Chest
Volume 140, Issue 2, August 2011, Pages 351-358
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Original Research
COPD
Mechanism of Greater Oxygen Desaturation During Walking Compared With Cycling in Patients With COPD

https://doi.org/10.1378/chest.10-2415Get rights and content

Background

Patients with COPD exhibit greater oxyhemoglobin desaturation during walking than with cycling. The purpose of this investigation was to investigate differences in ventilatory responses and gas exchange as proposed mechanisms for this observation.

Methods

Arterial blood gas and lactate levels were measured in 12 patients with COPD (aged 68 ± 6 years) during incremental treadmill and cycle exercise. The primary outcome to assess the ventilatory response to exercise was Pao2. The primary outcome to assess impairment in exercise gas exchange was the difference between partial pressures of alveolar and arterial oxygen (Pao2 − Pao2).

Results

Pao2 in patients was significantly lower at peak exercise for treadmill walking (51.4 ± 6.8 mm Hg) compared with cycling (60.4 ± 10.7 mm Hg) (P = .002). The initial increase in Pao2 with cycling occurred prior to the onset of the anaerobic threshold. At peak exercise, Pao2 was significantly higher with cycling compared with walking (P = .004). The anaerobic threshold occurred at a lower oxygen consumption during cycling than walking (P = .001), and peak lactate levels were higher with cycling (P = .019). With progressive exercise, Pao2 − Pao2 increased similarly during treadmill and cycle exercise.

Conclusions

The higher Pao2 during cycling minimized the magnitude of oxyhemoglobin desaturation compared with walking. The enhanced respiratory stimulation during cycling appears due to an initial neurogenic process, possibly originating in receptors of exercising muscles, and a subsequent earlier onset of anaerobic metabolism with higher lactate levels during cycling.

Section snippets

Subjects

Inclusion criteria were a diagnosis of clinically stable COPD,12 ≥ 10 pack-year history of smoking, and a decrease of oxygen saturation by pulse oximetry (Spo2) of ≥ 4% during ambulation. Exclusion criterion was any comorbidity that limited ability to exercise. Each patient was recruited from the outpatient clinic and signed informed consent.

Study Design

The study was approved by the Committee for the Protection of Human Subjects at Dartmouth College. At two visits, 2 to 3 days apart, patients were assigned

Subjects

Sixteen patients were screened and signed informed consent. Twelve patients completed the study. In two patients, a catheter could not be placed in the radial artery. One patient withdrew consent and another patient experienced an exacerbation prior to visit 1. Descriptive characteristics of the four female and eight male patients are summarized in Table 1. Four patients had stage II disease, and eight patients had stage III disease based on GOLD (Global Initiative for Chronic Obstructive Lung

Discussion

Our results of serial measurements of arterial blood specimens combined with metabolic exercise responses provide novel physiologic data to investigate mechanisms for greater oxyhemoglobin desaturation during walking compared with cycling. We found that patients with COPD exhibited higher ventilatory responses, as reflected by

a, Pao2, and
e/
o2, during cycle exercise. This higher Pao2 during cycling minimized the decrease in arterial oxygen tension due to ventilation-perfusion mismatching

Acknowledgments

Author contributions: Dr Mahler: contributed to developing the research protocol, supervising data collection, reviewing the analysis, and preparing the manuscript.

Dr Gifford: contributed to data collection, performing the statistical analyses, and reviewing the manuscript.

Ms Waterman: contributed to review and revision of the research protocol, data collection, review of the analysis, and review of the manuscript.

Mr Ward: contributed to review and revision of the research protocol, data

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    Funding/Support: The authors have reported to CHEST that no funding was received for this study.

    Reproduction of this article is prohibited without written permission from the American College of Chest Physicians (http://www.chestpubs.org/site/misc/reprints.xhtml).

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