The Role of Mechanical Economy and Efficiency as Key Drivers of Cardiac Rehabilitation Exercise Performance Improvement in a Patient With Heart Failure and an Implanted CRT-D: A Case Report and Literature Review.

Heart failure with reduced ejection fraction leads to exercise intolerance due to central and peripheral dysfunction. Improvements in exercise performance are not fully captured by traditional measures like peak oxygen consumption alone. A 57-year-old male with a complex cardiac medical history of hypertrophic cardiomyopathy with left ventricle non-compaction, Wolff-Parkinson-White syndrome, transient ischemic stroke, and heart failure with reduced ejection fraction with an implanted cardiac resynchronization therapy defibrillator underwent cardiac rehabilitation. Cardiopulmonary exercise testing revealed a non-proportional improvement in peak oxygen consumption (0.3 METs, 9%) versus peak workload (48 W, 61%). Gross mechanical economy and efficiency improved at all physiological thresholds: at ventilatory threshold by 11%, at respiratory compensation point by 60% and 41%, and at peak oxygen consumption by 47% and 33%, respectively. Mechanical economy and mechanical efficiency should be measured and reported for assessing rehabilitation outcomes in heart failure with reduced ejection fraction.