Fluid Flow Effects on Readings from the Mass Spectrometer with Diffusion Membrane Catheter

The diffusion membrane catheter coupled to the mass spectrometer has previously been used to measure partial pressures of gases in blood. However, measurement errors arise due to depletion of gases in the vicinity of the catheter. The nature of these errors in oxygen and argon partial pressure measurement is described for in vitro and in vivo studies. The in vitro setup consisted of a catheter having a silicone or Teflon membrane at its tip and located within a tube surrounded by flowing blood or water. The effects of tube diameter and bulk fluid flow rate on mass spectrometer readings of oxygen and argon partial pressures were examined. It was found that a log-linear relationship closely described the effect of flow rate on readings in the region of flow dependence. Also flow dependence for argon closely paralleled that for oxygen. The in vivo study which followed was an examination of the errors in mass spectrometer readings under conditions which cause varying flow rates in blood vessels. Substantial errors in oxygen readings were found which could be compensated for by using argon readings as a correction factor.