THE simple and accurate method for the evaluation of nasal airway capacity has been widely searched for since Zwaardemaker first wrote about his cold mirror back in 1889.1 Most of these attempts have been to make measurements of pressure changes either in the postnasal space or in a nasal mask during spontaneous respiration or during forced air flow either into or back from the nose.2-7
The method most in use today was originally presented by Lehmann in 1939, and further developed by Aschan et al, Craig, Solomon et al, Cass and Ogura. A transducer is used to measure the difference in pressure between the nasopharynx and a nasal mask and the actual amount of air flow is measured during spontaneous respiration using a pneumotachometer. These two values are plotted opposite each other on an oscilloscope screen and the line obtained has a slope which is representative of the