ECG and Pulse Oximetry: History and Types
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In this chapter, we will discuss the history of ECG and pulse oximetry, the timeline and variations through time of the concepts used. We all also discuss the types of pulse oximetry and the electronics used with their requirements.
1.1 History of ECG
The history of ECG is very wide, dating back to the 1600 with William Gilbert (that introduced the electrica concept for objects holding static electricity) (1).The most important founders of the electrocardiogram concept were Emil Reymond and Willem Einthoven. In 1843, Emil Reymond was the founder of the electrocardiograph concept by using a galvanometer to state that muscular contraction has action potentials. He also identified the types of waves by using the P, Q, R, and S waves. His studies inspired many physicians to continue and develop his work further. The evolution of concepts continued until the discovery of P, Q, R, S and T waves by Willem Einthoven in 1895. Einthoven also invented a modified galvanometer and used in for electrocardiogram recording. As a reward for his work, he won a Noble price in 1924 for inventing the electrocardiograph (1).
As stated before, the history of ECG is very wide, therefore we will limit the observation to the movement done between 1843 and 1942 as shown in the following table:
Table 1: ECG Timeline
Year Scientist Concept
1842 Carlo Matteucci heart beat is accompanied by electric current
1843 Emil Dubois-Reymond Muscular contraction is accompanied by action potential.
Test carlo's concept on animals successfully
1856 Koelliker , Muller Record of the action potential concept
1869 Alexander Muirhead Might have recorded a human electrocardiogram
1872 Gabriel Lippmann Capillary Electrometer invented
1876 Marey EJ Electrical activity of animal recorded by the electrometer
1878 John Sanderson , Frederick Page Electrical current of the heart is recorded
Divide into two phases (later known as QRS and T)
1887 Augustus Waller First human electrocardiogram is published
1890 GJ Burch Arithmetic correction of the electrometer
1891 William Bayliss , Edward Starling Capillary electrometer improved
Discovery of deflections (later known as P,QRS,T) and delay (later know as PR interval)
1893 Willem Einthoven The term electrocardiogram introduced
1895 Deflections P,Q,R,S and T distinguished
1897 Clement Ader Galvanometer invented( Amplification system for the lines of telegraph )
1901 Willem Einthoven Galvanometer modified for ECG use
1902 ECG records using galvanometer published
1903 Commercial production of galvanometer discussed
1905 Telecardigram invented (transmission of ECG signal by telephone)
1906 Normal and abnormal ECG record published
Introduction of the U wave
1908 Edward Schafer First purchase of Einthoven's galvanometer
1910 Walter James, Horatio Williams Electrocardiography reviewed for the first time in America
1911 Thomas Lewis Publication of a book about heart beat mechanism
1912 Willem Einthoven Description of the Einthoven triangle (formed for the leads)
1920 Hubert Mann Derivation of mono-cardiogram (later known as vector-cardiogram)
1924 Willem Einthoven Nobel price won for the electrocardiograph invention
1928 Ernstine, Levine Introduction of vacuum-tubes for ECG amplification
Frank Sanborn First portable ECG invented
1932 Charles Wolferth and Francis Wood Description of the chest leads use in the coronary occlusion
1938 American heart and cardiac British association Standard positions of chest leads defined and added (V1 to V6)
1942 Emanuel Goldberge Addition of aVR, aVL and AVF to previous model
Final ECG model used today
1.2: History of pulse oximetry
The revolutionary paper by Comroe and Botelho was the founder movement that stated the need for a better method for the detection of hypoxaemia later known as pulse oximetry. The paper clearly underlined the unreliability of the cyanosis method currently used for the detection of arterial hypoxaemia. This was done by showing that if the oxygen saturation is reduced to 75% the cyanosis could not be detected. Another paper written by Lundsgaard and Van Slyke enhanced the movement. The paper showed the factors that enhance the cyanosis such as 5mg reduced hemoglobin per 100 ml capillary blood. The paper also showed that the subject, environmental factors and the tester affects greatly the detection of cyanosis. As a result, many type of instrumentation were developed to detect the presence of hypoxaemia. However, these devices were inaccurate due to the inability to detect the difference between arterial oxygen saturation and the arterial venous and capillary blood. This separation remains a problem until the microprocessor era where the separation was finally realizable.
