Pulse oximetry is a device used to monitor a patient's oxygenation saturation in various health care settings such as emergency departments and intensive care units. It provides valuable information to avoid hypoxia and ensure adequate cell function after traumatic injury or medical condition and provides feedback for supplemental oxygen treatment.
Oxygen and Cell Function
Physiologically, every living cell in the human body requires oxygen to function properly. During this process, the cells produce carbon dioxide as a byproduct. Under normal circumstances, oxygen is acquired each time a breath is taken into the lungs. As molecules of oxygen are absorbed, they attach to red blood cells circulating through the lung tissue. These red blood cells, numbering in the thousands, are then pumped by the heart around the body to small vessels called capillaries.
At the capillary level, each blood cell allows the oxygen molecules to be transported to the cells of the body in exchange for the carbon dioxide byproduct. The red blood cells, now with carbon dioxide attached, are again pumped to the lungs where the carbon dioxide is released into the lungs in exchange for new oxygen, and the cycle repeats.
Pulse Oximetry Basics
Pulse oximetry technology uses two light-emitting diodes (LEDs). One beams a red light and the other an ultraviolet light. Both lights have different wavelengths and are aimed at a photodiode, or device used to measure wavelengths, on the other side of the sensor. During a pulse, wavelength absorption is measured at its baseline and at its peak during a surge of blood after the heart contracts. Differences in absorption rates between the two waves by oxygen molecules on the red blood cell are measured and displayed as a percentage. This is why a pulse is necessary for the oximeter to work and why a pulse rate can be displayed.
Using the Probe
The reusable finger probe is a common sensor for pulse oximetry readings. It can be placed on a finger by simply squeezing the spring loaded clip ends, placing the probe over the tip of the desired finger, and releasing it.
The disposable finger probe is another type of sensor. Remove the adhesive backing, and place the sensor on the tip of the desired finger. Fold the halves so that the sensors match up with one on top of the finger nail and the other on the pad of the finger, and press firmly.
A third probe is called the ear probe. Apply it by squeezing the spring loaded clip ends open and place it on the patient's earlobe.
The last type is called the disposable neonate probe. Apply by removing the adhesive backing and place the sensor on the nail of a big toe, and completing the application by wrapping the remaining adhesive strip.
Interpretation
The purpose of using pulse oximetry is to measure the saturation percentage of oxygen on the red blood cells and determine the indirect efficiency of its delivery to the cells of the body. Typically, this level should be around 94 to 99 percent.
For the most accurate reading, the oximeter must be placed on a translucent part of the body, such as a finger, earlobe or the foot in infants.
Trends in pulse oximetry readings are necessary to identify a condition of low oxygen content in the blood, called hypoxia. Hypoxia is caused by a multitude of conditions including cardiac arrest, severe trauma, suffocation, seizure and lung diseases such as asthma and COPD and may require supplemental oxygen to treat.
Though some organs are more sensitive to hypoxia than others, it can be classified as mild, moderate or severe. Mild hypoxia will show a reading of 88 to 93 percent, moderate hypoxia will show a reading of 82 to 87 percent, and severe hypoxia will be less than 82 percent.
Troubleshooting and Maintenance
The pressure of atmospheric oxygen is higher at sea level than at high elevations. As a result, more oxygen is being attached to the red blood cells standing by the ocean than standing on a mountain top. So at high elevations, a value of 88 to 90 percent should be considered normal.
The pulse oximeter is designed for simplicity of use, but may be prone to problems. If the display is inaccurate or reads zero for the oxygen saturation or pulse, first check that the sensor cable is not damaged and correctly inserted into the monitor. If still no reading, check that the sensor is properly positioned and that the patient is not moving the sensor excessively.
Be aware that other variables also influence oximeter readings such as poor circulation to the fingers and excessive illumination from ambient light.
Placing the finger sensor on the extremity with a blood pressure cuff will not yield accurate reading during inflation.
Clean reusable probes after each patient use to avoid cross contamination and a buildup of material that may cause inaccurate readings.
References
- "Structure and Function of the Body"; Gary Thibodeau, PhD, Kevin Patton, PhD; 1997
- Oximetry. org: Principles of Pulse Oximetry Technology
- Amperor Direct: How to Interpret Pulse Oximeter Readings
- Associated Content: Hypoxia Causes, Symptoms, and Treatment Options
- Flightstat: Hypoxia, Oxygen, and Pulse Oximetry
