Tuesday, October 23, 2007
EEG Recording
The EEG recording can last from anything between 15 minutes to 1 hour or longer depending on the situation. Typically the patient will be lying down or sitting relaxed in a chair. Most of the recording is taken with the eyes closed, although the patient will be frequently asked to open them for short periods.
Most patients will be asked to carry out a period of deep breathing for approximately 3 minutes. This may produce some abnormal activity which would not be seen while the patient is relaxed. The physiological effect of deep breathing is to increase the amount of carbon dioxide (CO2) being removed from the bloodstream. This fall in CO2 produces a fall in blood pressure and at the same time blood vessels in the brain become constricted. This reduces blood flow and the delivery of oxygen and glucose to the brain. This in turn may produce some abnormal brain activity not seen in the resting record.
Photic stimulation is also carried out. A strobe lamp is placed 30 cm from the patient’s eyes. Brief flashes of light (2 - 5 seconds in duration) at a number of different flash frequencies are delivered to the patient with both eyes open and eyes closed. A continuous flash with increasing and decreasing flash frequencies is sometimes used.
Some patients who are sensitive to flashing lights may show abnormal activity in the EEG.
Throughout the test the recordist is constantly annotating the record with any patient movements, or tasks that they are carrying out.
Other signals may also be recorded in conjunction with the EEG such as heart rate (ECG), respiration, eye movements (EOG), and muscle activity (EMG).
EEG Analysis
The EEG reports consists of a number of different sections. The recordist may prepare a report describing the type of activity seen in the record together with changes produced by deep breathing and photic stimulation. They will also comment on the patient’s state during the recording. The physician will then interpret these changes with regard to the medical problem being investigated.
With an increase in the number of long recordings being carried out, many departments make use of detection algorithms such as spike and seizure detection. Although it is still necessary for the clinician to review the complete record, such programmes will mark and highlight sections of interest. The most efficient method of implementing these algorithms is for the detection to be carried out on-line.
Other methods of analysing EEG data include Power Spectrum Analysis. A Fast Fourier Transform (FFT) is performed on sections of EEG data to determine the power content of the four main frequency bands. The resulting waveforms can be displayed as a brain map which will show the scalp distribution of the power within each frequency band.
The amplitude of the different waveforms at a single point can also be displayed in a similar format.
This type of display provides a more objective analysis of the EEG activity compared to a subjective visual analysis by a physician.
Video monitoring
Simultaneous video monitoring of the patient during the EEG recording is becoming more popular. It allows the physician to closely correlate EEG waveforms with the patients activity and may help produce a more accurate diagnosis.
Domestic video recorders and cameras can be connected to an EEG machine using a time code generator. This records an accurate time signal onto the videotape. When the videotape and EEG are reviewed together the two signals are accurately synchronised together.
Video monitoring is always used for Long Term Monitoring recordings as the patient is unattended. The patient may also have an event button connected to the EEG machine so that times when the patient thought they were having an epileptic attack can be easily identified.
Sleep studies
The EEG is frequently used in the investigation of sleep disorders especially sleep apnoea. EEG activity together with other physiological signals such as heart rate, airflow, respiration, oxygen saturation and limb movement are measured simultaneously. These recordings are usually carried out overnight although some sleep studies can be carried out in the department during the day under strictly controlled conditions.
The EEG record can be broken down into epochs which are normally of 30 seconds duration. Using the EEG activity, each epoch is classified into one of 5 sleep stages. This is displayed visually as a Sleep Histogram.
Respiration and airflow are used to look for periods of apnoea which occur when the patient stops breathing. These are then correlated with the sleep stage in which they occurred and the level the oxygen saturation fell to during the apnoea.
The EEG recording can last from anything between 15 minutes to 1 hour or longer depending on the situation. Typically the patient will be lying down or sitting relaxed in a chair. Most of the recording is taken with the eyes closed, although the patient will be frequently asked to open them for short periods.
Most patients will be asked to carry out a period of deep breathing for approximately 3 minutes. This may produce some abnormal activity which would not be seen while the patient is relaxed. The physiological effect of deep breathing is to increase the amount of carbon dioxide (CO2) being removed from the bloodstream. This fall in CO2 produces a fall in blood pressure and at the same time blood vessels in the brain become constricted. This reduces blood flow and the delivery of oxygen and glucose to the brain. This in turn may produce some abnormal brain activity not seen in the resting record.
Photic stimulation is also carried out. A strobe lamp is placed 30 cm from the patient’s eyes. Brief flashes of light (2 - 5 seconds in duration) at a number of different flash frequencies are delivered to the patient with both eyes open and eyes closed. A continuous flash with increasing and decreasing flash frequencies is sometimes used.
