Up to date, expert answers to frequently asked questions (FAQ) about oxygen supply systems, respiratory care and pulse oximetry written by OCC & collaborators.
How pulse oximeters work
Some oximeters, sometimes. Blood pressure fluctuations occur with respiratory (or ventilator) cycles, and changes in the arterial waveform signal due to pulse pressure variation can often be more pronounced in hypovolemic patients. For this reason, pulse pressure variation (PPV) or pulse oximetry plethysmographic signal (POP) are sometimes used to try to assess volume responsiveness in patients.
Similar to a peripheral arterial pressure waveform, many pulse oximeters show a processed representation of the photoplethysmographic waveform. However, visual estimation of variations in the pulse oximeter waveform are unreliable. Some pulse oximeters will populate PPV or POP values that are automatically and continuously calculated based off of the waveform. However, each pulse oximeter manufacturer uses different algorithms to calculate these values and displays, making it very difficult to interpret accurately and clinically.
Some studies have stated that POP appears to be effective for predicting fluid responsiveness in ventilated patients, while others have found that it did not correlate well with arterial pressure waveforms. Overall, using pulse oximeters in isolation to evaluate volume responsiveness is not adequately validated or reliable in the clinical setting at this time. This is due to many limitations of this technique including poor or noisy signal, probe position, motion, vasoconstriction, and highly processed pleth signals which vary among manufacturers.
Keywords: pulse pressure variation, PPV, volume, fluid, POP
A pulse oximeter is a noninvasive monitoring device that can indirectly measure a person’s functional oxygen saturation (SpO2) and help with early detection of hypoxemia. Pulse oximeters work by either transmitting light through tissue perfused by blood (ideally a site with a dense capillary bed like the fingertip). They are called pulse oximeters because they use the pulsations to discriminate between arterial blood and other tissue or venous blood. The probe on the pulse oximeter has light-emitting diodes (LEDs) which shine at least two types of light, red and infrared. The device is positioned so that the light shines through a translucent part of a patient’s body, such as a fingertip or earlobe, and measures the changing absorbance at each wavelength. Oxygenated hemoglobin absorbs more infrared light, whereas deoxygenated hemoglobin absorbs more red light. Since the absorption of light at these wavelengths differs between oxygenated and deoxygenated blood, the device can determine the absorbances due to just the arterial blood and thereby determine a patient’s peripheral oxygen saturation.
Keywords: wavelengths, IR, infrared, red, LED
A pulse oximeter photoplethysmograph (commonly referred to as a ‘pleth’) is a graphical display of the pulse oximeter signal over time. Its appearance can vary widely under different clinical scenarios. In healthy patients, the graph should appear as asymmetric humps similar in appearance to an arterial pressure waveform though usually with less level of detail (i.e. the dicrotic notch may not be visible). The waveform should appear at intervals that match the heart rate and regularity.
Patient motion, tremor and poor perfusion are common factors that affect the pleth. The pleth is an extremely useful feature to help clinicians quickly determine the quality of the oximeter signal. Of note, in the face of a low signal (or low perfusion) some oximeters may auto adjust the scale of the display (y axis) to increase the visual amplitude of the pleth. This can be misleading to clinicians who must also pay attention to signal quality indicators when available.
References: Jubran, Crit Care 2015
Keywords: pleth, waveform, PPG
The term CO-oximetry refers to devices that use at least four wavelengths of light to measure not only oxy and deoxy-hemoglobin, but also other forms of hemoglobin (e.g. CO-Hb, Met-Hb). While most conventional pulse oximeters use two wavelengths of light and can only detect oxyhemoglobin or deoxy-hemoglobin (to provide a functional saturation, sO2), some pulse oximeters contain many more wavelengths and the ability to function as ‘pulse CO-oximeters.’ With more wavelengths, these devices can measure dyshemoglobins and provide “fractional” oxygen saturation – see image below. (Note: these devices are clearly marked with this function and often are considerably more expensive than other devices).
Despite this, the term ‘CO-oximeter’ is often used synonymously with ‘arterial blood gas analyzer’ because historically it was only multi-function arterial blood gas analyzers that possessed CO-oximetry functionality. Some pulse CO-oximeters have ~10 wavelengths of light, whereas benchtop CO-oximeters may use 128 to 256 wavelengths of light.
Of note, the terms “CO-oximeter” and “Hemoxemeter” are brand names coined by the companies Instrumentation Laboratories and Radiometer, respectively. The generic term for a device with these capabilities is a “multi-wavelength oximeter,” but the term “CO-oximeter” is widely used to refer to these devices, regardless of brand.
Keywords: CO-oximetry, arterial, carboxyhemoglobin, methemoglobin
Spectrophotometry is a type of quantitative measurement technique that is used to measure the reflection or transmission properties of a substance as a function of wavelength. Every type of substance absorbs light over a specific range of wavelengths. This type of measurement allows us to assess the intensity of light that a substance (such as hemoglobin in blood) absorbs, and therefore has clinical and bioengineering applications.
Clinically, spectrophotometry is used in pulse oximeters to determine the proportion of oxygenated hemoglobin in arterial blood. Since different wavelengths of light are absorbed by oxygenated and deoxygenated blood, pulse oximeters can use this technique to determine a patient’s peripheral oxygen saturation (SpO2).
Keywords: spectroscopy, spectrophotometry, wavelength, light
There are two main types of pulse oximeter: transmittance devices and reflectance devices. Transmittance pulse oximeters are the most common and work by transmission (i.e. shining a light) through tissue, usually a fingertip or ear. As the light passes through the body part, the amount of oxygen in the blood determines how much light is absorbed in the tissue. A light detector on the other side of the probe detects light that is not absorbed, and a microprocessor calculates the oxygen saturation in the blood. Reflectance pulse oximeters are placed on the skin surface and measure the light reflected off the tissues rather than through the tissue. In this way, absorption is measured to calculate oxygen saturation. Of note, reflectance devices are inherently more difficult to design to perform well.
References: Lifebox Pulse Oximetry Learning Module
Keywords: transmittance, reflectance, light, absorption
In 1940, J.R. Squire was the first to recognize that the differences in transmission of red and infrared light permitted oxygen saturation to be computed. In 1942, the British scientist Glen Millikan developed the first portable pulse oximeter to be used during pilot training in World War II. The device was placed on the earlobe and was used to monitor pilots’ oxygen saturation and safety during flight.
Thirty years later in 1972, the Japanese bioengineer Takuo Aoyagi developed conventional pulse oximetry by using the ratio of red to infrared light absorption in pulsating (arterial) blood to calculate oxygen saturation. This was later commercialized and gradually integrated into clinical practice in the 1980s and 1990s. Notably, the American Society of Anesthesiologists determined in 1986 that using pulse oximetry was to be recognized as the new standard of care. To learn more history about the development of the pulse oximeter, read here.
Keywords: first, invention, Millikan, Aoyagi