Assessment of oxygen uptake during the 6-minute walking test in patients with heart failure: preliminary experience with a portable device.

In patients with heart failure, the 6-minute walking test (6-MWT) is considered a useful measure of submaximal exercise capacity. Few data are available on oxygen uptake (VO2) during a standard 6-MWT. The aim of this study was to measure the 6-MWT VO2 by using a recently validated portable instrument in 26 patients (24 men, 2 women; mean age, 56 +/- 11 years) with mild to severe heart failure (New York Heart Association class II, 10 patients; III, 10 patients; IV, 6 patients; left ventricular ejection fraction: 22 +/- 6%). Peak VO2 was measured during a symptom-limited cardiopulmonary exercise test performed in a period of 1 to 3 days (10 watt/m increment). Peak VO2 was 15 +/- 4 ml/kg/m during the symptom-limited test and 12.9 +/- 4.4 ml/kg/m during the 6-MWT (p < 0.05), corresponding to 86% of peak VO2. Seven (27%) of 26 patients showed a 6-MWT VO2 equal to or higher than peak VO2. Anaerobic threshold (AT) was identified in 23 of 26 patients during maximal exercise and in 19 of 26 patients during the 6-MWT; VO2 at AT was similar in the two tests (12.2 +/- 3.5 ml/kg/m vs 11.9 +/- 4.2 ml/kg/m). The distance walked during the 6-MWT (mean, 418 +/- 20 m) significantly correlated with 6-MWT VO2 (r = 0.71) and peak VO2 (r = 0.63); the 6-MWT VO2 also showed a high correlation with peak VO2 (r = 0.86). Thus in patients with failing hearts, VO2 during 6-MWT (considered a classic submaximal exercise) is, on average, only 15% lower than peak VO2 and is largely supported by anaerobic metabolism (work above the anaerobic threshold).