Pulse oximetry started as a simple monitoring technique and evolved through 15 years to become mandatory with every anaesthetic. It has the ability to detect the difference between arterial blood and venous capillary blood due to the pulsatile characteristics of the arterial blood and the smooth flowing of the capillary blood. The pulse oximetry became mandatory in anaesthetic due to the many characteristic such as:
- having a safety monitor
- showing the amount of oxygenation in the patient and the circulation of the blood
- having an non-invasive nature
- having no morbidity
- low running cost
- low capital cost
On the other hand, pulse oximetry has been imposed to some unjust criticism as in the case of any new technology. As a result, pulse oximetry has been accused of morbidity despite being a non-invasive technique; it has been accused of causing tissue damage to the tissues adjacent to the probe. As a result, the Medical Devices Agency in England issued a safety action bulletin that contained a historical background, mode of operation, calibration problems, the characteristics of clinical uses and the technique limitation.
1.2.1 Hewlett-Packard ear oximeter
Johann Heinrich Lambert was the founder of the correlation that exists between the absorbant and the amount of light absorbed in 1760. His ideas were developed later on by August Beer in 1851. However, the first real adoption of pulse oximetry was the ear oximeter founded by Hewlett-Packard. The concept used in this oximeter is based on an incandescent source combined with narrowband interference filters to transmit eight different wavelengths. Fiberoptics are used to lead the transmitted light from pinna to the detector. The calculation of the arterial oxygen saturation is based on the eight wavelengths absorption. In order to approximate the arterial saturation .this calculation is based on an approximation of overall absorption. The ear is heated causing vasodilation and the capillary blow flow to increase. That phenomenon leads to the approximation of the arterial saturation. The main problem of the device was the constant need for calibration due to the large and hard to handle probe-head. However, this technique was the only technique that allows continuous measurement of oxygen saturation; therefore this technique was the founder of pulse oximetry
1.2.2 Prototype pulse oximeter
The founder configuration of pulse oximeter or the prototype used a light source and two bundles of fibers. The light source is made of halogen incandescent lamp to transmit the broad band energy to a fingertip probe. This transmission was done using a glass fiber bundle. Another bundle of fibers were used to return the transmitted energy to the apparatus. This returning energy is divided into two paths at the apparatus: one passing through a 650nm centered filter interface having a narrow bandwidth, and the second path passing through an 805 nm centered filter centered, that point is isopiestic hemoglobin. Then, a semiconductor sensor is used to detect the appropriate energy at the wavelengths passed through each filter. Finally, an analogue calculation is used to find the appropriate value of the oxygen saturation. This is clearly shown in the figure bellow.
