Theoretical model for myocardial functional characterization: application to a group of patients evaluated before and...

The functional improvements resulting from coronary revascularization (CABG) in patients with depressed ventricular function may be described by the use of a model combining global or local quantification of myocardial perfusion, viability, and contraction. An illustration of this model, with data provided by conventional radionuclide studies as they are performed routinely in many centers, is presented and the limitations of this approach for predicting the results of CABG are discussed.

The model is based on three independent variables, which can be approximated in this preliminary study by parameters derived from standard stress and redistribution/reinjection thallium-201 single-photon emission computed tomography (SPECT) acquisitions with quantification of the tracer uptake defects and from a planar gated blood pool left ventricular ejection fraction (LVEF) measurement: Perfusion is assumed to correspond to 100-stress defect (in percentage), viability is 100-redistribution/reinjection defect, and contraction 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-CABG LVEF <40%, a significant improvement in LVEF was demonstrated (28.2% +/- 8.5% before CABG vs 35.8% +/- 7.3% after CABG), which is accompanied by a significant decrease of the stress thallium defects (34.8% +/- 13.8% vs 25.6% +/- 10.6%), whereas the average (but not the individual) 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 spectrum of responses to revascularization. It appears that the border between reversible and nonreversible thallium defects does not match the limit between ischemic myocardium (with no contraction alteration and therefore without contraction improvement potential) and hibernating myocardium, which is able to recover mechanical function and therefore is responsible for the improvement of global LVEF.

Thallium SPECT is far from ideal for use as an independent characterization of perfusion and viability because hibernating myocardium may be present in both the fixed and reversible parts of thallium defects. Prediction of functional recovery is conditioned by an accurate identification of viable but underperfused and noncontracting myocardium. In the future, with the use of adequate study protocols that are able to measure viability without interference of perfusion and perfusion independent of viability, the proposed model may be able to characterize regional function as a cluster of representative points for each territory and to delineate areas of the theoretical volume corresponding to a potentially recoverable situation.