Chest
Critical CarePulse Oximetry: Uses and Abuses
Section snippets
HISTORY
Although the clinical use of pulse oximetry has become popular only recently, the technology has existed for over 50 years. The measurement of oxygen saturation using light absorption was first proposed in the 1930s. The development of the Clark electrode in the 1950s, which allowed for easy measurement of PaO2, slowed further work on oximetry. The ear oximeter was introduced in the 1960s for noninvasive monitoring of oxygenation, but it was bulky and required heating the ear to arterialize it
PRINCIPLES
The principle of oximetry is based on Beer's law, which states that the concentration of an unknown solute dissolved in a solvent can be determined by light absorption (Fig 1). This relationship is expressed mathematically by Beer's equation (equation 1):
where is the amount of incident light, D is the distance through which light travels, C is the concentration of a substance (ie, hemoglobin); a is the extinction (absorption) coefficient, and Lout is the intensity of
ACCURACY
The measurement of oxygen saturation by the CO-oximeter is the standard by which pulse oximetry is calibrated. An early study by Yelderman and New4 examined the accuracy of pulse oximeters over the range of 70 to 100 percent saturation and found excellent correlation between the pulse oximetry and CO-oximetry measurements (correlation coefficient of 0.98; slope, 1.03; p<0.0001). The response of pulse oximetry to profound hypoxemia (saturations less than 70 percent) was studied by Severinghaus
LIMITATIONS
Although pulse oximetry represents a significant advance in noninvasive monitoring, situations exist where the pulse oximeter may give misleading or inaccurate information, as shown in the following tabulation:
Dyshemoglobinemias
Carbon monoxide
Methemoglobin
Fetal hemoglobin
Dyes and pigments
Methylene blue
Indocyanine green
Bilirubin
Low perfusion
Anemia
Increased venous pulsations
External light sources
Pulse oximeters measure oxygen saturation. This is related to PaO2 by the
SELECTION OF PULSE OXIMETER
The selection of a particular model of oximeter depends on the features desired for a particular setting. Most commercially available pulse oximeters display oxyhemoglobin saturation and heart rate. If the pulse oximeter is to be employed during the transport of patients, the battery life and recharge time and audio alarms will be important. In the intensive care unit, ECG synchronization may be useful. Other features to be considered when selecting a unit include accuracy, recording
CLINICAL SETTINGS
The use of pulse oximetry in the operating room and the recovery room have become the standard of care.24 Standards for monitoring during anesthesia set up by the Harvard Medical School System now include continuous oxygen monitoring by oximetry. This move was prompted by numerous reports of preventable adverse outcomes. McKay and Noble25 noted numerous episodes of hypoxemia in patients undergoing surgical procedures. Hypoxic episodes were not limited to high-risk patients or any specific
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