Reanalysis of the 12-Minute Walk in Patients With Chronic Obstructive Pulmonary Disease – Discussion
More traditional ways to assess the aerobic capacity of a patient with COPD include treadmill and bicycle ergometry. The 12-min walking test has three specific advantages: (a) no sophisticated equipment is necessary; (b) the ability of a patient to walk is more clinically relevant than their maximal performance; (с) the act of walking is familiar to all, whereas some individuals may have difficulty riding a bicycle or using a treadmill. Despite these positive aspects, there are drawbacks from assessing the functional capacity of an individual with the 12-min test alone. These include: (a) it can only be used with moderate or severe disease because walking distance is not limited by mild disease; (b) with the 12-min walk there is no simultaneous measurement of minute ventilation, Vo2, or Vco2; and (c) it is difficult to objectively measure the effort involved while performing the test. With a progressive exercise test (ie, treadmill or bicycle ergometer), the heart rate and ventilation in relation to a predicted maximum are recorded. With the walking test, a Borg score or a visual analogue score for dyspnea is frequently obtained. canadian health&care mall
Previous studies have demonstrated significant correlations between Vo2max and the 12-min (r = 0.52; p < 0.01 )2 and 6-min (r = 0.42; p < 0.01) walking distances. We are unaware of similar studies on the 4-min and 2-min walks. Our results confirmed these findings (r = 0.49 for 12-min test; r = 0.51 for 6-min test; r = 0.48 for 4-min test; r = 0.45 for 2-min test). In addition, we found a significant correlation between the Vco2max and 12-, 6-, 4-, and 2-min tests (r = 0.38, r = 0.40, r = 0.36, r = 0.35, respectively). All correla tions were enhanced when the gas volumes were normalized for patient weight. This supports the conclusion of Gray-Donald et al that walking is more work for a person of greater girth, and hence a distance walk is not necessarily interchangeable in its significance with cycle ergometry. To best compare the results, it is necessary to adjust for weight. We believe that the explanation as to why the correlations between walking and Vo2max are better than for walking and Vco2max is related to the fact that some of our patients reached their anaerobic threshold. If the anaerobic threshold is reached, then the Vco2 is not linearly related to the workload.
McGavin et al2 found that the Vo2max significantly correlated with the FEV1 (r = 0.65, p < 0.01) and the FVC (r = 0.67, p < 0.01). Our results support the relationship with the FEVl (r = 0.54), but we find no correlation with FVC (r = -0.17). This is most likely due to our patients having a greater FVC (3.40 ± 0.61 L) than McGavin’s patients did (2.84 ± 0.93 L).
Visual analog scores previously have been used to assess breathlessness during a 6-min walk. The Borg scale, likewise, is a measure of effort-induced dyspnea which we used during the walking tests in our study. As the 12-min walking test progressed, it is not surprising that the mean Borg scores increased (Fig 2) with each 2-min interval. Killian and Jones have reported that dyspnea as assessed by the Borg scale is directly dependent on the duration of the exercise.