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Physics and Monitoring in ICU – Part 1
Written by – Dr Madhuri Anupindi
Describe the physical principles of ultrasound
Ultrasound is sound whose frequency is above the range of human hearing. Sound is a mechanical wave produced by vibration of the medium and can be represented by a sinusoidal wave. The wavelength is the distance between two consecutive points of the same phase, the amplitude is the maximal distance covered by each particle during an oscillation and the frequency is the number of cycles occurring in 1 second measured in hertz. Medical ultrasound uses high frequency sound ranging from 2 – 20MHz to aid in the diagnosis and treatment of patients.
Ultrasound waves are generated from multiple small piezoelectric crystals within the transducer. When an alternating current is applied, these crystals deform and vibrate at high speeds, converting the electrical energy into an ultrasound wave. This process also works in reverse, where the ultrasound waves are received by the crystals and converted into electrical energy. The ultrasound waves are emitted as short pulses at regular intervals. Pulse duration is the length of the pulse measured in time, a short pulse duration results in better axial resolution, or the ability to distinguish two closely spaced objects along and parallel to the ultrasound path. The pulse is comprised of a range of frequencies and 2000 – 4000 pulses are emitted per second which is the pulse repetition frequency. These pulses propagate through different tissues and then return to the transducer as reflected echoes. These echoes are then converted back into electrical impulses by the transducer crystals and are amplified and processed to form an ultrasound image presented on the screen.
As ultrasound waves transverse through the body, several factors cause it to lose energy and undergo a reduction in amplitude. This is termed attenuation and is determined by the tissue involved, the distance travelled and the beam frequency. There are four main processes which result in attenuation:
What is the Doppler effect and how does it relate to ultrasound?
The doppler effect is the observed change in frequency of a sound wave due to the relative movement between an emitting sound source and the receiver. This can be illustrated by thinking about the sound of an ambulance’s siren as it approaches and then passes. The pitch of the siren is higher as the ambulance approaches and lower as it drives by. The pitch of the siren has not actually changed, it just appears to have changed due to the relative position of the sound source and the receiver. This is because waves emitted by an object travelling towards an observer get compressed resulting in a higher frequency whereas when the source is moving away the waves are stretched out resulting in a lower frequency. The Doppler effect will not occur if the source and observer both move in the same direction at the same speed or if the source is at 90 degrees to the receiver.
The Doppler effect is used in ultrasound to detect and measure blood flow. When a transducer is placed over a blood vessel, the reflected wave will have a higher frequency compared to the initial wave if the flow of blood is towards the transducer. This is a positive Doppler frequency shift. Conversely, if the flow of blood is moving away from the transducer, a lower frequency will be reflected, that is a negative Doppler shift. When flow is perpendicular, no Doppler shift is detected. The Doppler shift frequency is proportional to the velocity of the blood flow and the cosine of the angle between the transducer and the blood flow. The optimal Doppler shift is when the transducer is parallel to the blood flow.
Outline the principles underlying pulse oximetry
Pulse oximetry uses the principles of light absorbance as described by the Beer-Lambert Law to calculate the oxygen saturation of haemoglobin. The Beer-Lambert law states that the measured absorbance for a single compound is directly proportional to the concentration of the compound and the length of the light path through the sample. Therefore, the amount of light absorbed is proportional to the amount of haemoglobin present and the width of the artery that the light is travelling through. The second principle used in pulse oximetry is that oxyhaemoglobin and deoxyhaemoglobin absorb light of different wavelengths in different amounts. Deoxyhaemoglobin absorbs more of red light which has a wavelength of about 660nm whereas oxyhaemoglobin absorbs more infrared light which has a wavelength of around 940nm. Therefore, the amount of red and infrared light absorbed by the blood will depend on the proportion of deoxy and oxyhaemoglobin present.
The third principle utilised is that pulse oximeters measures pulsatile blood. In the finger, light can also be absorbed by other tissue such as skin which would otherwise confound the measurement of oxygen saturation. Pulse oximeters compare the pulsatile signals, that is those that have changing absorbance levels over time with signals that are non-pulsatile such as skin and venous blood. A ratio or R is then created of the pulsatile and non pulsatile elements at the two different wavelengths measured. This R value is then calibrated by using a graph of empirical values derived measuring oxygen saturations of healthy volunteers who were asked to breathe lower and lower concentrations of oxygen. Measurements were made of the pao2 and oxygen saturation and a calibration graph created. It is important to note that the volunteers were not allowed to desaturate below 75% so the remaining calibration was calculated empirically.
