should be 50% for all grafts and territories, and ideally >65%. If both the PI and DF% give correlative good measurements, then, the certainty of graft quality and distal coronary perfusion can be objectively assessed with greater comfort. In no instance has the author seen a PI > 7 with a normal DF%, and if DF < 25%, there is a problem with the graft or target. Although the author uses a flow meter on each coronary revascularization case, it adds value under circumstances that needs confirmation (immediately) of graft flow for a difficult anastomosis, or endarterectomy, and/or vein patch. Even though graft flows have had excellent intraoperative PI and DF% characteristics, the author could observe, years later, vein-graft closure; therefore, it is still unclear how transit-time flow meters help guide long term patency. Subanalysis of the randomized onversus off-pump coronary artery bypass surgery trial (ROOBY) may provide insight on graft patency based on intra-operative flow meter measurements and correlative 1-year angiographic analysis [2]. Clearly, other factors exist to determine graft patency, and risk-factor-modifying medication and dual antiplatelet therapy are important; yet, only time will tell whether intraoperative flow meters help guide long-term results.