A Quantitative Impedance Pneumograph

Abstract

An impedance pneumograph capable of quantitatively measuring respiratory volumes is described. Physically, the pneumograph is one of several 3/8 x 5/8 x 7/8 inches modules incorporated in a miniature biotelemetry system, with separate modules being utilized to process signals derived from a number of physiological parameters such as EEG and EKG in addition to respiration. Considerable experimentation was performed regarding optimization of the electrodes used in this impedance pneumograph as well as the frequency and amplitude of the excitation voltage applied between the electrodes. The result was a fairly simple and inexpensive device which uses a higher frequency (300 KHz) excitation voltage than is normally used in impedance pneumography. The two electrodes, constructed of silver coated nylon for flexibility, are insulated from the subject by a layer of polyethylene film, thus forming a capacitive coupling of the electrode to the subject. This technique effectively eliminates changes in skin-to-electrode resistance, largely responsible for baseline drift encountered with previous impedance systems, so that this pneumograph responds only to variations in the actual impedance between the two electrodes. Empirically, a nearly perfect linear correlation was found to exist between the transthoracic impedance measured by this impedance pneumograph and pulmonary volume. The two insulated electrodes are pasteless and therefore easily applicable and non-irritating. They are held in position by an elastic, quick-donning, vest-like garment.