San Francisco, CA - A new intraoperative graft angiography system called SPY™ (Novadaq Technologies) is helping cardiac surgeons to check the quality of coronary artery bypass grafts. The new SPY system is better than the most commonly used technique for graft assessment, transit-time flow (TTF) measurement, according to a clinical trial presented recently at the American Association for Thoracic Surgery annual meeting in San Francisco.

Dr Nimesh Desai (Sunnybrook and Women's College Health Sciences Center, Toronto, ONT) presented the results of the study.

Desai told heartwire: "Bypass surgery is one of the only major cardiac procedures where there is no imagery to verify the quality of the job done. The SPY system provides superior diagnostic accuracy compared with TTF. When we first saw the SPY technology a few years ago, we were excited by the quality of images we were able to get."

The SPY system was approved for use in the US earlier this year and is available in Europe, Canada, and Japan.

Dr David Taggart (John Radcliffe Hospital, Oxford, UK), who along with Desai has one of the most extensive experiences of using the SPY system to date, told heartwire: "CABG surgery is one of the few interventions where there is little or no intraoperative quality control. This is a major issue, particularly as surgical cases become technically more challenging. Until recently, there has been no user-friendly system available to check graft quality in the OR that produces high-quality images that are also easy to interpret. The SPY imaging system meets these criteria."

Desai explains that up to 12% of bypass grafts are not working immediately after surgery, and the cause of these early graft failures is often technical errors that could be corrected if they were seen at the time of the operation.

The ideal "gold standard" for assessing grafts would be to perform angiography immediately following surgery, but this is rarely done because of the risk of bleeding and also because of cost constraints, he says. "The standard for the past 35 years has been not to do anything postoperatively but to perform a standard stress test or echo three months after the bypass operation."

More recently, the technique of TTF measurement has been adopted to verify the quality of grafts, "mainly by those performing off-pump surgery who are concerned about the patency of grafts," says Desai. TTF uses an intraoperative ultrasound probe to measure the volumetric rate of flow through the graft, which only takes a few minutes, and Desai estimates it is used in approximately 10% of bypasses performed in North America.

However, there are problems with TTF, Desai notes. "It doesn't give you a picture of the actual anatomy, and if there is any flow whatsoever you get a signal, even though the graft could be 90% blocked. And if you do find a problem, you don't know whether it's at the proximal or the distal end of the anastomosis." In trials comparing TTF with angiography immediately after surgery, it was shown that TTF picks up total occlusions in grafts but not those blocked to a lesser degree.

The SPY system involves intravascular administration of fluorescent indocyanine green (ICG) dye and visualization of the graft with dispersed laser light. It enables surgeons to view, record, replay, print, and achieve real-time images of the coronary arteries and bypass grafts. Desai estimates that using the SPY system adds about eight to 10 minutes to each operation.

In the study reported by Desai and colleagues last week, 106 patents undergoing isolated CABG both on and off pump with no contraindications for postoperative angiography were randomly assigned to be evaluated with either SPY then TTF (Medtronic Medistim™ System) or TTF then SPY. Of the 106 patients, 46 underwent x-ray angiography on postoperative day four.

The primary end point of the trial was to determine the sensitivity and specificity of the two techniques vs reference standard x-ray angiography to detect graft occlusion or >50% stenosis in the graft or perianastomotic area.

In total, 139 grafts were reviewed with all three techniques, and 12 grafts (8.2%) were demonstrated to have >50% stenosis or occlusion by the reference standard. SPY was found to provide superior diagnostic accuracy for detecting clinically significant graft errors vs TTF measurement.

SPY vs TTF

Technique Sensitivity to detect >50% stenosis or occlusion (%) Specificity to detect >50% stenosis or occlusion (%) SPY 83.3 100 TTF 25 98.4

The p value for the overall comparison of sensitivity and specificity between SPY and TTF was 0.011. The difference between sensitivity for SPY and TTF was 58% (95% CI 30%-86% [p=0.023]).

Taggart told heartwire that in a trial performed by his team in around 100 patients assessing graft patency with both techniques, "for 80% of the grafts, both were okay." But in around 10% of cases where TTF was used, "we would have ended up revising the graft." Both techniques "are useful," he says, "but the SPY system is easier to use and more reliable." This study has been submitted to the Journal of Thoracic and Cardiovascular Surgery for publication, Taggart adds.

Desai told heartwire there is one limitation to SPY. "You can't see radial-artery grafts very well," he says. This is because the SPY system uses laser light in the infrared range that is dispersed by the thick muscular pedicle of the radial artery.

However, he says that his team still uses the SPY system for radial-artery grafts and that "you can still see the most important part, the anastomosis. We try to get around any problems by removing some of the tissue of the graft."

Taggart says his team uses a slightly different approach when using radial arteries in that they remove some of the tissue from them before grafting (a technique known as skeletonizing), so this circumvents the problem.

Desai says the SPY system has been used in more than 300 CABG procedures at his institution and that they bought the system at a cost of around Can $100 000 in 2002. However, he says he believes company now charges a per-procedure fee.

Novadaq told heartwire that it charges between $450 and $750 per procedure and that this is "in line" with the procedural costs of TTF.

Taggart says his institution is the only one using the SPY system in the UK and that they have carried out around 300 procedures. However, he doesn't know anything about the costs of the technology, as Novadaq provided the equipment for research purposes. "We are getting an updated version of the technology soon," he added.

Desai has no conflict of interest. Taggart is on the scientific advisory board of Novadaq and has options to buy shares.

	Source Desai ND, Miwa S, Kodama D, et al. A randomized comparison of intraoperative angiography and transit-time flow measurement to detect technical errors in coronary artery bypass grafts. American Association for Thoracic Surgery 2005 meeting; April 10-13, 2005; San Francisco, CA.