Carbon monoxide is endogenously produced in the human nose and paranasal sinuses

Carbon monoxide (CO) has recently emerged as an endogenously produced gaseous mediator that, like nitric oxide (NO), appears to be involved in both upper and lower airway inflammation. In healthy subjects a large part of the exhaled NO seems to originate from the nasal airways, and the paranasal sinuses have been described as a dominating site for NO production.

The current study was designed to investigate whether CO could also be produced in the nose and paranasal sinuses.

The occurrence in the nasal mucosa of the enzyme heme oxygenase, the rate limiting step for CO production, was analyzed with use of immunocytochemistry. CO in exhaled and sampled air was measured with an infrared analyzer. Forty-two healthy subjects and two patients with a tracheostoma volunteered for the study.

Heme oxygenase-like immunoreactivity was seen in the respiratory epithelium, in connection with seromucous glands and in the vascular smooth muscle of the nose. When CO was continuously sampled from one nostril during normal breathing through the mouth, stable levels of CO could be measured within 40 seconds in all subjects tested (n = 33). Repeated measurements indicated only minor variations in the values obtained. Sampling through a drainage tube inserted into the maxillary sinus revealed CO levels comparable to the levels obtained by sampling through the nose (n = 6). Breathing through the nose increased the CO levels obtained in the exhaled air (n = 33, P <. 001).

These results imply that the nose and paranasal sinuses contribute to the CO production of the human airways.