The accuracy of spirographic recording at high respiratory rates.

Spirometers were designed and used originally for the measurement of metabolic rate or such respiratory constants as vital capacity. Such measurements are made at low rates of respiration (up to about 40 breaths per minute) or with single inspiratory or expiratory efforts. There has been no reason to suspect the accuracy of records made under these conditions. Since Hermannsen (1933) first described the maximum breathing capacity test (M.B.C.) many investigators have used the spirometer to record tidal air at considerably higher rates of breathing. During an investigation into the M.B.C. of a series of subjects D'Silva and Mendel (1950) noted that at respiratory rates higher than 55 breaths per minute the oscillatory movements of the water in their spirometer were of such violence that very often water was aspirated into the tubes connecting the subject to the instrument, and that aspiration invariably occurred at these high respiratory rates if the tidal air was large. The large movements of the water column which they noted must have been either the cause, or the result, of considerable pressure changes within the spirometer bell, and it seemed that these pressure changes might affect the excursions of the bell and so invalidate the M.B.C. estimations. It was decided therefore to investigate the response of the spirometer at various respiratory rates in order to determine: (a) the magnitude of the errors produced, if any, and (b) whether it would be possible to construct a calibration curve for the instrument so that the errors could be corrected.