We appreciate the interest and comments by Mahmood in regards to our recent article (1). In this study we have shown not only that cardiac energetics are impaired early in the aortic stenosis (AS) disease process, being present in a group of patients with moderate aortic stenosis, but also that transition to systolic failure in severe AS is not dependent on further energetic decline. Whilst Mahmood points out correctly that energetic impairment was not a “necessary correlate of systolic failure as assessed by LV ejection fraction (LVEF)” this does not imply that it is unrelated. We would like to clarify that as ATP demands increase with AS severity, a reduction in the capacity to deliver ATP to the myocardium, as established early in the process of AS, could increase susceptibility to systolic failure when the increasing ATP demand exceeds supply.