A METHOD FOR MEASURING THE CO2 DEAD SPACE VOLUME IN FACIAL VISORS ANDRESPIRATORY PROTECTIVE DEVICES IN HUMAN-SUBJECTS

The external CO2 dead space volume (V-d) in facial visors and respirat ory protective devices is difficult, but important, to measure in huma n subjects. The lack of proper methods for its assessment has hampered the development of standards and the improvement of visor and device design. We have improved and evaluated a method for measuring V-d and the mean inspired fraction of CO2 (F-I,F-CO2) in human subjects wearin g facial visors or respiratory protective devices. The method is based on indirect measurements of inspiratory volumes using a calibrated re spiratory inductive plethysmograph (RIP), and measurements of F-CO2 wi th a mass spectrometer. The accuracy of the RIP method was assessed in eight subjects and its repeatability was studied during nasal and ora l breathing, at rest and after bicycle ergometer work. We related the V-d,V-RIP results to two known external V(d)s (With the effective V-d equal to the geometric volume; 15 ml and 320 mi), through which the su bjects breathed, using RIP and a pneumotachometer (PTM) simultaneously (V-d,V-RIP and V-d,V-PTM) The repeatability of V-d and mean F-I,F-CO2 results was determined from duplicate recordings during the wearing o f a welding visor. Initial RIP calibration was accepted if the inspira tory tidal volume error was less than or equal to 10%. This resulted i n an acceptable V-d,V-RIP error (less than or equal to 20%; V-d = 320 ml) in six of eight subjects. The validation technique allowed us to s eparate the V-d,V-RIP error into a volumetric error and an error relat ed to CO2 measurement. Poor RIP volumetric accuracy over the initial p ortion of inspiration was detrimental to V-d accuracy. Using the weldi ng visor, V-d and mean F-I,F-CO2 were less at rest than after work and less during nasal breathing than during oral breathing. The intra-ind ividual variability of V-d and mean F-I,F-CO2 were lower during nasal breathing than during oral breathing. To summarize, the improved RTP b ased method can accurately (less than or equal to 20% error) assess V- d and mean F-I,F-CO2 in facial visors and respiratory protective devic es in standardized work situations. A meticulous RIP calibration proce dure and repeated validations of RIP volume and CO2 measurement accura cy must, however, be applied.