Factors Modifying the Accuracy of ABG Results
Arterial blood gases (ABGs) are necessary for the investigation, monitoring and clarification of mechanisms of gas exchange, and of acid–base disorders. The design of the electrodes in the blood gas analyser is based on the model of the electro-chemical c
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Factors Modifying the Accuracy of ABG Results
Contents 12.1 12.2 12.3 12.4 12.5 12.6 12.7 12.8 12.9 12.10
Electrodes........................................................................................................................ Accuracy of Blood Gas Values ....................................................................................... The Effects of Metabolizing Blood Cells ....................................................................... Leucocyte Larceny .......................................................................................................... The Effect of an Air Bubble in the Syringe .................................................................... Effect of Over-Heparization of the Syringe .................................................................... The Effect of Temperature on the Inhaled Gas Mixture ................................................. Effect of Pyrexia (Hyperthermia) on Blood Gases ......................................................... Effect of Hypothermia on Blood Gases .......................................................................... Plastic and Glass Syringes ............................................................................................
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12 A. Hasan, Handbook of Blood Gas/Acid-Base Interpretation, DOI 10.1007/978-1-4471-4315-4_12, © Springer-Verlag London 2013
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Factors Modifying the Accuracy of ABG Results
Electrodes
Arterial blood gases (ABGs) are necessary for the investigation, monitoring and clarification of mechanisms of gas exchange, and of acid–base disorders. The design of the electrodes in the blood gas analyser is based on the model of the electrochemical cell. Two half-cells are immersed in an electrolyte solution. An external connection which includes an ammeter, completes the circuit. Chemical reactions that consume electrons occur at each half-cell in solution. The half-cell at which the stronger of the two reactions occurs, becomes the cathode; the other half cell (which is therefore negative relative to the first half-cell) becomes the anode. The temperature of the chamber is held constant: usually at 37 °C. The chemical reactions produce a measurable flow of electrons through the external circuit. The blood is analysed by three separate electrodes (see opposite page). Most blood gas machines measure pH and PCO2 but rather calculate HCO3− from the Henderson-Hasselbach equation using pH and PCO2. This of course means that examination of the pH and PCO2 alone conveys the greatest part of the information contained within the ABG printout. O2 electrode (The Clark electrode)
The working of the O2 electrode is based on the principle of polarography. The electrode includes a silver anode and a platinum cathode immersed in potassium chloride solution. A semi-permeable membrane separates this solution from the blood sample. O2 molecules diffuse into the cell and react with the cathode. The number of electrons produced by this reaction is propo
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