Mass spectrometric measurement of end-tidal xenon concentration for clinical stable xenon/computerized tomography cerebral blood flow studies

Abstract

We have demonstrated the feasibility of using a compact dedicated mass spectrometer to monitor end‐tidal xenon concentration in human subjects during stable xenon computerized tomography measurements of regional cerebral blood flow. End‐tidal carbon dioxide concentration is monitored simultaneously and noninvasively without degrading the dynamic response to xenon. For clinical regional cerebral blood flow studies we employed a Nuclide 3‐60‐G Sectorr mass spectrometer with a 3 in radius, 60° magnetic sector and a variable (0‐5000 V) ion accelerating potential. The required high vacuum (10⁻⁷ Torr) was achieved and maintained by means of a turbomolecular pump. A needlemetering valve was incorporated into an anesthesia mask connector, and exhaled gases were transported to the mass spectrometer via a 6 ft length of Teflon tubing (1/16 in i.d.). Molecular flow conditions between the sample and analysis chambers were provided by use of a gold foil leak (0.0005 in. hole). At an inlet pressure of 400 m Torr (achieved by means of the needle valve), the inlet system was characterized by a gas transport lag‐time of 1.3 s and a rise‐time constant of 85 ms. Xenon (doubly charged ion: m/z 68) and carbon dioxide (doubly charged ion: m/z 22) were monitored alternately at 75 ms intervals. Our experience with mass spectrometry has demonstrated the feasibility of using a compact dedicated instrument for accurately and non‐invasively monitoring end‐tidal xenon concentration in a clinical setting.