title: Oxygen cost of exercise is increased in heart failure after accounting for recovery costs.
abstract: OBJECTIVE
  The oxygen cost during exercise has been reported to be decreased in patients with congestive heart failure (CHF), implying an increased efficiency (lower oxygen uptake [VO(2)] per Watt [VO(2)/W]); however, these studies ignored the oxygen debt that is increased in heart failure.
  
  
  METHODS
  The primary aim of this research was to evaluate the total oxygen cost (work VO(2)/W) during exercise and recovery in patients with heart failure as compared with healthy adults.
  
  
  METHODS
  We performed a retrospective analysis comparing the exercise VO(2)/W, the recovery VO(2)/W, the work VO(2)/W, and the VO(2)/W relationship above and below the ventilatory threshold (VT) in 11 healthy control subjects and 45 patients with CHF.
  
  
  RESULTS
  The exercise VO(2)/W was decreased by 29% (p < 0.0001) in patients with CHF; however, the recovery VO(2)/W was increased by 167% (p < 0.0001) and the work VO(2)/W was increased by 14% in patients with CHF (p = 0.014). The VO(2)/W slope increased above the VT (+ 27%, p = 0.0017) in both normal subjects and patients with CHF, suggesting a decrease in efficiency above the VT. There was an inverse correlation (r = 0.646, p < 0.0001) between exercise VO(2)/W and recovery VO(2)/W, implying that subjects with a low exercise VO(2)/W were not efficient but rather accumulated a large oxygen debt that was repaid following completion of exercise.
  
  
  CONCLUSIONS
  Heart failure is associated with lower exercise VO(2)/W; however, the patient with heart failure is not efficient, but rather accumulating a large oxygen debt (recovery VO(2)/W) that is repaid following exercise. In addition, the work VO(2)/W (including both exercise and recovery) is increased in patients with heart failure in comparison to control subjects, and correlates inversely with the percentage of predicted VO(2). The large recovery VO(2)/W is likely due to impaired oxygen delivery to exercising muscle during exercise. The increase in the work VO(2)/W is probably due to changes in skeletal muscle fiber type that occur in patients with heart failure (type I to type IIb).
authors.affiliation: Division of Cardiology, Department of Medicine, University of Washington School of Medicine, Seattle, 98195, USA.
authors.forename: Steven H
authors.initials: SH
authors.lastname: Mitchell
authors.forename: Natalie P
authors.initials: NP
authors.lastname: Steele
authors.forename: Kenneth M
authors.initials: KM
authors.lastname: Leclerc
authors.forename: Mark
authors.initials: M
authors.lastname: Sullivan
authors.forename: Wayne C
authors.initials: WC
authors.lastname: Levy
mesh_terms: D000328:Adult; D000368:Aged; D016022:Case-Control Studies; D005080:Exercise Test; D005260:Female; D006333:Heart Failure / Q000378:metabolism*; D006801:Humans; D007700:Kinetics; D008297:Male; D008875:Middle Aged; D010101:Oxygen Consumption; D012189:Retrospective Studies