The 6-minute walk test in chronic respiratory failure: does observed or predicted walk distance better reflect patient functional status?

BACKGROUND

Acquiring 6-min walk test (6MWT) data from patients undergoing noninvasive mechanical ventilation due to chronic hypercapnic respiratory failure is limited. We aimed to assess whether the actual 6-min walk distance (6MWD) or the percent predicted 6MWD is a better reflection of the respiratory function of patients using home noninvasive ventilation (NIV) due to chronic hypercapnic respiratory failure.

METHODS

This was a cross-sectional observational study. The 6MWT was performed in subjects using home NIV. Diagnoses were grouped as COPD, obesity hypoventilation syndrome (OHS), kyphoscoliosis, and parenchymal lung disease. Sex, age, and body mass index (BMI) were used to calculate ideal 6MWD. Male: 1,140 m - (5.61 × BMI) - (6.94 × age), and subtract 153 m for the lower limit of normal. Female: 1,017 m - (6.24 × BMI) - (5.83 × age), and subtract 139 m for the lower limit of normal. The 6MWD and percent-of-predicted 6MWD were compared relative to arterial blood gas, spirometry values, and diagnosis.

RESULTS

The 6MWT was performed in 144 subjects, median (IQR) age 62 y (55-71 y). The male/female ratio, median (IQR) 6MWD, and percent-of-predicted 6MWD values were: COPD 32/6, 316 m (226-390 m), and 59.4% (42.5-68.9%); OHS 24/28, 303 m (240-362 m), and 73.0% (63.0-82.0%); kyphoscoliosis 16/7, 420 m (318-462 m), and 70.5% (56.0-75.2%); and parenchymal lung disease 19/12, 333 m (273-372 m), and 67.1% (46.7-74.7%). The correlation of percent-of-predicted 6MWD with spirometry and arterial blood gas values were better than with the actual 6MWD.

CONCLUSIONS

The percent-of-predicted 6MWD was better correlated with respiratory function than actual 6MWD for subjects using home NIV due to chronic hypercapnic respiratory failure with COPD, OHS, kyphoscoliosis, and parenchymal lung disease.