Instrument to measure pulmonary function

Claim 1

A portable instrument for indicating the respiratory volume of a patient, comprising means for defining an air passageway, a mouthpiece connected to one end of said passageway defining means, the internal dimensions of the mouthpiece and air passageway being such as to not restrict the flow of air expired by the patient through the air passageway, a diverging air passageway extension connected to the other end of said passageway defining means to substantially eliminate back pressure, a flow rate measuring means including a measuring element comprising a thermal sensitive grid filament disposed across the passageway, said filament being responsive to the average flow rate of the expired air passing through said passageway, integrating means connected to the output of said measuring means for integrating the flow rate reading per unit time of the measuring means, indicating means connected to the integrating means for indicating the integrated output of the integrating means to thereby provide an indication of the volume of air expired by the patient per unit time, and means for heating the measuring element above normal body temperature to minimize the effects on the measuring element of minor temperature variations of the expired air.