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INTRODUCTION

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Standards of basic anesthesia monitoring include measurement of adequate inspired oxygen delivery with an oxygen analyzer and continual exhaled carbon dioxide (ETCO2) with capnography.

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The oxygen sensor is placed on the inspiratory limb of the anesthesia machine. Blood concentrations and depth of anesthesia is assumed by monitoring expired gas analysis of oxygen, carbon dioxide, and anesthetic agent concentration. Several systems are available to monitor inspired and exhaled oxygen, carbon dioxide, and volatile inhalation agents. These technologies include: (1) electrochemical analysis—polarographic and galvanic cells (O2); (2) paramagnetic (O2); (3) magneto-acoustic (O2); (4) mass spectrometry (O2, N2, CO2, N2O, and gases); (5) Spectral analysis (a) infrared (CO2, N2O, and gases) and (b) Raman scattering (O2, N2, CO2, N2O, and gases); and (6) piezoelectric crystal (quartz) oscillation.

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OXYGEN ANALYZERS

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Currently, three types of oxygen analyzers are available: polarographic (Clark electrode), galvanic (fuel cell), and paramagnetic. The polarographic and galvanic electrochemical sensors differ in the composition of their electrodes and electrolyte gels. The cathode and anode electrodes are embedded in an electrolyte gel separated from the sample gas by a semipermeable membrane (usually Teflon). As oxygen reacts with the electrodes, a current is generated that is proportional to the partial pressure of oxygen in the sample gas. The components of the galvanic cell are capable of providing enough chemical energy so that the reaction does not require an external power source.

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  1. Polarographic—The electrode has a gold (or platinum) cathode and a silver anode. Unlike the galvanic cell, a polarographic electrode works only if a small voltage is applied to two electrodes. The amount of current that flows is proportional to the amount of oxygen present. The units can provide fast oxygen analysis within 1 minute, but has a higher failure rate compared to the galvanic cell.

  2. Galvanic—Fuel cell monitors are used on many anesthesia machines in the inspiratory limb. The cell contains a lead anode and gold cathode bathed in potassium chloride. At the gold terminal, hydroxyl ions are formed that react with the lead electrode (thereby gradually consuming it) to produce lead oxide, causing current, which is proportional to the amount of oxygen being measured, to flow. Because the lead electrode is consumed, monitor life can be prolonged by exposing it to room air when not in use. It has a slow response time of 3 minutes but lasts longer. Predictors of galvanic cell exhaustion include underreading of high oxygen concentration, failure to remember calibration, “blipping out,” and color changes.

  3. Paramagnetic—The paramagnetic oxygen analyzer plots oxygen concentration continuously breath by breath as a real-time waveform and displays as an oxygraph. The oxygraphy waveform has four phases similar to capnography, although displayed in a reverse manner. The device gives a digital display ...

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