The COVID-19 pandemic in the UK has placed considerable strain on NHS diagnostic services . Accordingly, there is a strong emphasis on appropriately utilising available diagnostic resources, such as CPET, ensuring that only those patients who are likely to gain benefit from specialised testing are referred. In the context of post-COVID patient follow-up, CPET has been suggested as a potentially useful tool to guide clinical management plans; however, it constitutes a resource intensive procedure due to time, staffing, equipment and consumable costs.
The findings of the present study show a continued reduced functional status in the majority (83%) of the studied post-COVID patients as defined by CPET at up to 12 months. This proportion appears to be higher than the reported in relation to SARS, where 41% of participants demonstrated a reduced peak VO2 at 3 months post infection . There is also a reported reduction in peak VO2 of 34.5% and 55% in COVID-19 patients respectively within the same time frame [9, 10]. The difference in this proportion may be attributed to the studied cohorts, since these studies were prospective in design and included a range of disease severity, while our patient cohort consisted of a selected group of post-COVID patients with persistent symptoms of breathlessness out-keeping with other radiological and physiological findings. Similarly, in a select group of 10 patients post COVID-19, all patients demonstrated a reduced peak VO2 .
Our study findings demonstrate that there is an initial and sustained impairment of functional status up to 12 months post COVID-19 in patients with persistent, ongoing breathlessness. This is predominately due to physical deconditioning which we postulate is related to muscle impairment and extended periods of inactivity. We have not identified a significant prevalence of any other clinical features within our study cohort, such as breathing pattern disorder or exercise induced desaturation. Where limitations other than physical deconditioning were identified, these conditions had previously been diagnosed and the performance of CPET served to confirm these causes of exercise limitation rather than identify them. Other studies have suggested that, although more simplistic and cheaper, field exercise tests including the 6MST and 6MWT are able to identify a reduced exercise capacity [7, 8]; however, they lack the ability to accurately discriminate whether or not this is due to physical deconditioning or any other underlying pathology.
Our findings are consistent with the suggestion that functional impairment post COVID-19 is predominantly due to physical deconditioning based on previous research that reports a significant positive impact of pulmonary rehabilitation on exercise capacity in a group of elderly COVID-19 patients . Considering these findings, we suggest that referral for diagnostic CPET should only occur if symptoms are exercise related. Nevertheless, before CPET can be utilised as a first line diagnostic test more evidence is required across all severity COVID-19 survivors, demonstrating that in the majority of patients physical deconditioning is the primary reason for symptoms and reduction in exercise capacity.
The appropriate triaging of patients to receive CPET is important given the need to manage extensive backlogs of operative patients  that have accumulated during the COVID-19 pandemic. Historically, CPET is primarily used for preoperative risk stratification and with a backlog of elective surgery, we argue that CPET should be prioritised for those patients who will gain the most benefit (e.g. those being assessed for major cancer surgery).
It is necessary to consider the limitations of our study, most notably the relatively small sample size and single centre nature of the study. However, despite this small sample size, we feel that this pilot study suggests that further research should initially focus on better understanding the physical deconditioning associated with the post-COVID-19 syndrome, rather than on CPET. Once we better understand both the physical deconditioning and recovery from this deconditioning, we should be better placed to identify atypical patients who may be presenting with an alternative problem, likely with a distinct underlying and potentially pre-existing pathology.
In conclusion, post-COVID-19 syndrome may present with profound symptoms and have a substantial negative impact on the quality of life for a large number of patients. It is therefore essential that these patients are supported through optimal diagnostic services and therapies, as clinically required. We believe that generalised CPET testing may not add significant additional clinical information that can be used to aid patient investigation and management in the context of post-COVID management, and that such investigations should be more focused to allow proper utilisation of NHS resources and restoration of routine NHS services.