Some patients who are sensitive to flashing lights may show abnormal activity in the EEG.
Throughout the test the recordist is constantly annotating the record with any patient movements, or tasks that they are carrying out.
Other signals may also be recorded in conjunction with the EEG such as heart rate (ECG), respiration, eye movements (EOG), and muscle activity (EMG).
EEG Analysis
The EEG reports consists of a number of different sections. The recordist may prepare a report describing the type of activity seen in the record together with changes produced by deep breathing and photic stimulation. They will also comment on the patient’s state during the recording. The physician will then interpret these changes with regard to the medical problem being investigated.
With an increase in the number of long recordings being carried out, many departments make use of detection algorithms such as spike and seizure detection. Although it is still necessary for the clinician to review the complete record, such programmes will mark and highlight sections of interest. The most efficient method of implementing these algorithms is for the detection to be carried out on-line.
Other methods of analysing EEG data include Power Spectrum Analysis. A Fast Fourier Transform (FFT) is performed on sections of EEG data to determine the power content of the four main frequency bands. The resulting waveforms can be displayed as a brain map which will show the scalp distribution of the power within each frequency band.
The amplitude of the different waveforms at a single point can also be displayed in a similar format.
This type of display provides a more objective analysis of the EEG activity compared to a subjective visual analysis by a physician.
Video monitoring
Simultaneous video monitoring of the patient during the EEG recording is becoming more popular. It allows the physician to closely correlate EEG waveforms with the patients activity and may help produce a more accurate diagnosis.
Domestic video recorders and cameras can be connected to an EEG machine using a time code generator. This records an accurate time signal onto the videotape. When the videotape and EEG are reviewed together the two signals are accurately synchronised together.
Video monitoring is always used for Long Term Monitoring recordings as the patient is unattended. The patient may also have an event button connected to the EEG machine so that times when the patient thought they were having an epileptic attack can be easily identified.
Sleep studies
The EEG is frequently used in the investigation of sleep disorders especially sleep apnoea. EEG activity together with other physiological signals such as heart rate, airflow, respiration, oxygen saturation and limb movement are measured simultaneously. These recordings are usually carried out overnight although some sleep studies can be carried out in the department during the day under strictly controlled conditions.
The EEG record can be broken down into epochs which are normally of 30 seconds duration. Using the EEG activity, each epoch is classified into one of 5 sleep stages. This is displayed visually as a Sleep Histogram.
Respiration and airflow are used to look for periods of apnoea which occur when the patient stops breathing. These are then correlated with the sleep stage in which they occurred and the level the oxygen saturation fell to during the apnoea.
EEG Applications
One of the major roles of EEG is as an aid to diagnose epilepsy. Abnormal patterns such as spikes, sharp waves and/or spike and wave complexes can be seen. The type of activity and the area of the brain that it is recorded from will assist the physician in prescribing the correct medication for that type of epilepsy.
Patients with epilepsy that can not be controlled by medication will often have surgery in order to remove the damaged tissue. The EEG plays an important role in localising this tissue. Special electrodes can be inserted through the cortex or alternatively a grid of electrodes placed directly on the surface of the cortex. These recordings, often called Long Term Monitoring for Epilepsy (LTME), can be carried out for periods ranging from 24 hours to 1 week. The EEG recorded will indicate which areas of the brain should be surgically removed.
EEG studies can also be used in patients who are deeply unconscious, to distinguish between brain death and possible reversible conditions.
Electrocerebral inactivity (ECI) or electrocerebral silence (ECS) is defined as no EEG activity over 2 µ V in amplitude when recording from electrodes on the scalp, that are 10 cm or more apart.
Using the 10/20 International System of electrode placement, the average distance between electrodes in an adult is 6 to 6.5 cm. Activity recorded using these distances and at a normal display sensitivity may suggest ECS. However if the same activity was recorded using longer inter-electrode distances, some activity might be seen. Therefore some double distance electrode linkages are recommended for example FP1-C3, F3-P3, C3-O1 etc.
Display sensitivities of a minimum of 2 µ V/mm are required. However digital EEG systems have the added advantage of having sensitivity values of 1.5 and 1 µ V/mm. This 50-100 % increase in sensitivity will allow a more confident assessment of the presence or absence of a 2 µ V signal.
The EEG is also used to investigate other conditions that may affect brain function such as strokes, brain injuries, liver and kidney disease and dementia.
