Mapping tissue oxygenation in the beating heart with near-infrared spectroscopic imaging

Abstract

Coronary artery disease (CAD), defined as partial or complete blockage of one or more arteries supplying blood to the heart, led to 40,000 deaths in 1998 in Canada alone. While hundreds of thousands of bypass surgeries are performed every year to treat CAD, there is currently no routine method to monitor blood or tissue oxygenation in order to gauge the success of the procedure. To provide this information, we have combined the chemical sensitivity of spectroscopy with the spatial resolution of imaging to generate maps of regional cardiac oxygenation with better than 1 mm 2 resolution. Since near-IR spectroscopic imaging has recently been used to map blood and tissue oxygenation in the arrested heart, the aims of the present study were (I) to map oxygenation in the beating heart, and (II) to determine the sensitivity of this approach in resolving different degrees of partial ischemia (reduced blood supply). Spectroscopic reflectance image sets were acquired (650–1050 nm at 10-nm intervals) for isolated, beating, blood-perfused porcine hearts ( n=4), in which the left anterior descending (LAD) artery was cannulated. Flow through the LAD artery was decreased stepwise to 50, 20 and 0% of normal flow and then restored to 100%, while flow through the rest of the tissue remained normal. Spectroscopic image sets were acquired at each step. Oxy- and deoxy-hemoglobin/myoglobin (Hb+Mb) levels were determined with a least squares spectral fitting algorithm. Oxygenation images (calculated using a ratio between the two Hb+Mb values) clearly revealed decreased oxygenation in the area at risk below the cannulation. Different levels of arterial flow were also clearly reflected by differences in oxygenation images.