Cerebral blood flow and oxygen metabolism measured with the Kety-Schmidt method using nitrous oxide.
BACKGROUND
The Kety-Schmidt method is the reference method for measuring global cerebral blood flow (CBF), cerebral metabolic rates (CMR) and flux, especially where scanners are unavailable or impractical. Our primary objective was to assess the repeatability of the Kety-Schmidt method in a variety of different approaches using inhaled nitrous oxide (N2O) as the tracer, combined with photoacoustic spectrometry. A secondary objective was to assess the impact of this tracer on the systemic vascular concentration of nitrite (NO2(-)).


METHODS
Twenty-nine healthy male volunteers underwent 61 CBF measurements by breathing a normoxic gas mixture containing 5% N2O until tension equilibrium. Paired blood samples were collected from an arterial and a jugular bulb catheter in the saturation or desaturation phase, by continuous or the discontinuous sampling. N2O concentration was measured with photoacoustic spectrometry after equilibration of blood samples with air. CBF was calculated by the Kety-Schmidt equation. CMR of oxygen (CMRO2) was determined by the Fick principle. NO2(-) in plasma and red blood cells (RBC) was measured by ozone-based chemiluminescence.


RESULTS
The most robust approach for CBF measurement was achieved by discontinuous sampling in the desaturation phase [CBF, 64 (95% confidence interval, 59-71 ml)] 100 g/min; CMRO2 1.8 (1.7-2.0) micromol/g/min). The tracer did not influence plasma or RBC NO2(-) (P>0.05 vs. baseline).


CONCLUSIONS
These findings confirm the reliability and robustness of the Kety-Schmidt method using inhaled N2O for the measurement of global CBF and CMR. At the low tracer concentration used, altered NO metabolism is unlikely to have affected cerebral haemodynamic function.
Taudorf
S
S
Department of Infectious Diseases, Centre of Inflammation and Metabolism, Rigshospitalet, Faculty of Health Sciences, University of Copenhagen, Copenhagen, Denmark.
Berg
R M G
RM
Bailey
D M
DM
Møller
K
K
D000328:Adult; D000465:Algorithms; D001812:Blood-Brain Barrier; D001921:Brain / Q000378:metabolism*; D002245:Carbon Dioxide / Q000097:blood; D002560:Cerebrovascular Circulation; D004912:Erythrocytes / Q000737:chemistry; D006400:Hematocrit; D006801:Humans; D006863:Hydrogen-Ion Concentration; D016857:Hypocapnia / Q000097:blood / Q000503:physiopathology; D008297:Male; D009569:Nitric Oxide / Q000378:metabolism; D009573:Nitrites / Q000097:blood*; D009609:Nitrous Oxide / Q000493:pharmacokinetics; D010100:Oxygen / Q000097:blood; D010101:Oxygen Consumption; D010313:Partial Pressure; D010949:Plasma / Q000737:chemistry; D015203:Reproducibility of Results; D013057:Spectrum Analysis / Q000379:methods; D055815:Young Adult
D009573:Nitrites; D002245:Carbon Dioxide; D009569:Nitric Oxide; D009609:Nitrous Oxide; D010100:Oxygen