Imaging during bypass surgery

To restore blood flow to the heart in patients with clogged arteries, cardiac surgeons often perform coronary artery bypass grafting. As the procedure’s name implies, new blood vessels are grafted onto the coronary arteries to create new routes of blood flow, bypassing the occlusion. However, grafts sometimes fail and, thus, do not permit blood flow. Various methods can help surgeons determine whether grafts will allow blood flow — that is, if they are “patent” — and researchers at John Radcliffe Hospital in Oxford, UK, have reviewed these methods.

They state that, although coronary angiography — an x-ray of blood vessels of the heart — remains the gold standard for imaging grafts, it usually is not available in operating rooms because it is invasive, increases surgery time and requires extra personnel, and because some of the contrast agents can damage the kidneys. Therefore, alternative procedures are needed.

The best alternative techniques are intraoperative fluorescence imaging and transit-time flowmetry. Fluorescence imaging uses indocyanine green, a fluorophore that binds to plasma proteins. The fluorophore emits at 830 nm when excited at 806 nm, and a CCD video camera records the fluorescence signal. The fluorophore and laser have proven safe after years of clinical use.

In transit-time flowmetry, a probe with a reflector is placed on the graft and between two ultrasound transducers. One transducer produces ultrasonic waves in the direction of blood flow, while the other does so in the opposite direction. The difference in time that it takes for the waves to travel between the transducers yields the approximate flow volume.

Intraoperative fluorescence imaging and transit-time flowmetry provide consistently accurate information about blood flow in grafts, although neither can detect minor abnormalities unrelated to blockage. Whereas flowmetry quantifies blood flow, fluorescence imaging shows whether blood flow is excellent, satisfactory or poor. However, fluorescence imaging is more sensitive and less prone to false-positives that lead surgeons to redo grafts unnecessarily. (The Annals of Thoracic Surgery, June 2007, pp. 2251-2257.)