We thank Dr. Kirsch for his comments regarding the use of work vs oxygen uptake (V̇o2) to assess exercise tolerance. Most of the time, these two measures are in close agreement in terms of percent predicted and can be used interchangeably as objective measures of exercise tolerance. Dr. Kirsch correctly points out some of the reasons for the discrepancies between these two measures, such as the selection of appropriate predicted values, especially in obese individuals, and the performance of different types of exercise. Most studies relating exercise capacity to important outcomes such as survival or the ability to tolerate lung resection surgery use V̇o2 as the measure of exercise capacity because it reflects the physiologic health of the individual in terms of their global ability to utilize oxygen. Our suggestion that work may be a more appropriate measure of exercise tolerance is based on our view of exercise expressed as power output (work per unit of time) rather than oxygen utilization. Indeed, many subjects achieve a percent predicted for work that is higher than their maximal percent predicted V̇o2, likely indicating a motivational ability to sustain exercise beyond the anaerobic threshold. In real-world terms of the ability to perform various tasks and activities, the amount of work someone is able to do seems more relevant than the amount of oxygen they can consume. In this regard, defining exercise tolerance in terms of work is also more appropriate when prescribing exercise or explaining the results of exercise testing to patients. As we stated in the editorial, we invite more discussion in this area of the definition of exercise tolerance.