Which Part Is More Suitable For Blood Oxygen Saturation Measurement

Which part is more suitable for blood oxygen saturation measurement

Pulse oximetry, as a non-invasive way to measure a patient's blood oxygen saturation, is largely dependent on the perfusion of vascular tissue. Therefore, its probe is usually placed on the high blood vessel density finger, earlobe or forehead. If peripheral perfusion is reduced, the measurement will suffer, resulting in inaccurate blood oxygen readings.

In these locations, the clip-on oximeter relies on perfusion from the radial artery to the digital artery, while the forehead relies on the supraorbital artery for SpO2 measurements. The forehead vasculature has limited vasoconstriction capacity compared to the finger vasculature, so in conditions of high sympathetic output and low peripheral perfusion, such as heart failure, placement of the oximeter on the finger may not be as accurate as on the forehead.

A few years ago, Nellcor researcher Bebout and his colleagues found that during peripheral vasoconstriction, fingers detect hypoxemia with a lag of about 90 seconds compared to forehead sensors. More recently, their work has been extended to include assays comparing hypoxemia in radial blood proximal to the ear, forehead near the supraorbital and digital arteries. In the study, subjects were placed in a cold room and thermal images were used to show differences in vasoconstriction and perfusion at different sites. Over time, thermal scans and readings showed that ear sensors (measuring external carotid artery branches) and digital sensor receptors had the greatest effect on thermoregulatory vasoconstriction and were slower to respond to changes in central oxygenation.

In a test involving more than 180 people, the finger-clip oximeter was able to accurately measure blood oxygen saturation and heart rate in patients with coronary heart disease during exercise stress testing, but was less accurate in patients with heart failure, while This may be due to reduced peripheral perfusion in patients with heart failure due to low cardiac output. Since the pulse oximeter determines the oxygen saturation of arterial blood by first detecting the arterial waveform and filtering the non-arterial blood readings. Therefore, pulse oximetry does not work well in these hypoperfusion conditions where arterial waveforms are attenuated. Acral skin, such as fingertips, is greatly affected by increased sympathetic tone, resulting in a more pronounced decrease in perfusion. Increased sympathetic tone in patients with heart failure may be a major contributor to poor performance when a oximeter is placed on the finger.

Meanwhile, a study of patients with a low resting cardiac index showed that a forehead-based oximeter was more accurate than a digital oximeter-based probe in determining oxygen saturation. Meanwhile, studies of surgical and trauma patients at risk of peripheral poor perfusion have shown that forehead oximetry probes are more accurate in measuring oxygen saturation. Although finger pulse oximeters have been shown to be accurate in low perfusion conditions, patients in transit are affected by motion and ambient temperature, and the use of forehead oximetry probes to measure oximetry compared with finger placement Fewer wrong measurements and failures.

The same results were seen in clinical research experiments, where patients tended to experience mild hypothermia and vasoconstriction during and after surgery. The researchers' optimal tissue for oxygen saturation measurements was the site that exhibited the least vasoactivity and that blood flowed to the forehead through the supraorbital artery, an area less susceptible to vasoconstriction caused by poor perfusion. In a companion study working at MacLeod Hospital, its researchers examined the performance of sensors on the forehead, earlobes and fingers of patients, initially at normothermia and vasoconstriction, then deliberately hypothermic and vasodilating. In both states, the forehead was found to be "lower in vasoactivity," so the researchers believe that the forehead may be the best location for pulse oximetry.

Although the researchers found that the forehead may be the most accurate in low perfusion conditions, they did not find this in previous experiments. The researchers found that the reason for this may be that the headband used to secure the forehead sensor was not used in the earlier study, but was used in the more recent study. They concluded that proper sensor placement and use of the headband are important considerations for the success of the forehead sensor. Proper placement means placing the sensor just above the eyebrow so that it is centered slightly outside the iris. Considering its cost effect, the researchers emphasized that the forehead sensor is not suitable for general use, and more should be used in low perfusion conditions.

Therefore, the finger is still the best place to use the blood oxygen probe for blood oxygen saturation measurement in most cases, and in the case of low saturation, in order to ensure the accuracy of the measurement, we can use the forehead blood oxygen probe.