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Gleeson and colleagues in hershey, pa, examined the aspiration of oropharyngeal contents during sleep in 10 normal male volunteers aged 22-55 years. The subjects were studied under sleep laboratory conditions on two separate occasions approximately one month apart. When stable sleep was reached, 99mTc was infused into the nasopharynx via a small catheter at a rate of 2 mL per hour. Radionuclide scans of the chest were performed immediately upon awakening.
The subjects slept well; they were asleep (as determined by EEG) 85.7% ± 2.6% of the time they spent in bed. Three subjects had a positive scan following their first sleep study, and four had a positive scan after their second. Two subjects had evidence of aspiration on both occasions. Thus, five of the 10 volunteers had evidence of aspiration after at least one night of sleep. The volume of tracer solution aspirated ranged from 0.011-0.129 mL with detection, in each case, in either the mid-lung or upper lobe regions. There was no significant difference detected between those who aspirated and those who did not with regard to age or sleep characteristics.
A normal cough reflex is the major means of defense against aspiration of oropharyngeal contents, the means by which infection occurs in most cases of pneumonia. The sleeping state is associated with an impaired cough reflex. Previous studies have examined the incidence of aspiration in normal adults during sleep. In one, a gauze packet containing 111In chloride was fixed to their teeth. While 10 of 14 subjects with a history of prior community-acquired pneumonia had radioisotopic evidence of aspiration during sleep, such evidence was present in only one of 10 normal subjects (Kikuchi R, et al. Am J Resp Crit Care Med 1994;150: 251-253). In a second study, small boluses of a solution of 111In chloride were injected into the noses of the subjects. There was evidence of aspiration during sleep in seven of 10 subjects with impaired consciousness, but similar evidence was also found in nine (45%) of 20 normal controls after sleep.
The calculated volume of tracer solution aspirated in the current study is, obviously, a minimal estimate of the total volume of oropharyngeal secretions aspirated. Nonetheless, using these minimal volumes and data indicating that the density of aerobic bacteria in normal oropharyngeal secretions is approximately 106 per mL, Gleeson and colleagues calculated that "the minimum quantity of potentially pathogenic organisms aspirated is on the order of 104 to 105." This is a quantity that may be sufficient to lead to clinical pneumonia, depending upon the organism and upon other host factors.
If aspiration is so common during sleep, why don’t we all wake up with pneumonia every morning? Obviously, there is sufficient redundancy in our airway protective mechanisms, including effective mucociliary clearance, the presence of antibacterial proteins, and a host of other factors that allow us to overcome such inocula. However, if some of these other factors are impaired, or if we aspirate a highly virulent organism, or if we aspirate a lesser virulent organism in very large numbers, infection may ensue. (Dr. Deresinski is Clinical Professor of Medicine, Stanford, and Associate Chief of Infectious Diseases, Santa Clara Valley Medical Center.)