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Abstract
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).
Coefficient of determination (r2) for the relationships between lung function and the change from baseline for breathlessness (ΔBorg) and SpO2. Linear regression was used for ΔSpO2 and ΔBorg, and exponential analysis for FEV1%pred
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