Carbon dioxide and oxygen partial pressure measurements in the cerebrospinal fluid in a conventional blood gas analyzer: analysis of bias and precision

• Published in: Journal of the neurological sciences 1997-03, Vol.147 (1), p.5-8
• Main Authors: Venkatesh, Balasubramanian, Boots, Robert J.
• Format: Article
• Language: English
• Subjects: Bias; Biological and medical sciences; Blood Gas Analysis - instrumentation; Calibration; Carbon dioxide; Carbon Dioxide - cerebrospinal fluid; Cerebrospinal fluid; Cerebrospinal fluid (biochemistry, cytology, circulation, manometry). Intracranial pressure determination; Electrode; Humans; Investigative techniques, diagnostic techniques (general aspects); Medical sciences; Monitoring, Physiologic - methods; Observer Variation; Optode; Oxygen; Oxygen - cerebrospinal fluid; Partial Pressure; Partial pressures; Precision; Reproducibility of Results; Sensors; Tonometry, Ocular - standards
• ISSN: 0022-510X; 1878-5883
• DOI: 10.1016/S0022-510X(96)05304-X

Cerebrospinal fluid (CSF) gas tension measurements have been used as a marker of cerebral oxygenation in animal models and in human studies. Discrepancies in the measurement of PCO 2 and PO 2 in non-blood solutions by standard blood gas analyzers have been described. The CSF is a physiological non-blood fluid with an ability to carry only dissolved O 2 or CO 2. The aim of this experiment was to determine the bias and precision of CSF pCO 2 and pO 2 measurements of a contemporary blood gas analyzer and a continuous gas sensor. CSF from human patients was tonometered to known PCO 2 and PO 2 using standard calibration gases in a bubble tonometer. Following equilibration, a continuous measurement of the partial pressure of gases in the CSF in the tonometer solution was made using a calibrated Paratrend 7 gas sensor (Biomedical Sensors, Bucks., UK) inserted into the equilibration chamber of the tonometer and continuous CSF pH, PCO 2 and PO 2 measurements recorded at 1-min intervals. After equilibration, a 3-ml CSF sample was aspirated anaerobically from the tonometer and analysed in duplicate using an ABL 620 (Radiometer, Copenhagen) blood gas analyser. The measured pCO 2 and pO 2 from the ABL 620 blood gas analyser and the Paratrend 7 sensor were paired with the expected values to calculate bias and precision. Small offsets in PCO 2 and large offsets in PO 2 measurement were seen with the ABL 620 blood gas analyser. This study brings into question clinical decisions based on CSF PO 2 measurements. The implications for calibrating CSF sensors against CSF gas measurements are discussed.