Published on 13/02/2015 by admin
Filed under Anesthesiology
Last modified 22/04/2025
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Klaus D. Torp, MD
Oximetry involves the measurement of oxyhemoglobin (HbO2) concentration based on the Lambert-Beer law. Fractional oximetry, which measures arterial O2 saturation (SaO2), is defined as HbO2 divided by total hemoglobin (Hb). Total Hb is calculated as the sum of HbO2, reduced or deoxyhemoglobin (HHb), methemoglobin (metHB), and carboxyhemoglobin (COHb). In contrast, functional oximetry, which measures O2 saturation using pulse oximetry (SpO2), is defined as HbO2 divided by the sum of HbO2 and HHb. In clinical practice, SpO2 is measured using a pulse oximeter to estimate SaO2.
< ?xml:namespace prefix = "mml" />SaO2 = HbO2/(HbO2+HHb+metHb+COHb)
SpO2 = (HbO2/HbO2+HHb)
HHb absorbs more light in the red band (600 to 750 nm) than does HbO2, whereas HbO2 absorbs more light in the infrared band (850 to 1000 nm) than does HHb. The conventional pulse oximeter probe contains two light-emitting diodes (LEDs) that emit light at specific wavelengths: one in the red band and one in the infrared band. Typical wavelengths are 660 nm and 940 nm. When the probe is placed on the patient, the light emitted from the LEDs is transmitted or reflected (depending on the site of the sensor) through the intervening blood and tissue and is detected by sensors built into the probe. The amount of transmitted light is sensed several hundred times per second to allow precise estimation of the peak and trough of each pulse waveform. At the pressure trough—during diastole—light is absorbed by the intervening arterial, capillary, and venous blood, as well as by the intervening tissue. At the pressure peak—during systole—additional light is absorbed in both the red and infrared bands by an additional quantity of purely arterial blood, the pulse volume. The typical pulse amplitude accounts for 1% to 5% of the total signal. Pulse oximeters isolate the pulsatile components from the blood volume signal (photoplethysmogram) and calculate the red over infrared red ratio, which is then used to calculate SpO2 by using an algorithm, based on a nomogram, built into the software of the pulse oximeter. Isolation and measurement of the pulsatile component allows individuals to act as their own controls and, thus, eliminates potential problems with interindividual differences in baseline light absorbance. The “calibration curve” used to calculate SpO2 was derived from studies of healthy volunteers.
The process to identify the pulse, which is initiated with application of the probe to the subject, includes sequential trials of various intensities of light to find those strong enough to transmit through the tissue without overloading the sensors.
Pulse oximeters have generally been found to be accurate to within 5% of in vitro oximeters, in the range of 70% to 100%. The most widely used “gold standard” for comparison is the IL282 co-oximeter. Sensors are calibrated to the site of application (digit, ear, forehead, etc.). Applying them to a different site may give false SpO2 readings even when showing an acceptable plethysmographic waveform. In discussing the accuracy of pulse oximeters, the terms bias and precision are used. Bias is the mean value of SaO2 minus SpO2. Precision is the standard deviation of the bias.
There are two potential problems with the accuracy of pulse oximetry below values of 70%. First, as stated previously, pulse oximeters have been calibrated using studies of healthy volunteers (an Olympic athlete, in one case). Therefore, it is unlikely that much data have been collected for calibration at low saturation levels. Second, the absorption spectrum of HHb is maximally steep at 600 nm. Therefore, any slight variance in the light emitted from the 660-nm LED has significant potential to introduce measurement error into the system. Because decreasing levels of SpO2 lead to an increasing proportion of HHb, there is the potential for increasing inaccuracy as SpO2 decreases. These potential problems are unlikely to be of much clinical significance. For example, it is unlikely that a treatment decision would be based on whether an SpO2 is 50% versus 60% at a given time. Some studies have reported poor accuracy of pulse oximeters at SpO2 values of less than 70%.
Most pulse oximeters average pulse data over 5 to 8 seconds before displaying a value. Some oximeters allow for an override of this lag by providing for a shortened averaging interval or allowing a beat-by-beat display. Response time is also related to probe location and perfusion. Desaturation response times range from 7.2 to 19.8 seconds for ear probes, from 19.5 to 35.1 seconds for finger probes, and from 41.0 to 72.6 seconds for toe probes.
Pulse oximeters depend on a pulsatile waveform to calculate SpO2. Therefore, under conditions of low or absent pulse amplitude, the pulse oximeter may not accurately reflect SaO2 or may not provide a reading at all (e.g., during cardiac arrest, proximal blood pressure cuff inflation, tourniquet application, hypovolemia, hypothermia, vasoconstriction, or cardiac bypass). In addition, pulse oximeters are more sensitive to movement artifact during low-pulse–amplitude states.
