Finns det daglig variation i resistans i de nedre luftvägarna hos häst?
Among researchers it has been established since decades that humans and animals have an internal biological clock that controls certain physiological mechanisms. One example regarding humans is the impairment of lung function during the night, causing the phenomenon ?nocturnal asthma? among asthmatics. Only a few studies concerning respiratory circadian rhythm in horses have been done previously and they showed a daily variation in horses with a chronic or an acute respiratory disease. The technique used in those studies was the conventional technique, which is invasive and less sensitive than the technique used in this study. The aim of this study was to investigate whether variation in resistance in the lower airways of healthy horses could be seen in a comparison between morning and afternoon measurements. The resistance of the lower airways was measured in 6 healthy horses in the morning and afternoon with forced oscillation technique. To reduce the influence of the upper airways the horses? heads were fixed during the tests and they were given no sedation. Measurements were made during 4-5 days in the frequency range of 1-5 Hz. A significant diurnal variation in resistance of the lower airways was only found at the frequency of 4 Hz, manifested as an increased resistance in the morning compared to the afternoon. To be able to determine with certainty that there is a natural circadian rhythm in the lower airways in the horse, a more extensive study is required than was possible within the framework of this study.