Background The 6MWT is a simple, reproducible test of exercise performance. Lettieri et al. 1 proposed a composite index of the product of the lowest oxygen saturation (SpO2) and distanced walked – the distance–saturation product (DSP). Pimenta et al. 2 calculated the differences between actual SpO2 and 100% every two seconds, and produced the desaturation-distance ratio (DDR-1). Ijiri et al. 3 simplified Pimenta’s methodology, using the SpO2 at the end of each minute (DDR-2).
Aim To compare the different methods of calculating the composite index.
Methods We retrospectively analysed 48 6MWT datasets, obtained using standard methodology. SpO2 was recorded every second using a Minolta 300i pulse oximeter and analysed using Visi-Download (Stowood Scientific, Oxford). The DSP and DDR’s was calculated by each method. Additionally, data using Pimenta’s method, was calculated using one second data intervals (DDR-3). Data are given as median (IQR).
Results 41/48 patients were male. Group age was 63 (53 to 73) years, FEV1%pred 68.4% (41.3 to 82.3), distance walked 390 m (321.3 to 477.5), baseline SpO2 95% (93 to 96) and decline in SpO2 was -4% (-2 to -8). DSP was 324.6 (278.8 to 419.4) m%, DDR-1 was 8.90 (4.98 to 13.27), DDR-2 was 16.83 (9.14 to 24.8) and DDR-3 was 8.90 (4.99 to 13.66). Bland-Altman analysis of DDR-1 vs DDR-3 gave a bias of -0.0083 and 95% limits of -0.042 to 0.025. Correlations between each index and FEV1%pred, SpO2 and Borg were significant for FEV1%pred and SpO2, but not significant for Borg (Table 1).
Conclusions 1) there is no significant difference between DDR-1 and DDR-3, both providing an accurate assessment of changes in SpO2 during exercise and allowing for the different storage capabilities of pulse oximeters; 2) the simplest index (DSP), showed poorer correlations cpmpared to the DDR’s, perhaps reflecting the simplicity of the index; 3) the conceptual idea of a composite index of distance walked and changes in SpO2 during a 6MWT needs further investigation in a range of different clinical settings.
References 1 Lettieri CJ, Nathan SD, Browning RF, et al. The distance-saturation product predicts mortality in idiopathic pulmonary fibrosis. Respir Med. 2006;100:1734–1741
2 Pimenta SP, Rocha RB, Baldi BG, et al. Clinics 2010;65:841–846
3 Ijiri N, Kanazawa H, Yoshikawa T, Hirata K, et al. Application of a new parameter in the 6-minute walk test for manifold analysis of exercise capacity in patients with COPD. Int J Chron Obstruct Pulmon Dis. 2014;9:1235–1240