The earlobe and forehead appear to be areas that are least sensitive to a decreased pulse. If the SpO2 decreases without an obvious physiologic cause (e.g., asystole) and changing the site of the sensor does not produce the desired result, changing to a different brand of pulse oximeter, with a different signal-processing algorithm, may provide a reading. There has been some question of the accuracy of pulse oximeter readings in the face of an arrhythmia in which not all electrocardiographic complexes produce a sufficient stroke volume, creating a pulse deficit. However, a relationship between pulse deficit and bias has not been identified.
Conventional pulse oximeters use only two wavelengths of light; therefore, conventional pulse oximeters can accurately measure only HbO2 and HHb. The presence of a third or fourth species of hemoglobin (e.g., metHb or COHb) can interfere with accurate measurement by causing changes in the absorbance of light in the critical red and infrared regions.
The COHb is interpreted by the pulse oximeter as a mixture of approximately 90% HbO2 and 10% HHb. Thus, at high levels of COHb, the pulse oximeter will overestimate true SaO2, as may occur in patients with recent CO exposure (e.g., house fire, combustion engine exhaust, or cigarette smoking).
metHb is formed when the heme iron is oxidized from the ferrous (Fe2+) to the ferric (Fe3+) state. metHb is very dark and tends to absorb equal amounts of red and infrared light, resulting in a red:infrared ratio of 1. When extrapolated on the calibration curve, a ratio of 1 corresponds with a saturation of 85%. Thus, as metHb increases, SpO2 approaches 85% regardless of the true level of HbO2. Drugs capable of causing methemoglobinemia (defined as greater than 1% metHb) include nitrates, nitrites, chlorates, nitrobenzenes, antimalarial agents, amyl nitrate, nitroglycerin, sodium nitroprusside, and local anesthetic agents. High levels of metHb create mitochondrial hypoxia caused by the diminished O2-carrying capacity of blood and a leftward shift in the HbO2 dissociation curve. Recent advances in pulse oximetry, some of which use more than seven wavelengths (Rainbow SET Technology, Masimo Corp., Irvine, CA) allow approximate measure of levels of COHb and metHb, but these newer pulse oximeters do not correct for COHb and metHb in the SpO2 readings.
Pulse oximeters underestimate the SaO2, especially at low saturation values, in patients with anemia; however this finding has little clinical significance because an intervention would likely take place before the SaO2 would fall to a low level. Some new pulse oximeters have the capability of measuring total hemoglobin either continuously or as a spot check device.
Injections of methylene blue produce a large and consistent spurious decrease in SpO2, with readings remaining below baseline for 1 to 2 minutes. Injection of indocyanine green decreases SpO2 to approximately 80% to 90%, which lasts for a minute or less. Injection of indigo carmine decreases the SpO2 the least, with the decrease lasting approximately 30 seconds.
Elevated serum bilirubin concentrations, per se, do not affect the accuracy of the pulse oximeter.
Inaccurate SpO2 readings have been reported to occur because of interference by the light of surgical lamps, fluorescent lights, infrared-light–emitting devices, and fiberoptic light sources.
Obtaining an accurate pulse oximetry reading may not be possible in deeply pigmented individuals because of a failure of LED light transmission.
Electrocautery results in decreased SpO2 readings caused by interference from the wide-spectrum radiofrequency emissions, which affect the pulse oximeter probe. Interference is usually of little clinical significance unless electrocautery is used for a lengthy time in patients who have decreased or unstable O2 saturation. Some manufacturers have attenuated this problem by improving the electrical shielding of sensors and cables.
Repetitive and persistent motion artifact of any kind tends to cause the SpO2 to display the local venous SaO2 level. The susceptibility to motion artifacts may also depend on the signal-processing algorithm of the pulse oximeter brand.
The presence of fingernail polish may cause a low of SpO2 value, although this effect is not constant and may be related to the number of layers and the color of the polish. Acrylic nails usually have no significant effect, but depending on the color may also result in a lower SpO2 reading. Placing the sensor sideways on the finger may produce a reading, but the preferred action would be to remove the nail polish or choose another site.
While some pulse oximeters measure COHb and met Hb, In addition to measuring SpO2, COHb, and metHb, most pulse oximeters also provide plethysmography, which can provide useful information regarding the patient’s volume status by assessing the respiratory variation, where the plethysmographic amplitude changes during the respiratory cycle. Plethysmographic waveforms however are highly processed and filtered so that a visual estimate of the extent of the respiratory variation of the pulse amplitude may not be correct. However some devices now provide a numeric index of that variability, which correlates with change in intravascular volume status and fluid responsiveness in hypotensive patients during positive pressure ventilation. Some pulse oximeters also provide measurements of perfusion in a digit, which can be used to assess a change in sympathetic tone during general and regional anesthesia.
Complications are very rare and generally minor, including mild skin erosions and blistering, tanning of the skin with prolonged continuous use, and ischemic skin necrosis.
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