A method for measuring the C0₂ dead space volume in facial visors and respiratory protective devices in human subjects

Abstract

The external CO₂ dead space volume (V_d ) in facial visors and respiratory protective devices is difficult, but important, to measure in human 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.CO₂) in human subjects wearing facial visors or respiratory protective devices. The method is based on indirect measurements of inspiratory volumes using a calibrated respiratory inductive plethysmograph (RIP), and measurements of FCO₂ with a mass spectrometer. The accuracy of the RIP method was assessed in eight subjects and its repeatability was studied during nasal and oral breathing, at rest and after bicycle ergometer work. We related the V_d,RIP results to two known external V_d s (with the effective V_d equal to the geometric volume; 15 ml and 320 ml), through which the subjects breathed, using RIP and a pneumotachometer (PTM) simultaneously ( V_d,RIP and V_d.PTM ). The repeatability of Vd and mean F_ICO₂, results was determined from duplicate recordings during the wearing of a welding visor. Initial RIP calibration was accepted if the inspiratory tidal volume error was ≤ 10%. This resulted in an acceptable V_d,RIP error (≤ 20%; V_d = 320 ml) in six of eight subjects. The validation technique allowed us to separate the V_d,RIP error into a volumetric error and an error related to CO₂ measurement. Poor RIP volumetric accuracy over the initial portion of inspiration was detrimental to V_d accuracy. Using the welding visor, V_d and mean F₁co₂ were less at rest than after work and less during nasal breathing than during oral breathing. The intra-individual variability of V_d and mean -F₁,co₂, were lower during nasal breathing than during oral breathing. To summarize, the improved RIP based method can accurately (≤ 20% error) assess V_d and mean F₁ CO₂ in facial visors and respiratory protective devices in standardized work situations. A meticulous RIP calibration procedure and repeated validations of RIP volume and CO₂ measurement accuracy must, however, be applied.

Keywords:

• Carbon dioxide
• Ergonomics
• Human subjects
• Mass spectrometry
• Respiration
• Respiratory inductive plethysmography
• Welding visors