Assessment of Six-Minute Walk Test Among Discharge-Ready Severe COVID-19 Patients: A Cross-Sectional Study

Background Among patients hospitalized for severe pneumonia due to coronavirus disease (COVID-19), clinical stability and normal resting peripheral oxygen saturation (SpO2) levels are widely used as a discharge criterion after recovery. It is unknown whether a test to assess the functional exercise capacity, like a six-minute walk test (6MWT), can add to the appropriateness of discharge criteria. Methods A cross-sectional study was conducted at a tertiary care COVID-19 hospital in India from 01st to 31st May 2021. All patients considered fit for discharge after recovery from "severe" COVID-19 pneumonia were subjected to 6MWT. Fitness for discharge was assessed by clinical stability and resting SpO2 above 93% for three consecutive days. Patients were considered to have failed the 6MWT if there was ≥4% fall in SpO2 or if they could not complete the test. Serum samples were analyzed for levels of C-reactive protein (CRP), interleukin-6 (IL-6), and lactate dehydrogenase (LDH) at the time of discharge. Results Fifty-three discharge-ready patients with a mean age of 54.54 ± 14.35 years with a male preponderance (60.38%) were analyzed. Thirty-three (62.26%) patients failed the 6MWT with a median six-minute walk distance (6MWD) of 270 m (60-360). A total of 45 (84.91%) patients had a fall in SpO2 during the test. The median change in SpO2 (∆SpO2) was 5% ranging from -6% to 8%. Serum LDH was significantly higher among patients who failed the 6MWT with a median LDH of 334 IU/L (38.96-2339) versus 261 IU/L (49.2-494) (p = 0.02). The difference was not significant for CRP or IL-6. There was no statistically significant correlation between the inflammatory markers with either 6MWD or (∆SpO2). Conclusion Two-thirds of the patients considered fit for discharge after recovery from severe COVID-19 pneumonia failed 6MWT, implying reduced functional exercise capacity and exertional hypoxia. Serum LDH levels were higher in these patients but not in other inflammatory markers. None of the inflammatory markers at discharge correlated with 6MWD or ∆SpO2 of 6MWT.


Introduction
India experienced the most vicious phase of the coronavirus disease (COVID-19) pandemic from March to May 2021, with a massive upsurge in the number of cases and fatalities. With over 26 million confirmed cases and deaths surpassing 274,000, the healthcare system in the country was put under unprecedented strain [1]. It became imperative to introduce effective triaging, admission, and discharge criteria to grapple with the challenge. Clinical stability and normoxemia at rest were used as criteria to discharge patients after severe COVID-19 illness. Exertional tests have previously been proposed and evaluated to predict severe COVID-19 pneumonia to guide early admission [2][3][4][5]. Based on their experience with 26 discharge-ready COVID-19 patients, Fuglebjerj et al. had proposed that the six-minute walk test (6MWT) could be used potentially as a tool for the assessment of patients before discharge [6]. Elevated levels of laboratory markers such as d-dimer, interleukin-6 (IL-6), and IL-10, and lower levels of peripheral lymphocytes at discharge may be associated with poor outcomes [7,8]. We have analyzed the results of 6MWT among the patients fit for discharge as per the prevailing national guidelines and compared it with the laboratory markers of disease severity at discharge.

Study design
We conducted a cross-sectional study at a COVID-19 care center in Western India from 1 st to 31 st May 2021. The hospital was mandated to provide in-hospital care to reverse transcriptase-polymerase chain reaction (RT-PCR)-positive severe-COVID-19 cases. Severity was defined by peripheral oxygen saturation (SpO 2 ) at room air of <90% or respiratory rate of >30 breaths per minute at initial presentation. The Institutional Ethics Committee of Army Hospital Research and Referral (R&R) approved the study (approval number 07/2022), and informed consent was obtained from all enrolled patients. The study was done in accordance with the Declaration of Helsinki.

Patients
All consecutive patients planned to be discharged after 1 st May were enrolled in the study. Patients were considered fit for discharge if they had attained clinical stability and had resting SpO 2 > 93% on three consecutive days at room air, based on the prevailing national guidelines. Such patients were subjected to 6MWT, in accordance with current accepted standards and protocol [9,10]. The total distance walked and the change in peripheral oxygen saturation and pulse rate from beginning to the end of the test were recorded. Any patient with hemodynamic instability, recent acute coronary syndrome or angina, acute pulmonary embolism, acute myocarditis, or any other condition that, in the attending physician's opinion, could affect the exercise performance or get aggravated by 6MWT were excluded from the study [10]. Also, individuals with baseline SpO 2 < 94% (due to a pre-existing cardio-pulmonary illness) or unwilling to participate were excluded from the study [3]. Those individuals who were unwilling to get the inflammatory markers (i.e., IL-6, C-reactive protein [CRP], and lactate dehydrogenase [LDH]) evaluated and those whose tests could not be done within 24 hours of the 6MWT were also excluded. Between 1 st May and 31 st May 2021, 53 patients who were found fit for discharge as per the prevailing guidelines and had no contraindications to perform 6MWT were included in the study.

Methodology
Baseline pulse and SpO 2 were recorded for all the patients. Patients were requested to walk at their own pace in a hallway of length 30 m with marks at every 1 m under strict medical supervision. SpO 2 was monitored continuously during the test with the help of a portable pulse oximeter. The test was stopped prematurely if SpO 2 dropped below 90% [3]. Other reasons for premature termination of the test included intolerable dyspnoea, fatigue, chest pain, diaphoresis, or uneasiness, as assessed by the attending physician. SpO 2 , pulse, distance, and time walked were recorded at the end of the test. The patients were considered to have failed the 6MWT if they had a fall in SpO 2 ≥ 4% or if the test was prematurely terminated. The study population was also subjected to IL-6, CRP, and LDH assessment before 6MWT within a maximum time gap of 24 hours.

Statistical analysis
The findings were analyzed using the John's Macintosh Project (JMP) 16.1 (Cary, North Carolina). The continuous data were assessed for normal distribution using the Shapiro-Wilk test. Variables with normal distribution were represented as mean ± standard deviation (SD) and assessed using parametric tests, i.e., the student's t-test. The variables without normal distribution were described as median (range) and evaluated using nonparametric tests, i.e., the Wilcoxon test. The p < 0.05 was considered significant. The correlation between the continuous variables was analyzed using logistic fit (Fit Y by X).    The comparison for days of hospitalization among the two groups is depicted in Figure 3. On fit Y by X analysis (logistic fit), none of the inflammatory markers correlated well with the 6MWT parameters (Figures 4, 5).

FIGURE 4: The correlation of inflammatory markers at discharge with six-minute walk distance using fit Y by X bivariate model: (A) CRP, (B) LDH, and (C) IL-6
6MWT: Six-minute walk test; CRP: C-reactive protein; LDH: Lactate dehydrogenase; IL-6: Interleukin-6.  The differences in different study variables among the patients who passed or failed 6MWT are enumerated in Table 2

Discussion
In this single-center study of 53 discharge-ready patients admitted for severe COVID-19 illness, we found that around two-thirds of patients failed 6MWT. Among the inflammatory markers, LDH levels, but not CRP or IL-6 levels, were significantly different between those who passed or failed the 6MWT. However, none of the inflammatory markers correlated with 6MWT parameters on bivariate analysis.
6MWT is a reproducible and validated test for assessing exercise capacity and predicting outcomes, including mortality, in chronic lung diseases like chronic obstructive pulmonary disease (COPD) [9][10][11]. 6MWD is the most robust variable assessed during 6MWT and correlates best with pulmonary function tests and outcomes. Change in 6MWD is ideally suited for studying the effect of clinical interventions in chronic respiratory diseases [11]. Apart from 6MWD, SPO 2 fall during 6MWT gives valuable information about the severity of underlying respiratory physiology and correlates well with pulmonary function tests, particularly diffusion capacity [12][13][14].
Although widely advocated [3][4][5], exertional desaturation tests have not yet been validated in patients with COVID-19. Preliminary studies have shown good correlations of 6MWD and ΔSPO 2 with disease severity, diffusion capacity, and lung volumes [2,15,16]. 6MWT has been studied in the pre-hospital setting (to assess the need for hospitalization), hospital setting (to determine the discharge readiness), and post-hospital setting (to determine the impact of post-COVID illness) [3,4,6,15,17,18]. In this study, we assessed the test's application in discharge-ready patients after recovery from severe COVID-19 illness. In another study that evaluated 26 discharge-ready patients, without any chronic pulmonary disease or heart failure, at a university hospital in Denmark, it was shown that 13 (50%) patients had exertional desaturation (SPO 2 < 90%). They also found that these patients with exertional desaturation perceived less dyspnea compared to patients of idiopathic pulmonary fibrosis with similar exertional desaturation [6]. In our study, 62.26% of patients failed the 6MWT, questioning the aptness of the prevailing discharge guidelines.
We also found that LDH levels were significantly higher in those discharge-ready patients that failed 6MWT than those that did not. A similar finding was not reproducible with other studied markers, i.e., CRP and IL-6. LDH is an established marker of severe COVID-19 pneumonia [19,20]. The rise of serum LDH in patients with severe illness is attributable to three mechanisms. First, apoptosis of cells during viral replication leads to the release of intracellular LDH; second, dysregulated and exaggerated inflammation in these patients leads to the release of LDH from organs and tissues; and lastly, in hypoxic patients, LDH is upregulated because of its role in oxygen homeostasis. Hypoxia leads to increased lactate generation through the glycolytic cycle, and LDH is upregulated in pyruvate fermentation and metabolic regulation of hypoxic response [19,21]. All three mechanisms are likely operational during acute COVID illness; LDH is elevated along with other inflammatory markers. However, after recovery from acute illness, other inflammatory markers would progressively settle down. Higher LDH in discharge-ready patients with exertional hypoxia can be explained by hypoxia-induced upregulation of LDH.
Our study was limited by the small sample size and a control group's absence. We also did not have follow-up data on these patients that could have guided the incorporation of 6MWT in discharge criteria. Longitudinal studies are suggested to validate the role of 6MWT in assessing discharge readiness after recovery from severe COVID-19 illness.

Conclusions
Around two-thirds of the patients ready for discharge from the hospital after recovery from severe COVID-19 pneumonia had exertional intolerance in the form of significant exercise desaturation or inability to complete a 6MWT. On bivariate analysis, none of the inflammatory markers at discharge correlated with 6MWT parameters. Serum LDH, but not the other inflammatory markers, was significantly elevated in patients who failed 6MWT at the time of discharge. The major implications of the study are the inadequacy of current discharge criteria to identify complete recovery, the requirement of an additional period of convalescence to allow the patient to restore normalcy of respiratory physiology, and the lack of surrogate biomarkers to identify individuals with exertional hypoxia.

Additional Information Disclosures
Human subjects: Consent was obtained or waived by all participants in this study. Institutional Ethics Committee of Army Hospital Research and Referral (R&R), Delhi Cantonment issued approval 07/2022. Approved in current format. Animal subjects: All authors have confirmed that this study did not involve animal subjects or tissue. Conflicts of interest: In compliance with the ICMJE uniform disclosure form, all authors declare the following: Payment/services info: All authors have declared that no financial support was received from any organization for the submitted work. Financial relationships: All authors have declared that they have no financial relationships at present or within the previous three years with any organizations that might have an interest in the submitted work. Other relationships: All authors have declared that there are no other relationships or activities that could appear to have influenced the submitted work.