This primary prototype had many disadvantages such as:
- Having a heavy probe
- Having an hard to manage Fiberoptics cable
- Having an inaccurate filters letting some undesired wavelengths to pass through the tissues of the fingers
- Having a biohazard on the finger, in some cases the finger could burn
- Not fully respecting the beer-Lambert law
- Insensitivity with low pulse pressure
- Having a tendency to change in the analogue electronics part
1.2.3 Traditional pulse oximeter
The current pulse oximeter uses light - emitting diodes with a semiconductor photo detector to generate two wavelengths of 660 nm and 940 nm. Therefore this design provides a small and efficient probe to be attached to the ear or the finger and a small cable to connect the probe and the main unit. However, the pulse oximeter used with a magnetic resonance scanner has a different design. The main unit contains all the electronic components and optical fibers are used to transmit the light energy to and from the patient
1.2.4 Complete history of pulse oximetry
- Beer'Lambert law in 1851
- Discovery of oxygen carrier in blood as a form of pigment by Georg Gabriel Stokes in 1864
- Purification of the pigment and naming it hemoglobin by Felix Hoppe in 1864
- Detailed study of the reflection spectra of the hemoglobin and the finger by Karl von Veirordt in 1876
- Detailed study of the absorption spectra by Carl Gustav Hufner in 1887'90
- Measure of the oxygen saturation in fish using spectroscopy by August Krough and I Leicht in 1919
- Study of the light transmitted throughout human tissues using quantitative spectrophotometry by Ludwig Nicolai in 1931
- Measurement of the oxygen saturation of blood through laboratory tubes Kurt Kramer in 1934
- Measurement of the spectrum of concentrated hemolysed and non-hemolysed blood by David Drabkin and James Harold Austin in 1935
- Continuous monitoring of oxygenation is achieved by passing red and infrared light throughout the finger web by JR Squires in 1940. This was done by creating bloodless area of calibration by compression of tissues
- Revolutionary change in the concept of oximeter leading to the development of the Millikan oximeter by Glen Alan Millikan in 1940-42
- Creation of Wood's ear oximeter by Earl Wood in 1948'50
- Ability to differentiate between hemoglobin, carboxyhemoglobin and methemoglobin by the creation of 'CO-oximeter' in 1960
- Creation of the ear oximeter having eight wavelengths by Robert Shaw in 1964
- Marketing of the newly created ear oximeter by Hewlett-Packard in 1970
- Separation of the arteries absorption from the tissues absorption using the pulsatile nature of the absorption signal by Takuo Aoyagi in 1971
- Development of prototype pulse oximeter containing luminous light source , filters and analogue electronics by Aoyagi in 1974
- Commercialization of the pulse oximeter in 1975
Chapter II: Pulse Oximetry Characteristics
The pulse oximeter separates the variation of oxygenation absorbance of the human boundary. The pulse oximeter uses the reflection from the skin and tissues or the transmission through the human boundary to perform spectrophotometry. The most common used technique is the transmission technique, but the reflection technique is also used in intrapartum monitoring.
2.1 Transmission pulse oximetry
The human parts that must be chosen as extremity are the earlobe, toe, noise or typically the finger. The chosen part should have a short optical path length to have a translucent nature at the wavelengths used. The wavelengths used should have the range of 600 nm to 1300 nm and in the same range of the absorption spectrum due to the fact that each spices of hemoglobin have a unique absorption as shown in the figure bellow.
As a result from the formulas we can show that the minimum number of used wavelengths should be greater or equal to the number of unknowns. As a result the commonly used pulse oximetry uses two wavelengths for the two unknowns' oxygenated hemoglobin and deoxygenated hemoglobin. In addition, the wavelengths used must be monochromatic and have a low cost. In the design, a sensitive detector must be used to prevent high levels energy that causes tissue damage from passing through. Thus, there is a need to separate the saturation value for arterial hemoglobin. In order to separate the saturation, computing power is used for arterial hemoglobin saturation extraction.
In addition to that, spectrophotometry requires the use of a laser due to the requirement of a single wavelength/color source as energy source. Therefore two lasers are used each having a different wavelength in order to transmit the energy to the patient boundary using optical fibers. Due to the presence of the laser, the pulse oximetry will have a high cost, a fragile nature and requires safety implications.
However, the fiber optic cables were rejected in the later designs after the discovery of the possibility of the use of LED as an energy source. As a result, the overheating of the tissues problem was removed and the narrowband filters were removed from the design thus reducing the cost and fragility of the design. In addition, the number of photodector was reduced to a single device due to the possibility of switching the LEDs on and off quickly.
Energy sources used in pulse oximetry are monochromatic ideally with the option of using the expensive semiconductor lasers. Early pulse oximeter used similar wavelengths of 660 nm for red light and 940 nm for near infrared. Therefore, LEDs of 660nm and 940 nm were used in these designs. However, modern devices used additional wavelengths.
