A Guide to Pulse Oximeters

With chronic health problems, such as emphysema, congestive heart failure and various lung diseases, patients will have difficulty breathing and experience a significant drop in energy levels and the ability to perform simple everyday activities. A pulse oximeter is a device used to detect if the decrease in energy is due to a lack of oxygen in the blood. Here is a useful guide to pulse oximeters and their function.

What is a pulse oximeter?

An alternative to taking a blood sample, pulse oximeters provide a noninvasive way to measure and monitor blood oxygen saturation. The monitor works by attaching to a finger or other thin, translucent body part and calculating blood oxygen levels. Normal blood oxygen level should be 95% or higher. If blood oxygen drops below 92%, low blood oxygen is indicated. Pulse oximeters also show the heart rate, which in the normal range should be 60-80 beats per minute. It’s not unusual for oxygen saturation to decrease and heart rate to increase during exercise, but the overall oxygen saturation should still be at 90% or higher. For patients with COPD (chronic obstructive pulmonary disease), asthma, and other lung diseases, the tool is a useful way to measure the oxygen needs of a patient, monitor blood oxygen levels, and assist in detecting and diagnosing a worsening condition.  

How a pulse oximeter works

A pulse oximeter measures the peripheral oxygen saturation which, though not the same as arterial oxygen saturation, is quite often close enough to safely and easily measure blood oxygen saturation. Normally a clip-like sensor is placed a thin body part, such as a finger or earlobe. In infants, the monitor is placed on a foot. The device works by sending two light wavelengths through that body part to a photodetector, which measures light or other electromagnetic activity. By calculating the change in the absorption of each wavelength, it measures the pulsing of the arterial blood alone, and can exclude venous blood, skin, bone, muscle, fat, and even nail polish, in its calculations. The device is particularly useful in clinical settings as a quick, non-invasive way to measure blood saturation. This method of measurement is called transmissive pulse oximetry, as opposed to reflectance pulse oximetry, which doesn’t require a thin body part and can be used on the feet, forehead, or chest.

The function of detecting blood oxygen

The pulse oximeter actually shows the percentage of the blood that is saturated with oxygen by measuring the percent of the hemoglobin, which is the blood protein that is carrying the oxygen. That percentage should normally be between 95 and 99 percent. By using an electronic processor and two small light-emitting diodes (LEDs) that are facing a photodiode through a thin body part, the blood oxygen is detected. This is because one of the LEDs is red and one is infrared, and the amount of light absorption between the two differs greatly in blood that is full of oxygen and blood that is not. The hemoglobin carrying the oxygen will absorb more infrared light and lets more of the red light go through, while the hemoglobin lacking oxygen will absorb more red light, letting infrared light pass through. Between the cycles of the LEDs, the photodiode will respond to the two lights, red and infrared, separately.  The blood oxygen saturation level reading is determined by analyzing the ratio of oxygenated hemoglobin to deoxygenated hemoglobin.

Home-Use Pulse Oximeters

There are also pulse oximeters that have been developed for personal use at home. This is particularly helpful for patients with COPD, asthma, and other lung diseases who need to frequently monitor their oxygen levels and adjust oxygen flow during various activities. It is critical, however, that the oximeter be reliable and accurate, especially when patients are active and their blood oxygen levels are decreasing. The home-use pulse oximeters is also a valuable tool in diagnosing whether a patient’s condition is worsening.

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