mechanism(s). For instance, a low peak V′O2 due to severe mechanical constraints in a patient with COPD probably signals a higher pre-operative risk than a similar value obtained in a detrained patient with ample mechanical-ventilatory reserves. With the exception of the V′E–V′CO2 relationship [59], little attention has been given to the ability of the submaximal variables from the incremental test to predict poor outcome. The best metric of ventilatory inefficiency for prognosis estimation is currently unclear [26]. For instance, it is possible that additional information is gained from the V′E–V′CO2 intercept or V′E/V′CO2min rather than the V′E–V′CO2 slope, particularly in mechanically limited patients with COPD and lung cancer. A negative V′E intercept in a patient with combined heart or pulmonary vascular disease signals a particularly high ventilatory drive, which is a potent marker of poor prognosis in cardiovascular disease. CPET to assess the effects of interventions: the key unmet clinical needs Table 1 presents a list of research areas that should be considered in the future investigation of the effects of interventions. Constant WR tests are rarely used in practice (with the exception of clinical trials), partially because little is known about how to integrate this information into decision making for individual patients. The surmounting complexities in determining an individual’s CP are unlikely to be solved in clinical populations [103] feasible approaches to the individualisation of exercise intensity for endurance tests are therefore warranted [114, 115]. Such pragmatic approaches might reduce the pre-intervention variability on Tlim, thereby decreasing the sample size for interventional studies while aiding the interpretation of changes [116]. Establishing robust criteria for CPET responders versus non-responders would be valuable to titrate medications with a variable effect on exercise tolerance, such as bronchodilators in COPD [117] and β-blockers in heart failure with reduced ejection fraction [58]. In addition, submaximal variables from the incremental test have been largely neglected in assessing the effects of interventions. This unmet need is particularly relevant in practice as the incremental test is the only testing modality that is reimbursed in many countries. Conclusion As with any other topic, the potential usefulness of a clinical method of investigation should be cautiously analysed in light of its current limitations. Such a sober and unbiased approach is a crucial step in order to open real perspectives of improvement. Despite the relevant advances outlined here, the authors recognise that CPET remains largely undervalued and, therefore, underused in respiratory medicine worldwide. We hope that at least part of the challenges facing contemporary CPET may be addressed in the next 10 years (table 1), thereby expanding its clinical application in respiratory medicine. At this time, and considering the contemporary trends of increasing obesity, sedentarism, polypharmacy and psychogenic causes of dyspnoea, it seems plausible that CPET will be more frequently used in association with other noninvasive to minimally invasive [94, 109, 111, 118, 119] and, in some selected cases, invasive methods [92]. In this context, the assessment and management of patients with exercise intolerance and dyspnoea would benefit greatly from a new generation of modular “metabolic” systems with the capabilities of measuring key cardiovascular (e.g. noninvasive stroke volume and cardiac output), pulmonary gas exchange (e.g. estimated PaCO 2 and dead space ventilation) and sensory responses (continuous dyspnoea readings). The field would also greatly benefit from xx
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