ECG and Pulse Oximetry: History and Types

ECG and Pulse Oximetry History and TypesIn this chapter, we ordain p showtime the history of ECG and trice oximetry, the timeline and variations finished time of the notions utilise. We all as well discuss the types of pulsate oximetry and the electronics used with their requirements.1.1 History of ECGThe 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 measurable 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 determine 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 denudation of P, Q, R, S and T waves by Willem Einthoven in 1895. Ein thoven also invented a flip-flop galvanometer and used in for electrocardiogram recording. As a reward for his work, he won a Noble hurt 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 amid 1843 and 1942 as shown in the following tableTable 1 ECG TimelineYear Scientist Concept1842 Carlo Matteucci heart beat is accompanied by electric contemporary1843 Emil Dubois-Reymond herculean contraction is accompanied by action potential.Test carlos concept on animals successfully1856 Koelliker , Muller Record of the action potential concept1869 Alexander Muirhead might set out recorded a human electrocardiogram1872 Gabriel Lippmann Capillary Electrometer invented1876 M ary EJ Electrical activity of animal recorded by the electrometer1878 John Sanderson , Frederick Page Electrical current of the heart is recordedDivide into both phases (later known as QRS and T)1887 August us Waller First human electrocardiogram is publish1890 GJ Burch Arithmetic correction of the electrometer1891 William Bayliss , Edward Starling Capillary electrometer improved baring of deflections (later known as P,QRS,T) and delay (later know as PR interval)1893 Willem Einthoven The term electrocardiogram introduced1895 Deflections P,Q,R,S and T distinguished1897 Clement Ader Galvanometer invented( Amplification system for the lines of telegraph )1901 Willem Einthoven Galvanometer modified for ECG use1902 ECG records using galvanometer published1903 Commercial production of galvanometer discussed1905 Telecardigram invented (transmission of ECG preindicationize by telephone)1906 Normal and abnormal ECG record publishedIntroduction of the U wave1908 Edward Schafer First purchase of Einthovens galvanometer1910 Walter James, Horatio Williams Electrocardiography reviewed for the first time in America1911 Thomas Lewis Publication of a book most heart beat mechanism1912 Willem Eintho ven Description of the Einthoven triangle (formed for the behaves)1920 Hubert Mann Derivation of mono-cardiogram (later known as vector-cardiogram)1924 Willem Einthoven Nobel price won for the electrocardiograph invention1928 Ernstine, Levine Introduction of vacuum-tubes for ECG elaborationFrank Sanborn First portable ECG invented1932 Charles Wolferth and Francis Wood Description of the chest leads use in the coronary occlusion1938 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 modelFinal ECG model used today1.2 History of impetus oximetryThe extremist report card by Comroe and Botelho was the founder movement that stated the need for a better method for the detection of hypoxaemia later known as impetus 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 color 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 posture 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 worry 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 authorization 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 move of the capillary blood. The beatnik oximetry became mandatory in anaesthetic due to the many characteristic such ashaving a condom monitorshowing the gist of oxygenation in the patient and the circulation of the bloodhaving an non-invasive naturehaving no morbiditylow running addresslow capital costOn the other hand, pulse rate 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 patronage being a non-invasive technique it has been accused of causing tissue molest 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 oximeterJohann Heinrich Lambert was the founder of the co rrelation that exists between the absorbant and the amount of silly 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 spring combined with narrowband interference filters to transmit eight distinct wavelengths. Fiberoptics are used to lead the transmitted descend from pinna to the detector. The unhurriedness 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 oximetry1.2.2 Prototype pulse oximeterThe 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 pushing to a fingertip probe. This transmission was done using a trash 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 discharge through a 650nm centered filter interface having a narrow bandwidth, and the second path passing through an 805 nm centered filter centered, that show is isopiestic hemoglobin. because, a semiconductor sensor is used to detect the charm 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 kind bellow.This primary prototype had many dis services such asHaving a heavy probeHaving an hard to annihilate Fiberoptics cableHaving an inaccurate filters letting some undesired wavelengths to pass through the tissues of the fingersHaving a biohazard on the finger, in some cases the finger could pruneNot fully respecting the beer-Lambert lawInsensitivity with low pulse pressureHaving a tendency to change in the analogue electronics part1.2.3 Traditional pulse oximeterThe 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 high-octane 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 magnetised resonance scanner has a unalike design. The main unit contains all the electronic components and optical fibers are used to transmit th e light energy to and from the patient1.2.4 Complete history of pulse oximetryBeerLambert law in 1851Discovery of oxygen carrier in blood as a form of blusher by Georg Gabriel Stokes in 1864Purification of the pigment and naming it hemoglobin by Felix Hoppe in 1864Detailed study of the reflection spectra of the hemoglobin and the finger by Karl von Veirordt in 1876Detailed study of the absorption spectra by Carl Gustav Hufner in 188790Measure of the oxygen saturation in fish using spectroscopy by August Krough and I Leicht in 1919Study of the light transmitted throughout human tissues using quantitative spectrophotometry by Ludwig Nicolai in 1931Measurement of the oxygen saturation of blood through laboratory tubes Kurt Kramer in 1934Measurement of the spectrum of concentrated hemolysed and non-hemolysed blood by David Drabkin and James Harold Austin in 1935Continuous