Transcutaneous blood gas measurement using a mass spectrometer

We use a Perkin Elmer MGA 1100 mass spectrometer modified for blood gas tension measurements to study transcutaneous measurements of oxygen, carbon dioxide, nitrogen and helium in normal subjects. We use an aluminum sampling chamber heated to 42--44 degree C and connected to the mass spectrometer by about 2 m of stainless steel tubing. The sampling area of 5 cm2 is covered with a polytrifluorochloroethylene membrane 18 micrometer thick (Allied Chemical Co., Aclar 33c). The membrane is supported by sintered stainless steel disc with a thermistor incorporated for measurement of "membrane" temperature. The chamber is heated with a printed circuit heater (Minco) and is temperature regulated by a servo circuit. We have used the system to measure, O2, CO2 N2 and He via the heated skin in four normal subjects, breathing air and then 100% oxygen. We also observed the response to a single breath of helium. Helium appears at the skin within 10--15 sec. The time constant of its disappearance is about 70 sec. The average uncorrected gas tensions which we obtained breathing air were 11.2 kPa for oxygen, 5.1 kPa for carbon dioxide and 67.1 kPa for nitrogen. The corresponding values breathing oxygen were 41.1 kPa for oxygen, 4.4 kPa for carbon dioxide and 13.1 kPa for nitrogen.