Pulse oximeters consist of an opaque probe which minimises ambient light and contains light emitting diodes that emits rapid sequences of red light at 660nm and infrared at 940nm. The pulse oximeter also contains a photodetector on the other side of the probe which records the amount of light received and can compare it to the amount of light received when the red light was on, the infrared light was on and when neither was on. In this way it can calculate the amount of ambient light (the light detected when neither of the LEDs were on) and thus negate its effect. The pulse oximeter also contains a signal amplifier and an integrated circuit controller and processor which can calibrate and analyse the data. This is then displaced on a user interface, ideally as a continuous trace.
List the limitations of pulse oximetry
Limitations of pulse oximetry include difficulties with signal measurement, interference with absorbance, errors in calibration and limitations of technique.
Errors and difficulty in signal measurement
Interference with absorbance
Errors in calibration
Limitations of technique
What is the effect of an elevated level of carboxyhaemoglobin and methaemoglobin on pulse oximetry values.
Carboxyhaemoglobin has similar absorbances of the red light (660nm) to oxygenated haemoglobin. This can result in the pulse oximeter seeing oxyhaemoglobin and carboxyhaemoglobin as the same when using the red light spectrum and therefore measure a combined absorbance. Carboxyhaemoglobin minimally absorbs infrared (940nm) light so this will not change the absorption. The red light absorption of the combined O2Hb and COHb will be maintained or slightly increased. However, as deoxygenated Hb still absorbs red light better than either COHb or O2Hb, the reduction in deoxygenated Hb in the presence of COHb results in a net effect of decreased red light absorption. Therefore when R, or the ratio of absorbance of the pulsatile and non-pulsatile elements at both wavelengths is calculated, it will be lower than in reality and a falsely elevated Sao2 is generated.
Methaemoglobin absorbs more light than either deoxy or oxyhaemoglobin at 940nm whereas at 660nm its absorption is similar to deoxyhaemoglobin. The pulse oximeter may therefore perceive an elevated concentration of deoxyhaemoglobin which can falsely lower the oxygen saturation. Furthermore, as methaemoglobin absorbs both red and infrared light well it results in an R value that approaches 1, which is equivalent to saturations of around 85%. Therefore, the higher the methaemoglobin concentration, the greater the tendency of the pulse oximeter to display a spo2 of 85% regardless of the actual oxygen saturation. If the actual Sao2 is less than 85% the Spo2 value will overestimate it, if it is more than 85% the Spo2 will underestimate it.
How is ETCO2 measured using infrared spectroscopy and what are the potential sources of error?
Infrared spectroscopy also uses the Beer-Lambert law to calculate the concentration of carbon dioxide present in expired air. Infra-red light with a wavelength of 4.26um is passed through expired air and the amount absorbed is measured. The amount of infra-red light absorbed is proportional to the concentration of carbon dioxide. This wavelength was chosen as it fits well with the peak absorption of Co2 and is well separated from other absorption peaks of other gases. A light-emitting diode emits a narrow band of infrared light which passes through a sample chamber of fixed size which contains expired Co2. A detector is present on the opposite side which measures the absorption of light at this spectrum. A processor then calibrates this information and ideally produces a graph of CO2 concentration over time or a capnograph trace.
Capnograph analysers can be either main stream or side stream. Mainstream analysers place the detector directly in the path of the expired gas whereas sidestream monitors remove a small fraction of the expired gas via additional tubing and the analyser is located away from the patient.
Sources of error include errors of sampling, calibration, interference and pathological.
What are the causes of an increased difference in PaCO2 and ETCO2 levels?
The normal PaCO2 – ETCO2 difference is around 2 – 5 mmHg and is due to alveolar dead space. Alveoli which are ventilated but not perfused will therefore have an expired gas mixture which is identical to inspired gas. This would then dilute the expired CO2 concentration (as inspired gas does not contain CO2) and increase the difference between the PaCO2 and ETOC2. Therefore, anything that increases alveolar dead space will increase the PaCO2 – ETCO2 difference. Causes of an increased difference include: