Continuous measurement of percutaneous oxygen saturation during 6-minute walk test is useful for stratification of the prognostic risk in patients with heart failure

Abstract Background The 6-minute walk test (6MWT) is an established test to assess the functional exercise performance and the 6-minute walk distance is strongly correlated with the functional capacity and the prognosis in patients with heart failure (HF). Also, the hypoxia during 6MWT is reported to be associated with severity of pulmonary diseases. However, no study has reported the clinical implication of sequential changes in percutaneous oxygen saturation (SpO2) during 6MWT was not investigated in HF patients. Purpose To prospectively clarify the association between sequential changes in SpO2 during 6MWT and prognosis in patients with HF. Methods In this prospective observational study, 50 patients admitted to our hospital with acute heart failure were enrolled. The 6MWT was performed at the time before discharge, after hemodynamic stability was confirmed. The average of SpO2 was calculated from data obtained at rest (3 minutes just before the test) and during 6MWT by employing the wearable pulse-oximeter which are designed to record and store the SpO2 value every seconds. Alternations in SpO2 (Δ SpO2) were analyzed by subtracting the average during 6MWT from that at rest. Patients were followed 1 year for composite outcome of heart failure-hospitalization and death). Results The mean age of participants was 78.8 years, and 23 (46%) were female. The mean EF was 46.1% and the number of the patients with EF≧50, 50>EF>40, 40≧EF were 22, 6, and 22, respectively. Mean NT-proBNP was 4374.5pg/ml. The mean walk distance was 250 ± 112m and the mean Δ SpO2 was 2.88 ± 4.3%. All cases were classified by walk distance and Δ SpO2; long- (>220m) vs. short-distance group (≦220m) and low- (<3.5%) vs. high-ΔSpO2 group (≧3.5%). There were no significant differences in cardiac parameters and respiratory function between long- and short-distance group as well as between low- and high-ΔSpO2 group. Kaplan-Meier analysis revealed that cardiac event rate was higher in the short-distance group than in the long-distance group (HR = 2.89, CI = 1.24-6.71, p = 0.014), and higher in the high-Δ SpO2 group than in the low-ΔSpO2 group (HR = 3.49, CI = 1.55-7.83, p = 0.002) (Figure 1A and 1B). When these classifications were combined, the patients with short-distance and low-ΔSpO2 showed remarkably high event rate of 91% (Figure 2). Conclusions This study showed that, compared to walk distance, ΔSpO2 was more predictive of prognosis of heart failure patients, and that the combination of ΔSpO2 and walk distance may identify the highest risk group. The elucidation of the usefulness of continuous SpO2monitoring during 6MWT as the marker for severity of heart failure and treatment effect warrants further investigation.Figure1Figure 2