monitoring of oxygenation is achieved by passing red and unseeable emission light throughout the finger web by JR Squires in 1940. This was done by creating bloodless area of calibration by compression of tissuesRevolutionary change in the concept of oximeter leading to the development of the Millikan oximeter by Glen Alan Millikan in 1940-42Creation of Woods ear oximeter by Earl Wood in 194850Ability to differentiate between hemoglobin, carboxyhemoglobin and methemoglobin by the creation of CO-oximeter in 1960Creation of the ear oximeter having eight wavelengths by Robert Shaw in 1964Marketing of the newly created ear oximeter by Hewlett-Packard in 1970Separation of the arteries absorption from the tissues absorption using the pulsatile nature of the absorption signal by Takuo Aoyagi in 1971Development of prototype pulse oximeter containing luminous light source , filters and analogue electronics by Aoyagi in 1974Commercialization of the pulse oximeter in 1975Chapter II Pulse Oximetry CharacteristicsThe pulse oximeter separates the variation of oxygenation absorbance of the human boundary. The pulse oximeter uses the reflection from the whittle 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 transmittance pulse oximetryThe 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 deoxygenate d hemoglobin. In addition, the wavelengths used must be monochromatic and have a low cost. In the design, a tender 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. ascribable 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.2.2 LEDsEnergy 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 uprise infrared. Therefore, LEDs of 660nm and 940 nm were used in these designs. However, modern devices used additional wavelengths.Doped Material Wavelength LightGa.28In.72As.6P.4 1250 nm InfraredGa 1100 nmGaAsSi 940 nmGaAs 900 nmGaAIAs 880 nmGaAIAs 810 nm Near InfraredGaPZnO GaAs.6P.4 780 to 622 nm RedGaAs.35P.65 622 to 597 nm OrangeGaAs.14P.86 597 to 577 nm YellowGaPN 755 to 492 nm GreenGaAs-phosphor (ZnS, SiC) 492 to 455 nm BlueGaN 455 to 390 nm VioletGaN GaS2 455 to 350 nm UltravioletStand ard 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 matte at 940nm allowing the calibration to be tolerant 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 .2.3 ProbeThe probe of a pulse oximeter consists of light emitting diodes as energy source having a perpendicular product through the extremity towards a semiconductor photo-detector. The mechanical design prevent mispositioning that cause errors in calibration2.3.1 Differential Amplifiers in the probeNowadays differential amplifier techniques are being used in the plethysmograph signal to enhance the common mode electric al 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 fingers 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 curren t passing through the LED must be varied to allow the variation in the vividness 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 probeLED used in pulse oximetry have a bandwidth between 10 and 50 nm and a 15 nm condense wavelengths variation.On the other hand, variations in the driving current cause errors at the red LED but doesnt 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 ways1. Selection of LED having an acceptable range of errors in the center wavelengths.2. Measurement and calibration of center wavelengths into actual wavelengthThe 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 probes wavelength to the one of the red LED.2.4 Photo-detectorIn 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 ambit 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 LEDs power and the small signal from the photo-detect or. 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 Electronics2.5.1 Electronics circuitryPulse oximetry makes use of different electronics circuitry for different purposesAmplifies the signal coming from the photo detectorSeparates 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 signalDivide the signal into arterial signal and other signalsConvert the infrared signals and the red signals into digital signals using AD conversion.Computation of the ratio red to infrared.Eliminates artifacts imagine the value of oxygen saturationDisplay of the computed valuesManaging the alarms settingsThe absorption of energy from the LED to the photo-detector creates the signal in the red and the infra red 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 symbolise 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 stageThe amplification is processed in different stagesThe low amplitude photo-detector signal is amplified.The LEDs are energized in an alternating range 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 kilocycle high-frequency switching, making the signal continuous and having different wavelength.The dark signal is subtracted f rom 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 methodsEarlier pulse oximetry used one of two methods to calculate the spo2 values. The first method is solving simultaneous BeerLambert 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 microprocessors 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 tissue2.6 Elimination of artifactsThe intact calculated saturation values include the real values with some handicap values created by artifacts. Therefore, statistical averaging methods are used in order to remove these artifacts2.6.1 Mechanical movement artifactsThe mechanical movement artifacts are processed with the Nellcor algorithm. The Nellcor algorithm consists of the following stepsDivide the output signal from the differential amplification stage into pulses.Check the pulses for motion artifactsIf the pulses do not contain motion artifacts, equivalence 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 pulseIf the pulse is not identical to the normal pulse, that pulse is rejectedIf the pulse is identical to the normal pulse, check if characteristics of the indentified pulse are physiologically possibleIf the characteristics of the identified pulse are not physiologically possible , that pulse is rejectedIf the characteristics of the identified pulse are physiologically possible, the pulse is compared to the average of the preceding pulsesIf the pulse is not equal to the average of the preceding pulses, that pulse is rejectedIf the pulse is equal to the average of the preceding pulses, the pulse is divided at di crotic notch . Then the whole pulse or the main component is selected for the calculation.Then, a filter based on confidence assessment is implementedFinally, the SpO2 value is calculated