Published on 13/02/2015 by admin
Filed under Anesthesiology
Last modified 13/02/2015
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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.
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