Desaturation during six-minute walk test (6MWT) is associated with increased mortality in idiopathic pulmonary fibrosis (IPF) and distance walked has been shown to have prognostic value in various forms of advanced lung disease. We defined a new index, the distance-saturation product (DSP), which is the product of total distance walked and final oxygen saturation (SpO2) during 6MWT. We hypothesized that for IPF, the DSP is a more reliable predictor of mortality than other physiologic variables.
Prospective analysis of serial 6MWTs with contemporaneous pulmonary function tests (PFTs), in IPF patients, seen between 1998 and 2004. Oxygen saturation and distance walked were recorded at baseline and throughout 6MWT. The DSP was calculated in each test. We compared PFTs, the occurrence of hypoxia at each minute during 6MWT, the DSP, and the decline in DSP over time in survivors and non-survivors.
63 patients (35 survivors, 28 non-survivors) were identified. Median period of observation was 23.3 months. Hypoxia was common, but occurred earlier during 6MWT and with greater magnitude among non-survivors. Desaturation >8% from baseline and the development of hypoxia (SpO2 <88%) within 3 minutes of walking were both more common among non-survivors (p<0.05). Final SpO2 declined by 10% in survivors and 14% in non-survivors (p<0.05). Average distance declined 10% in survivors and 21% in non-survivors (p<0.05). Mean DSP measured 256 (%-meters) among survivors vs. 176 in non-survivors (p=0.001). During follow-up, the average DSP declined in survivors by 23% compared to 45% in non-survivors (p=0.036). Neither initial PFTs nor changes in PFTs over time, differentiated survivors from non-survivors.
The DSP represents a simple, non-invasive test that is a sensitive predictor of mortality. Timing and degree of hypoxia during 6MWT also correlates with mortality. Each of these functional measurements was a better predictor of outcome than either PFTs or serial changes in PFTs over time.
Physicians should consider utilizing 6MWTs to identity subjects at increased for death. The DSP may be a useful marker of disease severity in IPF.
C.J. Lettieri, None.