The advantages of the CS-1 used in the present study were that it required limited space, could be conducted within a short time, and made real-time dynamic evaluation possible. Based on the results of the CS-1 on admission in COVID-19 patients with acute respiratory failure, the main findings of this study are as follows. First, there was a significant difference in ΔSpO2 during the CS-1 on admission between the intubation and non-intubation groups. Second, there was a significant correlation between hospitalization and ΔSpO2 during the CS-1. Finally, the cut-off point for ΔSpO2 among intubated patients was 9.5%.
At first, we examined the relationship between intubation or non-intubation and SpO2. There was no significant difference in the resting SpO2. In addition to the significant difference in ΔSpO2 during the CS-1 between the intubation and non-intubation groups in this study, the LDH level, lymphocyte count, ferritin level, CTSS on admission, and lowest SpO2 during the CS-1 on admission were also significantly different between the groups. The CS-1 could be used to evaluate a patient on admission quickly and conveniently; hence, the versatility of this evaluation will make it useful in clinical practice. The CS-1 involves a compound movement and was assumed to be useful in the detection of COVID-19 severity, which is a systemic disease with various degrees of severity [14]. We believed that the CS-1 is an efficient evaluation technique compared to the CS-30 because it could be performed while maintaining social distance to ensure infection management for patients with COVID-19. In addition to ΔSpO2, the CS-1 will be useful in determining intubation requirement in clinical practice.
Next, we observed a significant correlation between hospitalization and ΔSpO2 during the CS-1. There have been some reports about exercise-induced hypoxemia. Based on them, we considered the effect of ventilation disorder caused by pulmonary alveolus diffusion disturbance and the presence of a pulmonary infiltration shadow in both lungs, which is associated with COVID-19, mentioned for the first time in this study [7]. Mason et al, suggests that there is an increase in type II-alveolus epithelium cells because of a surfactant factor induced by COVID-19, causing a pulmonary diffusing capacity disorder, and this condition partially resembles pulmonary fibrosis [15]. The average CTSS on admission was 6.5 in this study, supporting the existence of pulmonary lesions and might reflect a diffusing capacity disorder. One of the disease severity indexes for pulmonary fibrosis is exercise-induced hypoxemia, where the 6MWT has been widely used as an exercise evaluation tool; Additionally, some studies showed a significant correlation between hypoxemia during the 6MWT and CS-30 in patients with interstitial lung disease [8, 11]. Because the CS-1 used in this study involved the same single rising movement as in the CS-30, we considered the detection of a similar significant decrease in SpO2 on exertion. Generally, necessary treatment increased along with disease severity during hospitalization, and our findings showed a significant correlation between hospitalization and ΔSpO2. It is interesting from the viewpoint of early treatment intervention and bed control that the CS-1 results on admission can predict hospitalization. Furthermore, the CS-1 may be used as a hospitalization criteria for patients with COVID-19 in the outpatient department, and it may be a useful remote rating system when new infectious diseases occur in the future.
Finally, we examined the cut-off point of ΔSpO2 among intubated patients. Attention to severity prediction and aggravation of hospitalization is needed in intubated patients with ΔSpO2 of 9.5% or more. This cut-off point may be useful for predicting hospitalization and disease severity in patients with COVID-19, for whom contact is restricted owing to infection management, and the CS-1 is accurate and convenient to perform during admission [5, 6]. Exploring patients’ respiratory conditions and determining the curative effect of treatment will be important when assessing ΔSpO2 in the future as well. This study has some limitations as follows. First, the CS-1, used as an exertion evaluation tool is a rating system devised based on the CS-30. Thus, there is no previous report to validate the usefulness of the CS-1 for COVID-19. Furthermore, we discussed the multidisciplinary use and correct and incorrect performances of the CS-1; eight patients did not perform the CS-1 in this study. The patients with COVID-19 who performed the CS-1 were not affected and could be evaluated safely, but there is no established selection protocol. Of note, two patients who completed the CS-1 died during hospitalization. The more general 6MWT has been used as a rating system, but we used the CS-1 because it provided a result after light exertion by the patient and could be performed with social distancing; this was beneficial considering infection management for COVID-19. We will examine the immediate effect of the CS-1 in future studies. In addition, because COVID-19 is like pulmonary fibrosis, the CS-1 could also be used for other diseases such as interstitial lung disease. Second, we mainly considered an alveolus diffusing capacity disorder as the condition of a patient with exercise-induced hypoxemia in this study; however, the possibility of platypnea-orthodeoxia syndrome, including right-to-left shunting affecting disease severity has also been reported [16, 17]. This is a future research topic because a plural pathologic examination was not performed in this study. Third, severe acute respiratory syndrome coronavirus 2 mutated over the 1.5 years during this study. However, the CS-1 results were relatable to the disease severity, and the CS-1 was considered adaptive to many variants of COVID-19. Therefore, we may be able to use the CS-1 for COVID-19, particularly in cases where the virus mutates, and for new infectious diseases in the future.