The Concept of SpO2

The percentage of oxygenated hemoglobin to total hemoglobin when the probe of the pulse monitor detects the small arteries of the patient’s finger (toe) is called SpO2. So what are the principles when we testing?

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The Monitoring Principle of Pulse Oximeter

The pulse oximeter is an instrument used to monitor the patient’s arterial blood oxygen saturation. According to the different probes designed by the instrument and the different objects to be monitored, a probe is placed on a translucent part of the body (often fingers, earlobes, toes, nose, forehead, etc.), one side of the detector is a diode, which emits a spectrum to the opposite side, and a spectrum detector is installed on the opposite side. When the oxygen content in the blood is low, the color of the blood becomes bluish, and the oximeter measures the blue component to determine the oxygen values.

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Patient-related Factors and Interventions Affecting the Accuracy of SpO2

Cold Finger (toe), Abnormal Skin or Color

The normal operation when testing SpO2 depends on the good perfusion of the tissue. When the fingertips are cold and the local arteries are contracted, it will show insufficient pulse signal, low blood oxygen saturation, or no results. Therefore, during the monitoring, the finger (toe) should be kept warm, and the room temperature should be kept at 25℃~28℃. If necessary, the hand should be covered with a quilt to keep warm.

Studies have found that skin pigment has an effect on SpO2. In the state of low blood oxygen saturation, SpO2 overestimates the actual SaO2 value, and this phenomenon is particularly obvious among blacks. But there are also experiments that show that skin pigment does not affect the accuracy of oxygen saturation. Therefore, the influence of skin pigment is still controversial.

Thick skin can affect the penetration of light. Nail polish or other organic dyes may interfere with the absorption of light waves of the pulse oximeter, making the reading inaccurate, especially blue nail polish. Red, purple, and transparent nail polish have little effect. When monitoring, it is advisable to select fingers with thin skin and no pigmentation for monitoring. Avoid fingers (toes) painted with nail polish or other colored dyes as the test site, or wipe them off before testing.

Monitor Excessive Activity

Monitoring the activity of the limbs can cause inaccurate readings and false alarms from the monitor, which is called motion artifacts. Movement or tremor of the limbs can cause motion artifacts. Therefore, in the process of monitoring SpO2, it is necessary to know whether the patient has chills or excessive activity.

Monitoring of Blood Supply Disorders

Frequent monitoring of blood pressure, excessive tightening of restraint belts in agitated patients, excessive flexion of limbs, and long-term fixed finger monitoring can block blood flow and affect the results of blood oxygen saturation monitoring. In nursing care, avoid the monitoring of the ipsilateral finger for blood pressure measurement, and replace the finger every 2 h to 4 h, and the restraint band should be tight.

Vasoactive Drugs

Long-term use of dopamine, norepinephrine, sodium nitroprusside and other drugs can constrict peripheral blood vessels and significantly reduce the pulse volume wave. To cause serious distortion or lack of oxygen saturation, drugs that improve peripheral blood circulation can be appropriately applied.

The Impact of Disease

Hyperbilirubinemia, methemoglobinemia, and carbon monoxide poisoning lead to decreased oxygenated hemoglobin, increased carboxyhemoglobin, diabetes or arteriosclerosis, severe vascular disease, and hypotension due to weakened arterial pulsation, and its accuracy is also affected to a certain extent.

And in the next article, we will analyze the factors and interventions related to blood oxygen saturation monitor.

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