Thorax 53:668-672 doi:10.1136/thx.53.8.668
  • Original article

Raised levels of exhaled carbon monoxide are associated with an increased expression of heme oxygenase-1 in airway macrophages in asthma: a new marker of oxidative stress

  1. Ildikó Horváth,
  2. Louise E Donnelly,
  3. András Kiss,
  4. Paolo Paredi,
  5. Sergei A Kharitonov,
  6. Peter J Barnes
  1. Department of Thoracic Medicine, National Heart and Lung Institute at Imperial College, Dovehouse Street, London SW3 6LY, UK
  1. Professor PJ Barnes.
  • Received 13 January 1998
  • Revision requested 6 March 1998
  • Revised 31 March 1998
  • Accepted 21 April 1998


BACKGROUND Chronic inflammatory diseases are associated with an increased production of oxidants. Induction of a stress protein, heme oxygenase (HO) HO-1, is a cytoprotective mechanism against oxidative cellular injury. HO-1 catabolises heme to bilirubin, free iron, and carbon monoxide (CO).

METHODS Exhaled CO and sputum bilirubin levels were measured and HO-1 protein expression in airway macrophages was determined by Western blotting in asthmatic patients as levels of oxidants are raised in asthma and may induce HO-1.

RESULTS Exhaled CO was significantly increased in 37 non-steroid treated asthmatic patients compared with 37 healthy subjects (5.8 (95% CI 5.20 to 6.39) ppm vs 2.9 (2.51 to 3.28) ppm; p<0.0001) but was similar to normal in 25 patients who received corticosteroids (3.3 (95% CI 2.92 to 3.67) ppm; p>0.05). In non-treated asthmatic patients more HO-1 protein was expressed in airway macrophages than in normal subjects. Bilirubin levels in induced sputum were also higher than in normal subjects. Inhalation of hemin, a substrate for HO, significantly increased exhaled CO from 3.8 (95% CI 2.80 to 4.87) ppm to 6.7 (95% CI 4.95 to 8.38 CI) ppm (p<0.05) with a concomitant decrease in exhaled nitric oxide levels, suggesting an interaction between the two systems.

CONCLUSIONS Increased exhaled CO levels and HO-1 expression may reflect induction of HO-1 which may be inhibited by steroids. Measurement of exhaled CO, an index of HO activity in non-smoking subjects, may therefore be clinically useful in the detection and management of asthma and possibly other chronic inflammatory lung disorders.