Noninvasive Myocardial Work Index: Characterizing the Normal and Ischemic Response to Exercise

Background: Myocardial work index (MWI) and work efficiency are new parameters for assessing left ventricular function. We aim to characterize the normal response to exercise in a mixed population and determine whether MWI can identify patients with inducible ischemia.

Methods: Patients were retrospectively enrolled from an existing database of exercise stress echocardiography. Inclusion criteria were a clinical indication of possible ischemia and technical suitability to calculate MWI. Exclusion criteria were abnormal baseline left ventricular function or inadequate image quality. Echocardiograms positive for ischemia were defined by independent visual assessment and compared with angiographic findings where available. Myocardial work index was determined using a proprietary algorithm and efficiency calculated as constructive work divided by the sum of constructive and wasted work.

Results: A total of 177 patients met inclusion criteria; 117 were excluded, leaving 40 normal and 20 positive tests. During normal exercise, global MWI increased 54% (from 2,296 to 3,523 mm Hg%) and efficiency remained at 96%. However, in patients with inducible ischemia, MWI decreased in affected segments, global MWI did not increase (2,069-2,070 mm Hg%), and global efficiency fell from 93% to 87%. The receiver operating characteristic curve for MWI had an area under the curve of 0.94.

Conclusions: During normal exercise, MWI increases and efficiency remains unchanged. However, during exercise-induced ischemia, MWI paradoxically decreases in affected segments, while globally MWI fails to increase and efficiency decreases. We have demonstrated that MWI can be applied to stress echocardiography to identify ischemia, but its utility remains uncertain. Further research that makes comparisons with an objective measure of functional ischemia is needed.