Pulse oximetry (SpO2) assesses oxygenation by measuring the arterial oxygen saturation of hemoglobin through a noninvasive sensor attached to the patient. Unlike oxygen saturation in arterial blood (SaO2), which can be directly measured by arterial blood gas analysis, SpO2 is an indirect measurement calculated by the pulse oximetry monitor. (Keep in mind, though, that many arterial blood gas analyzers report a calculated saturation, which is actually less accurate than that provided by the pulse oximeter.)
Primarily used for early detection of hypoxia, SpO2 provides ongoing analysis of a patient's oxygen saturation. The standard of care for patients undergoing anesthesia, it's also become commonplace in medical/surgical units and other acute care settings. The proper use of a pulse oximeter can ensure earlier detection of hypoxia, reduce the number of arterial blood gas samples needed, and save the patient from painful arterial punctures.
Like any technology, pulse oximetry has both benefits and limitations. In this article, I'll review what you need to know to get the most out of this basic monitoring tool.
On the move
Oxygen carried in blood is released to tissues by diffusion, moving from an area of higher partial pressure and concentration to an area of low partial pressure and concentration. Because tissues are constantly using oxygen for energy, the oxygen concentration in tissues is always low, stimulating hemoglobin to release oxygen to the tissues.
Oxygen is carried by the blood in two forms. Most (97% to 98%) is bound to hemoglobin; the remaining 2% to 3% is dissolved in plasma. Oxygen dissolved in plasma is measured as PaO2. Dissolved oxygen is the only form that diffuses across cell membranes and produces a partial pressure, which in turn drives diffusion.
When hemoglobin is saturated with oxygen, it's called oxyhemoglobin; this is what gives arterial blood its bright red color. When it releases oxygen to tissues, it becomes deoxyhemoglobin, which explains why venous blood is dark red.
How pulse oximetry works
Usually applied to a fingertip, the pulse oximeter sensor (peripheral probe) consists of two light-emitting diodes (LEDs) and a photodetector located directly opposite the LEDs. Besides the patient's finger, the toe, nose, forehead, or earlobe can be used as an application site, as long as tissue perfusion is adequate.
The LEDs emit two wavelengths of light: red light absorbed by deoxyhemoglobin and infrared light absorbed by oxyhemoglobin. The beams of light pass through the tissues to the photodetector. During passage through the tissues, some light is absorbed. The photodetector captures the beat-to-beat change in light absorption. The microprocessor can distinguish pulsatile arterial blood from nonpulsatile venous or capillary blood and other tissue pigments.
The microprocessor calculates the patient's SpO2 and pulse rate. A normal SpO2 range is 95% to 100%. Remember that although pulse oximetry gives a good estimation of adequate oxygenation, it doesn't provide information on ventilation.
Beware false readings
Anything that causes a decrease in blood flow or poor perfusion to the sensor application site, such as peripheral arterial disease, vasoconstriction, hypothermia, or hypotension, may cause a false reading by pulse oximetry. When choosing an application site, avoid sites distal to the BP cuff to avoid an interruption of blood flow from cuff inflation. Try applying the sensor to the patient's earlobe or forehead if he's mildly hypothermic.
The presence of carboxyhemoglobin or methemoglobin can also produce false readings. Carboxyhemoglobinemia may affect longtime heavy smokers and patients with carbon monoxide poisoning or smoke inhalation injuries, giving a falsely high SpO2 reading. Also remember that blood abnormalities such as anemia can affect the hemoglobin level; in this case, the pulse oximeter may indicate 95% oxygen saturation even though the total oxygen content is profoundly low. Hemoglobinopathies such as sickle-cell disease can alter the shape and function of red blood cells, causing either a falsely high or low SpO2 reading.
Movement can cause inaccurate SpO2 readings. If your patient has tremors from Parkinson's disease or seizures, apply the sensor to an area of the body least affected by the motion for more accurate readings.
If you're applying the sensor to a patient's finger or toe, nail polish or artificial nails may affect the accuracy of the reading. Remove polish before applying the sensor.
Getting a reading
Once you've applied the sensor, secure the cable to the pulse oximeter and turn on the power. Wait for evidence of a good pulse signal quality, depending on the type of device you're using. Correlate the pulse oximeter rate with the patient's palpated pulse.
Remember that SpO2 is only one patient-assessment tool and should be interpreted along with other patient data including vital signs, cardiac rhythm, and breath sounds.
Knowing the physiology and limitations of the device can help you head off complications if your patient's oxygenation starts to stall.
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