Doped Material Wavelength Light
Ga.28In.72As.6P.4 1250 nm Infrared
Ga 1100 nm
GaAsSi 940 nm
GaAs 900 nm
GaAIAs 880 nm
GaAIAs 810 nm Near Infrared
GaP:ZnO GaAs.6P.4 780 to 622 nm Red
GaAs.35P.65 622 to 597 nm Orange
GaAs.14P.86 597 to 577 nm Yellow
GaP:N 755 to 492 nm Green
GaAs-phosphor (ZnS, SiC) 492 to 455 nm Blue
GaN 455 to 390 nm Violet
GaN GaS2 455 to 350 nm Ultraviolet
Standard pulse oximetry have the isobestic point (805 nm) at which there are two wavelength concentrated at each side. As stated earlier, two wavelengths of 940 nm (infrared) and 660nm. The absorption spectra are flat at 940nm allowing the calibration to be immune to the variations in the peak wavelength. In addition to that, the difference between the absorption of reduced hemoglobin and the absorption of oxygenated hemoglobin at 660nm is large ,causing a flat curve and allowing the detection of changes in absorption caused by small changes in oxygen saturation .
The probe of a pulse oximeter consists of light - emitting diodes as energy source having a perpendicular output through the extremity towards a semiconductor photo-detector. The mechanical design prevent mispositioning that cause errors in calibration
2.3.1 Differential Amplifiers in the probe
Nowadays differential amplifier techniques are being used in the plethysmograph signal to enhance the common mode electrical and magnetic noise reduction.
The amplification is done between the conductor signal and the current pathway. This amplification is performed to prevent the electromagnetic interference (EMI) from affecting the probe or the lead. Due to the fact that, a small voltage signal cause the voltage generated by the EMI to be greater than the signal itself.
Two identical conductors from the detector to an amplifier are feed through the differential amplifier. The resulting output will be the absolute value of the signal from conductor 1 minus the signal from conductor 2. The advantage of using such a differential amplifier is that the induced voltage from the EMI will be two identical signals that will cancel each others.
The energy output of the photo detector must be immune to the variation in the finger's thickness, leading to a variable energy output from the LEDs. This criterion requires detectable and unsaturated energy levels that reach the semiconductor. In the other hand, the current passing through the LED must be varied to allow the variation in the intensity of the output over several orders of magnitude. This variation is necessary to prevent high level of energy from passing through the tissues, causing heat damage.
2.3.2 LED in the probe
LED used in pulse oximetry have a bandwidth between 10 and 50 nm and a 15 nm centre wavelength's variation.
On the other hand, variations in the driving current cause errors at the red LED but doesn't have any effect on the near infrared LED. These facts are related to the absorption spectra; it is flat near infrared region and steep in near the red region as shown in figure 3. This will lead to an increasing inaccuracy in pulse oximeter as the oxygen saturation decreases. This problem can be solved by two different ways:
1. Selection of LED having an acceptable range of errors in the center wavelengths.
2. Measurement and calibration of center wavelengths into actual wavelength
The calibration is usually performed by the use of a fixed resistor attached to the connector of the probe lead. This resistor will automatically set the probe's wavelength to the one of the red LED.
In pulse oximetry, a single photo-detector made of silicon photodiode is positioned perpendicularly to the LED in order to detect the energy from both LEDs. Due to the fact that semiconductors are sensitive to external energy and light, general semiconductors have their size increased. However, Semiconductor photo-detectors having their sensitivity varying with wavelength, take advantage of the limited photosensitivity to limits the choice of device and the scope of wavelengths. The silicon photodiode is characterized by the direct correlation between the output and the incident light and its wide dynamic range. On the other hand, phototransistors have more electrical noise, but more sensitivity than photodiodes.
The electrically screened flexible cable carries the LED's power and the small signal from the photo-detector. The cables also have the function of temperature detection of the probe and the skin using conductors. Finally, in order to be immune to the mechanical artifacts caused by movement, the cable must be flexible and light.
