HETEROGENEITY OF MYOCARDIAL PERFUSION PROVIDES THE PHYSIOLOGICAL-BASIS OF PERFUSABLE TISSUE INDEX

Assessment of viable from nonviable myocardium is critical for the car e of patients being considered for revascularization procedures. Recen tly, the perfusable tissue index (PTI) has been proposed as an index o f myocardial viability. Methods: Computer simulations were performed f or homogeneously and heterogeneously perfused tissue over a wide range of flows (0.04-6.4 ml/g/min) using both bolus and infusion inputs. Re sults: PTI estimated from simulated homogeneously perfused tissue did reflect the amount of tissue being perfused independent of absolute le vel of flow, type of input or model configuration, whereas PTI obtaine d from simulated heterogeneously perfused tissue was consistently lowe r than the simulated ''true'' PTI and varied with flow, type of input function and model configuration. Flow estimated with O-15-water was n ot significantly different from that measured with radio labeled micro spheres. Conclusion: Oxygen-15-water can diffuse into both acutely and chronically ischemic myocardium irrespective of its functional status . The results suggest that PTI is most likely an index of the heteroge neity of myocardial flow rather than an index of the amount of tissue being perfused. Its utility for delineating myocardial viability is th us related to the amount of tissue perfused that has low absolute leve ls of perfusion or high degrees of flow heterogeneity.