Breath-by-breath monitoring of respiratory gas concentrations during compression and decompression.

We compared six systems for sampling respiratory gases from within a hyperbaric chamber to provide online gas concentrations by a mass spectrometer (MS) outside the chamber. Gas at hyperbaric pressure was sampled through either a capillary and mass flow controller (system 1), a capillary and micrometering manual valve (system 2), or a capillary alone (system 3) by a vacuum pump outside the chamber. In each system, a small amount of decompressed gas was drawn into the MS for analysis while the balance vented into the room. Systems 4-6 were constructed from systems 1-3, respectively, by eliminating the vacuum pump. Using a square wave respiratory simulator, which generated known gas concentration profiles within the chamber, MS analyses were recorded from each sampling system at compression and decompression rates of 20 meters of sea water.min(-1) and chamber pressures from 1.0 to 6.0 atm abs. Inspiratory and end-expiratory concentrations of nitrogen (0.0 and 1.0%, respectively) and carbon dioxide (0.0 and 4.2%, respectively) were accurately determined at respiratory rates of up to 70 breaths.min(-1) with a sensitivity of 0.05% and without pressure artifact.