Metabolic Requirements during Six Minutes Walking Tests in Patients Affected by Chronic Obstructive Pulmonary Disease in Different Stages

Backgrounds: In Chronic Obstructive Pulmonary Disease (COPD) a multi factorial effort limitation becomes progressively relevant as the disease progresses in the consecutive stages. It is measured by both six minutes walking test (6MWT) and maximal cardiopulmonary incremental test (CPET). Aim: It is important to assess in each stage of disease the metabolic load during 6MWT referring to the outcome of CPET and to ascertain whether there is a significant relationship between the measures obtained by CPET and 6MWT. Methods: Four groups of fifteen patients affected by COPD in stage I to IV underwent 6MWT and maximal CPET in the same day and results were compared to a group of healthy people. Airflow obstruction was measured by whole body plethysmography, blood gases by gas analysis, maximal oxygen consumption and metabolic parameters by ergometer, lactic acid levels by analyzer. Results: Maximal oxygen consumption (V’O2max) and 6MWT are progressively impaired and related (V’O2max = 1.25  0.26, 1.152  0.4, 1.03  0.44, .85  0.2 l/m; 6MWD = 452  84, 446  82, 381  165, 200  100 respectively in GOLD I to IV stage). Oxygen consumption (V’O2) during 6MWT becomes stable after 3 4 minutes and reached at the end of the test close to those measured at decompensated metabolic acidosis anaerobic threshold (TDMA) (85 ± 0.4 l/m vs. 9  0.4 l/m) in stage I to III, while in COPD there is no difference between V’O2max and V’O2 during 6MWT (0.85  0.2 vs. 0.8  0.23 l/m). 6MWT is more suitable to determine oxygen desaturation than CPET (dSaO2 −4 ± 2% vs. −2 ± 1%). 6MWD, the workload performed in 6MWT and V’O2max are significantly related. Conclusion: 6MWT looks as a suitable sub maximal test related CPET. Metabolic requirements under 6MWT are close to TDMA and are obtained in a suitable, self paced, usual exercise, close to everyday experience and thus related to activity daily levels. As the disease worsens the differences between V’O2 during 6MWT and V’O2max wane.