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Transit-time flow predicts outcomes in coronary artery bypass graft patients
8 March 2010
MedWire News: Researchers report that a high pulsatility index (PI) on intraoperative measurement of transit-time flow (TTF) of arterial grafts is associated with poor outcomes in patients undergoing coronary artery bypass graft (CABG) surgery.
Their study of 336 consecutive patients who received a total of 1000 arterial grafts found those with grafts with a high PI had a significantly increased risk for major adverse cardiac events (MACE), despite having no other intraoperative measurements suggesting graft malfunction.
Lead author of the study Teresa Kieser (University of Calgary, Alberta, Canada) believes cardiac surgeons should routinely measure TTF of all arterial grafts, instead of relying on finger palpation assessment of pulse, to ensure the best possible outcome for patients.
Speaking with MedWire News, Kieser explained: “Until recently we haven’t had a device to assess grafts objectively and at the time we were writing the paper still only 20% of surgeons in North America were using TTF. You can use palpation, but ‘pulse does not equal flow’ – you cannot tell just by feeling the graft whether it is good or not.”
Measuring TTF gives the surgeon the chance to revise problematic grafts or identify other problems while the chest is still open, according to Kieser. “The bottom line is, if you use this and you have a good graft in the operating room, the chances are you are going to have a good result in your patient,” she told MedWire News.
For the study, Kieser and colleagues used TTF to measure flow in 990 (99%) of the grafts. They revised grafts suspected to be functioning poorly by usual clinical criteria, ie, electrocardiogram changes, hemodynamic instability, and new regional wall motion abnormalities on echocardiogram. Grafts that were abnormal on TTF measurement (PI >5, flow <15 cm3/min, and diastolic filling <25) were not revised unless the TTF corroborated clinical suspicions or was surprisingly abnormal.
As reported in the European Journal of Cardio-thoracic Surgery, satisfactory grafts were achieved in 916 (93%) of the grafts, and these had flows of 34 to 61 cm3/min, PI ≤5, and diastolic filling 62–85%. A total of 20 (2%) grafts were revised.
Over a mean of 3 years of follow-up, MACE (recurrent angina, perioperative myocardial infarction, postoperative angioplasty, re-operation and/or perioperative death) occurred in 10 (17.0%) of 59 patients with at least one graft with a PI >5 compared with 15 (5.4%) of 277 patients with at least one graft with PI ≤5.
After adjusting for the two other independent covariates, age and admission status, in multivariable analysis, the risk for MACE was 4.2 times higher in patients with a PI >5 graft than in those with a lower PI graft (p=0.002).
Neither flow nor diastolic filling independently predicted MACE, the authors note.
Further analysis revealed direct correlation between the territory of the high PI graft and territory causing angina, angioplasty, or myocardial infarction in seven of the 10 patients with at least one PI >5 graft who suffered MACE.
Kieser emphasized that abnormal TTF readings are not well defined, and there is no clear PI cutoff that signals a problem.
“If it’s under 3, you don’t need to worry. If it’s between 3 and 5, I like to try to invoke some reason, I look to see if there is competitive flow,” she explained. “So it’s a ‘zone’ you look at. For the most part, I really look at it closely if it’s over 5.”
Even then, a high PI does not necessarily mean graft revision is required, but at least the surgeon can take a further look, Kieser said.
“If the PI is 25 and there’s no flow – you know you have a really bad graft. But if you have a PI of 6 with a flow of 30 [cm3/min], it may be competitive flow.”
And redoing a graft does not always resolve abnormal TTF readings, but still in these cases Kieser said a high PI alerts her to focus more closely on the patient and look for other potential causes.
Given each TTF measurement takes around 15–20 seconds per graft, adding only 2 or 3 minutes in total to the operation time, and sometimes uncovers a problem in a graft that otherwise seems perfect, Kieser indicated that surgeons can and should adopt this approach.
Kieser is continuing to research TTF, to determine how to interpret readings for sequential arterial grafts, and eagerly anticipates the introduction of newly developed technology that generates a visual image of TTF readings, offering a tool similar to angiography.
“TTF gives you function but you also need anatomy – and this will give you both,” she said.
