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Pulse Oximetry
| Pulse oximetry is the standard of care for the continuous noninvasive monitoring of peripheral arterial hemoglobin oxygen saturation (Sp02) This practical, noninvasive, and reliable monitor provides an early warning of arterial hypoxemia that is often not appreciated by subjective observation. A light--emitting diode that measures absorption of specific wavelengths of light relative to the ratio of oxyhemoglobin and reduced hemoglobin is most commonly placed on the patient's finger or ear. A computer calculates Sp02 and displays this value along with a pulse pressure waveform and heart rate on a screen. Nevertheless, pulse oximetry has both physiologic and technical limitations. Because the technique uses light absorbency changes in a pulsatile vascular bed, any event that significantly decreases those pulsations (hypotension, hypothermia, vasoconstriction) will decrease the ability of the pulse oximeter to obtain and process a signal and thus calculate Sp02. Therefore, it is may be necessary to change sensor sites (finger, ear) to obtain an optimal signal. Motion artifact, as evidenced by a heart rate discrepancy between the ECG and pulse oximeter, may interfere with accurate calculation of Sp02 in awake, agitated, or shivering patients. Ambient light as well as other light sources (radiant warmers, fluorescent bulbs) can contaminate light-emitting diode signals. Nail polish can alter the spectra of emitted light. The presence of dysfunctional hemoglobins can alter the ability of the SpO2 to accurately reflect SaO2. Carboxyhemoglobin is read as oxyhemoglobin by pulse oximeters, producing a falsely high SpO2. This is the reason the Sp02 may exceed the SaO2 as measured by a laboratory co-oximeter. Methylene blue causes a spurious decrease in SpO2. A high methemoglobin concentration tends to result in a SpO2 reading of 85% regardless of the actual PaO2 or Sa02. Fetal hemoglobin has limited influences on the accuracy of the SpO2 measurement Complications from the use of pulse oximetry are most commonly caused by errors in data interpretation. Use of continual pulse oximetry during magnetic resonance imaging has been associated with skin burns. There are now MRI compatible pulse oximeters. |
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