2.5.1 Electronics circuitry
Pulse oximetry makes use of different electronics circuitry for different purposes:
- Amplifies the signal coming from the photo detector
- Separates the plethysmograph signals into red signals and infrared signals.
- Switching and controlling the current of the LED.
- Setting the gain of the signals to be equivalent to the other signal
- Divide the signal into arterial signal and other signals
- Convert the infrared signals and the red signals into digital signals using AD conversion.
- Computation of the ratio red to infrared.
- Eliminates artifacts
- Compute the value of oxygen saturation
- Display of the computed values
- Managing the alarms settings
The absorption of energy from the LED to the photo-detector creates the signal in the red and the infrared channels. This absorption is the assembly of different absorptions from various sources such as arterial blood and its pulsation, venous blood and tissues.
The initial amplification stage is implemented by analog electronics, whereas calculation of spo2 stage is implemented with a microprocessor, the photo-detector signal is treated by electronics or microprocessors. The output signal from the analog part is processed by an ADC to be suitable for the digital part or the microprocessor.
2.5.2 Amplification stage
The amplification is processed in different stages:
- The low amplitude photo-detector signal is amplified.
- The LEDs are energized in an alternating sequence with a short delay in between to allow the measurement of external light.
- The amplified signal is decomposed into three signals: red, infrared, and dark signal.
- The electronic filters remove the 1 KHz high-frequency switching, making the signal continuous and having different wavelength.
- The dark signal is subtracted from the DC levels to prevent problems from the energy source.
- The DC components of the infrared signal is equalized to the DC components of the red signal by changing the amplitude of a photo-plethysmograph signal .
- The red to infrared ratio is calculated from the amplitudes of the AC components.
2.5.3 Conventional Spo2 calculation methods
Earlier pulse oximetry used one of two methods to calculate the spo2 values. The first method is solving simultaneous Beer'Lambert law equations. However, this method have many limitations such as one unknown, absence of scattering and turbidity, and the need for the path length to be constant. Due to the many limitations, this method is considered inaccurate and therefore rejected. The second and common method uses the red to infrared ration with a look up table to find the spo2 values.
The thickness and size of the finger varies from one person to another, thus the optical density will also vary from one patient to another. However, the saturation of the semiconductor does not depend on the characteristics of the patient but only on the intensity of light. In order to have the same saturation, the same amount of light is applied to the patient regardless of the size and age. This can cause serious heat damage for children. The prevention of this problem is another microprocessor's role. The microprocessor implements a correction factor that controls the LED current and synchronizes the LEDs intensities. The resulting current should be the minimum amount of light energy allowing the calculation of pulse oximetry while not damaging the tissue
2.6 Elimination of artifacts
The intact calculated saturation values include the real values with some invalid values created by artifacts. Therefore, statistical averaging methods are used in order to remove these artifacts
2.6.1 Mechanical movement artifacts
The mechanical movement artifacts are processed with the Nellcor algorithm. The Nellcor algorithm consists of the following steps:
- Divide the output signal from the differential amplification stage into pulses.
- Check the pulses for motion artifacts
- If the pulses do not contain motion artifacts, compare the identified pulse to the normal pulse.
- If the pulse contains motion artifacts, higher standards for the quality of the light motion signal are applied. The resulting pulse should be compared to the normal pulse
- If the pulse is not identical to the normal pulse, that pulse is rejected
- If the pulse is identical to the normal pulse, check if characteristics of the indentified pulse are physiologically possible
- If the characteristics of the identified pulse are not physiologically possible , that pulse is rejected
- If the characteristics of the identified pulse are physiologically possible, the pulse is compared to the average of the preceding pulses
- If the pulse is not equal to the average of the preceding pulses, that pulse is rejected
- If the pulse is equal to the average of the preceding pulses, the pulse is divided at dicrotic notch . Then the whole pulse or the main component is selected for the calculation.
- Then, a filter based on confidence assessment is implemented
- Finally, the SpO2 value is calculated
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