THEORETICAL-MODEL FOR MYOCARDIAL FUNCTIONAL-CHARACTERIZATION - APPLICATION TO A GROUP OF PATIENTS EVALUATED BEFORE AND AFTER SURGICAL REVASCULARIZATION

Background. The functional improvements resulting from coronary revasc ularization (CABG) in patients with depressed ventricular function mag be described by the use of a model combining global or local quantifi cation of myocardial perfusion, viability, and contraction. An illustr ation of this model, with data provided by conventional radionuclide s tudies as they are performed routinely in many centers, is presented a nd the limitations of this approach for predicting the results of CABG are discussed. Methods and Results. The model is based on three indep endent variables, which can be approbated in this preliminary study by parameters derived from standard stress and redistribution/reinjectio n thallium-201 single-photon emission computed tomography (SPECT) acqu isitions with quantification of the tracer uptake defects and from a p lanar gated blood pool left ventricular ejection fraction (LVEF) measu rement: Perfusion is assumed to correspond to 100-stress defect tin pe rcentage), viability is 100-redistribution/reinjection defect, and con traction is 100(LVEF/70), assuming that a normal 70% LVEF corresponds to 100% contraction. In a group of 30 patients prospectively evaluated with this protocol and included in the study on the basis of a pre-CA BG LVEF <40%, a significant improvement in LVEF was demonstrated (28.2 %+/-8.5% before CAPG vs 35.8% +/- 7.3 % after CABG), which is accompan ied by a significant decrease of the stress thallium defects (34.8 +/- 13.8% vs 25.6% +/- 10.6%), whereas the average (but not the individua l) redistribution/reinjection defects remain almost stable (27.7% +/- 10.9% vs 25.7% +/- 10.1%). As reported in the three-dimensional model, pre-CABG and post-CABG representative points clearly demonstrate the functional improvements for the main variables, but there is a large s pectrum of responses to revascularization, It appears that the border between reversible and nonreversible thallium defects does not match t he limit between ischemic myocardium (with no contraction alteration a nd therefore without contraction improvement potential) and hibernatin g myocardium, which is able to recover mechanical function and therefo re is responsible for the improvement of global LVEF. Conclusions, Tha llium SPECT is far from ideal for use as an independent characterizati on of perfusion and viability because hibernating myocardium may be pr esent in both the fixed and reversible parts of thallium defects. Pred iction of functional recovery is conditioned by an accurate identifica tion of liable but underperfused and noncontracting myocardium. In the future, with the use of adequate study protocols that are able to mea sure viability without interference of perfusion and perfusion indepen dent of viability, the proposed model may be able to characterize regi onal function as a cluster of representative points for each territory and to delineate areas of the theoretical volume corresponding to a p otentially recoverable situation.