BACKGROUND: The Kety-Schmidt method (1) is the reference method for measuring global cerebral blood flow (CBF), metabolic rate (CMR) and net exchange with the blood in humans in settings where access to a scanner is limited or impractical. The method was originally developed with inhaled N2O as the tracer; both the saturation and desaturation mode as well as continuous and discontinuous blood sampling have been used, but the literature on the performance of the method is scarce. Photoacoustic spectrometry is a novel method for measuring N2O concentration. The aim of the present study was to characterise the Kety-Schmidt method using inhaled N2O in the washout phase, with continuous or discontinuous blood sampling, and with photoacoustic spectrometry for measuring N2O concentration. METHODS: Following ethical approval, a thorough physical examination, and informed consent, twenty-nine healthy male volunteers underwent 61 CBF measurements. Volunteers breathed a normoxic gas mixture containing 5% N2O until tension equilibrium, following which the N2O supply was terminated. Paired blood samples were collected simultaneously from an arterial and a jugular bulb catheter during washout, by continuous or discontinuous blood sampling. N2O concentration in blood samples was measured with photoacoustic spectrometry after equilibration of the samples with air; we calculated a modified coefficient of variation (mCV) for this measurement. CBF was calculated by the Kety-Schmidt equation; CMRO2 was determined according to the Fick principle. Parametric statistical analysis was performed, including the calculation of the repeatability coefficient and limits of agreement (2). RESULTS: The mean difference in CBF between discontinuous and continuous sampling was 4 ml/100g/min (limits of agreement of -23 to 34 ml/100g/min). CBF measurements based on discontinuous blood sampling provided the most reliable and reproducible values (Table). The mCV for N2O concentration measurements was 4.0%. CONCLUSION: The Kety-Schmidt method with inhaled N2O, discontinuous blood sampling and N2O measurement with photoacoustic spectrometry may be used for measurement of global CBF and CMR.
University of Cambridge (2008) Proc Physiol Soc 11, C12
Oral Communications: Cerebral blood flow and oxygen metabolism measured with the Kety-Schmidt method using inhaled N2O in healthy volunteers
S. Taudorf1, R. M. Berg1, K. Moller1,2
1. Center of Inflammation and Metabolism, Rigshospitalet, Copenhagen University Hospital, Copenhagen O, Denmark. 2. Intensive Care Unit 4131 / Dept. of Neuroanaesthesia, Rigshospitalet, Copenhagen University Hospital, Copenhagen O, Denmark.
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Where applicable, experiments conform with Society ethical requirements.