title: Mechanism of greater oxygen desaturation during walking compared with cycling in patients with COPD.
abstract: 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.
authors.affiliation: Section of Pulmonary and Critical Care Medicine, Dartmouth Medical School, Dartmouth-Hitchcock Medical Center, Lebanon, NH. Electronic address: Donald.a.mahler@hitchcock.org.
authors.forename: Donald A
authors.initials: DA
authors.lastname: Mahler
authors.affiliation: Section of Pulmonary and Critical Care Medicine, Dartmouth Medical School, Dartmouth-Hitchcock Medical Center, Lebanon, NH.
authors.forename: Alex H
authors.initials: AH
authors.lastname: Gifford
authors.affiliation: Pulmonary Function and Cardiopulmonary Exercise Laboratories, Dartmouth-Hitchcock Medical Center, Lebanon, NH.
authors.forename: Laurie A
authors.initials: LA
authors.lastname: Waterman
authors.affiliation: Pulmonary Function and Cardiopulmonary Exercise Laboratories, Dartmouth-Hitchcock Medical Center, Lebanon, NH.
authors.forename: Joseph
authors.initials: J
authors.lastname: Ward
authors.affiliation: Section of Pulmonary and Critical Care Medicine, Dartmouth Medical School, Dartmouth-Hitchcock Medical Center, Lebanon, NH.
authors.forename: Sasa
authors.initials: S
authors.lastname: Machala
authors.affiliation: Section of Pulmonary and Critical Care Medicine, Dartmouth Medical School, Dartmouth-Hitchcock Medical Center, Lebanon, NH; Psychological Applications, South Pomfret, VT.
authors.forename: John C
authors.initials: JC
authors.lastname: Baird
mesh_terms: D000368:Aged; D015308:Anaerobic Threshold; D005080:Exercise Test; D005652:Functional Residual Capacity; D006801:Humans; D019344:Lactic Acid / Q000097:blood; D010100:Oxygen / Q000097:blood*; D010108:Oxyhemoglobins / Q000032:analysis; D011653:Pulmonary Diffusing Capacity; D029424:Pulmonary Disease, Chronic Obstructive / Q000097:blood* / Q000503:physiopathology; D011659:Pulmonary Gas Exchange; D012123:Pulmonary Ventilation; D013147:Spirometry; D016138:Walking / Q000502:physiology*
chemical_list: D010108:Oxyhemoglobins; D019344:Lactic Acid; D010100:Oxygen