The Non-Invasive Advantage
There is no doubt that pulse oximetry represents a great advance in patient monitoring. It is a relatively inexpensive and above all, completely non-invasive technique.
Pulse oximetry is a continuous and non-invasive method of measuring the level of arterial oxygen saturation in blood. The measurement is taken by placing a sensor on a patient, usually on the fingertip for adults, and the hand or foot for infants. The sensor is connected to the pulse oximetry instrument with a patient cable. The pulse oximetry sensor collects signal data from the patient and sends it to the instrument. The instrument displays the calculated data in three ways:
- As a percent value for arterial oxygen saturation (SpO2).
- As a pulse rate (PR).
- As a plethysmographic waveform.
The Evolution of Pulse Oximetry
Development of non-invasive spectrophotometric techniques to monitor O2 saturation began during World War II. The development of high altitude aircraft created a need for pilots to be externally monitored for any physiological changes induced by extreme altitude. In response to this need, the first functional non-invasive spectrophotometer was developed in 1942. Its inventor, Glen Millikan, named this new device the “oximeter”.
Pulse oximeters have evolved from physiologic monitoring curiosities to common patient monitoring devices. New pulse oximetry technology couples spectrophotometry with pulse waveform monitoring and permits clinicians to continuously assess arterial O2 saturation in operating rooms, in intensive care units, during sleep studies (polysomnography), and at the bedside. Portable pulse oximeters and recorders have also become popular monitoring devices during emergency medical transport and outpatient assessment of gas exchange. Advantages to pulse oximeters, other than their non-invasiveness, include their well-documented accuracy, ease-of-application, and good patient tolerance.
Pulse Oximetry’s Abilities
Continuous pulse oximetric monitoring of arterial oxygenation can detect intermittent or chronic disruptions in gas exchange that may not be detected by random arterial blood sampling and analysis. Also, pulse oximeter measurements of O2 saturation do not carry the risk of morbidity and mortality associated with invasive arterial blood sampling. Another value of continuous monitoring is the ability to quantitatively determine the amount of time spent at any given level of arterial O2 saturation. This information can then be used to monitor the progression of gas exchange impairment or to evaluate the effectiveness of therapeutic interventions. With such widespread application of pulse oximetry technology, comprehension of the operating principles and the practical limitations of use can aid clinicians. The following section describes the fundamental principles used in pulse oximetry technology to acquaint clinicians with environmental and physiological conditions that can affect their use.
The Measurement Process
The measurement process is based on two factors:
- A pulsatile signal is generated by the heart in arterial blood, which is not present in venous blood and other tissues.
- Oxyhemoglobin and reduced hemoglobin have different absorption spectra. Also, it is important to note that both spectra are within the optical window of water (and the soft tissue).
Pulse oximeters measure oxygen saturation by means of a sensor attached to the patient’s finger, toe, nose, earlobe or forehead. Typically, the sensor uses two light-emitting diodes (LEDs) at wavelengths of 660nm and 940 nm (infrared) and a photodetector placed opposite them. The photodetector measures the amount of red and infrared light that passes through the tissue to determine the quantity of light absorbed by the oxyhemoglobin and hemoglobin. As the proportion of oxyhemoglobin increases in the blood, the absorbance of the red wavelength decreases, while the absorption of infrared increases. SpO2 is determined by calculating the ratio of red-to-infrared light absorbencies and comparing it with values in a look-up table or calibration curve, which is a standardized curve developed empirically by simultaneous measurement of SaO2 and light absorbencies.
SpO2 is physiologically related to arterial oxygen tension (PaO2) according to the O2Hb dissociation curve. Because the O2Hb dissociation curve has a sigmoid shape, oximetry is relatively insensitive in the detection of developing hypoxemia in patients with high baseline PaO2.
SpO2 measurements made by a pulse oximeter are defined as being accurate if the root-mean-square (RMS) difference is less than or equal to 4.0% SpO2 over the arterial oxygen saturation (SaO2) range of 70% to 100%, SpO2 accuracy should be determined by clinical study of healthy or sick subjects, whereby SpO2 measurements are compared with SaO2 measurements.
Other Pulse Oximeter Factors
Pulse oximeters can also measure pulse rate. The standard states that pulse rate accuracy should be defined as the RMS difference between paired pulse data recorded with pulse oximeter and a reference method.
There are several limitations of pulse oximetry:
skin pigmentation, ambient light, intravenous dyes, low perfusion and motion artifact.
As pulse oximetry technology has advanced, manufacturers have attempted to reduce the effect of some of the limitations mentioned above. Particular improvements have been made in the ability of oximeters to deal with low signal-to-noise conditions observed during periods of motion or low perfusion.
Regular functional checks should be carried out on equipment to ensure it is safe to use. This should include visual checks, especially checking for signs of damage.
Functionality of an oximeter can be checked using a pulse oximeter tester or simulator. These simulate the properties of a finger and its pulsatile blood flow. Their purpose is allowing testing of a pulse oximeter and the continuity of probes. They cannot be used to validate the accuracy of a pulse oximeter.