We read the article titled ‘Walking with Non-Invasive Ventilation Does Not Prevent Exercise Induced Hypoxaemia in Stable Hypercapnic COPD Patients’ by Walker et al. (Citation1) with great interest. They investigated the effect of NPPV without supplemental oxygen on a 6-minute walk test (6MWT-NPPV) compared to a 6-minute walk test with supplemental oxygen (6MWT-O2). They concluded that NPVV did not prevent exercise induced hypoxemia and determined that during 6MWT with NPPV partial oxygen pressure (PO2) significantly decreased from 66.8 ± 7.2 mmHg to 55.5 ± 10.6 mmHg (p < 0.02) whereas no significant changes happened in partial carbondioxide pressure (PCO2) (pre: 50.6 ± 7.5 mmHg vs. post: 53.0 ± 7.1 mmHg; p = 0.17) (Citation1). We would like to comment on some subjects regarding the methodologic aspects and results.
Our main concern is about the analysis of arterial blood gas prior to the exercise. They evaluated PaO2 level of COPD patients prior to 6MWT-NPPV with supplemental oxygen and blood gas analyses with NPPV without supplemental oxygen prior to exercise were not performed. Therefore comparision of PaO2 levels before and after exercise was not at the same manner in 6MWT-NPPV trial and the conclusion of additional hypoxemia in NPVV group after 6MWT in the present study is not possible. Whereas, additional need of supplemental oxygen for the study population is the fact as PO2 values of several subjects (6 of 10) decreased below a critical level of 55 mmHg after exercise with only NPVV. However absence of significant changes in lactate and PCO2 levels after exercise in NPVV run indicates the unloading respiratory muscles and improvement in ventilation perfusion mismatch even with only NPVV during exercise.
NPVV with supplemental oxygen aided exercise in COPD patients was also evaluated in previous studies (Citation2,3). Dreher et al. (Citation2) evaluated the feasibility of NPVV with supplemental oxygen during 6MWT and compared with oxygen alone in very severe COPD patients. Substantial decrease of PO2 from 72.4 ± 5.6 to 61.6 ± 7.9 mmHg (p < 0.001) during 6MWT when supplemental oxygen was used, but significant increase from 70.7 ± 8.1 to 81.2 ± 12.4 mmHg (p < 0.001) when NPPV was used in addition to supplemental oxygen were determined (Citation2). Moreover, lower walking distance with supplemental oxygen in the study by Dreher et al. (Citation2) than in the study by Walker et al. (Citation1) pointed out that the poorer exercise performance of subjects enrolled to the study by Dreher et al. (Citation2).
The differences between this study and present study regarding deoxygenation and exercise performance during 6MWT with supplemental oxygen may be due to the differences between patients’ characteristics. When we compared these two studies regarding patient selection criterias, inclusion and exclusion criterias were similar; however, presence of cor pulmonale or left ventricular heart failure were not indicated in both studies. As presence of cor pulmonale or left ventricular heart failure deteriorates oxygenation during exercise and exercise performance (Citation4,5), the number of enrolled patients with cor pulmonale or left ventricular heart failure might be different in these two studies. Further studies related to exercise performance and deoxygenation during exercise in very severe COPD patients should also consider presence or absence of cor pulmonale and other cardiovascular diseases.
Another important point is learning effect, which is related to test repetition and practice on the 6MWD especially in the evaluation of response to treatment or change over time. Performance of two 6MWTs is recommended, and the best 6MWD should be recorded (Citation6). Performance of two 6MWTs can decrease the sample size requirements for clinical trials, due to reduced variability in the pre- and post-intervention measures (Citation7). In the present study all subjects were familiarized with 6MWT during former hospital visits and one test for each day was performed (Citation1). Whether the learning effect is equally significant for individuals who have previously performed multiple 6MWTs is difficult to establish, as few data are available. Regarding the design of the present study, one test may also be sufficient for patients who have recently performed the test where the learning effect is smaller. However in order to highlight learning effect on 6MWTs, two tests on the same day with an interval of at least 30 minutes between tests could be performed in the present study.
Finally, bronchodilators for COPD and cardiovascular medications significantly improve exercise capacity (distance walked and/or dyspnea scale) (Citation8,9). Types of bronchodilator therapy, dose and number of hours taken before 6MWT on those two consecutive days and time period of day in which exercise performed were not mentioned in the present study (Citation1). These important points are also needed to be standardized in both trials of the present study.
Declaration of interest statement
The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.
References
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