Patients with epilepsy that can not be controlled by medication will often have surgery in order to remove the damaged tissue. The EEG plays an important role in localising this tissue. Special electrodes can be inserted through the cortex or alternatively a grid of electrodes placed directly on the surface of the cortex. These recordings, often called Long Term Monitoring for Epilepsy (LTME), can be carried out for periods ranging from 24 hours to 1 week. The EEG recorded will indicate which areas of the brain should be surgically removed.
EEG studies can also be used in patients who are deeply unconscious, to distinguish between brain death and possible reversible conditions.
Electrocerebral inactivity (ECI) or electrocerebral silence (ECS) is defined as no EEG activity over 2 µ V in amplitude when recording from electrodes on the scalp, that are 10 cm or more apart.
Using the 10/20 International System of electrode placement, the average distance between electrodes in an adult is 6 to 6.5 cm. Activity recorded using these distances and at a normal display sensitivity may suggest ECS. However if the same activity was recorded using longer inter-electrode distances, some activity might be seen. Therefore some double distance electrode linkages are recommended for example FP1-C3, F3-P3, C3-O1 etc.
Display sensitivities of a minimum of 2 µ V/mm are required. However digital EEG systems have the added advantage of having sensitivity values of 1.5 and 1 µ V/mm. This 50-100 % increase in sensitivity will allow a more confident assessment of the presence or absence of a 2 µ V signal.
The EEG is also used to investigate other conditions that may affect brain function such as strokes, brain injuries, liver and kidney disease and dementia.
EEG Activity
EEG activity can be broken down into 4 distinct frequency bands:
Beta activity > 13 Hz
Alpha activity 8 Hz-13 Hz
Theta activity 4 Hz-7 Hz
Delta activity <>
Alpha activity is also a normal activity when present in waking adults. It is mainly seen in the channels recorded from the back of the head. It is fairly symmetrical and has an amplitude of
40 µV to 100 µV. It is only seen when the eyes are closed and should disappear or reduce in amplitude when the eyes are open
40 µV to 100 µV. It is only seen when the eyes are closed and should disappear or reduce in amplitude when the eyes are open
Theta activity can be classed as both a normal and abnormal activity depending on the age and state of the patient. In adults it is normal if the patient is drowsy. However it can also indicate brain dysfunction if it is seen in a patient who is alert and awake. In younger patients, theta activity may be the main activity seen in channels recorded from the back and central areas of the head.
Delta activity is only normal in an adult patient if they are in a moderate to deep sleep. If it is seen at any other time it would indicate brain dysfunction.
Abnormal activity may be seen in all or some channels depending on the underlying brain problem.
Abnormal activity may be seen in all or some channels depending on the underlying brain problem.
There are a number of other waveforms which tend to be a little more specific to certain conditions. For example spike and wave activity indicates a seizure disorder and may be seen in the EEG even if the patient is not having an epileptic seizure. Other epileptic conditions may be diagnosed if spikes or sharp waves are seen.
Triphasic waves are sometimes seen if the patient has severe liver or kidney disease that is affecting brain function.
These are just brief descriptions of some of the simpler waveforms that may be seen in any one EEG recording. Combinations of any of the above patterns are possible which can make interpretation of the record difficult. Abnormal activity is not always specific to any condition and may suggest a few different diagnoses.
These are just brief descriptions of some of the simpler waveforms that may be seen in any one EEG recording. Combinations of any of the above patterns are possible which can make interpretation of the record difficult. Abnormal activity is not always specific to any condition and may suggest a few different diagnoses.
Digital EEG instruments
A digital EEG system converts the waveform into a series of numerical values. This process is known as Analogue-to-Digital conversion (ADC).
A digital EEG system converts the waveform into a series of numerical values. This process is known as Analogue-to-Digital conversion (ADC).
The values can be stored in the computer memory, manipulated and then redisplayed as waveforms on a computer screen. The rate at which the waveform data is sampled in order to convert it into a numerical format is known as the sampling rate.
The sampling rate is usually expressed in Hz, for example 240 Hz is 240 times per second. The minimum acceptable sampling rate is 2.5 times greater than the highest frequency of interest but most digital EEG systems will sample at 240 Hz.
The sampling rate is usually expressed in Hz, for example 240 Hz is 240 times per second. The minimum acceptable sampling rate is 2.5 times greater than the highest frequency of interest but most digital EEG systems will sample at 240 Hz.
Some recordings which involve recording activity directly from the brain surface, may have activity of a higher frequency, for example 200 Hz. Therefore some digital EEG systems will have optional sampling rates of 480 Hz available.
Sampling at rates lower than this will mean that when the signal is converted back to analogue form, it will not resemble the original waveform A second factor that affects the accuracy of the waveform is sampling skew. Sampling skew occurs when all channels are not sampled simultaneously. Many digital EEG systems sample channel 1 first, then sample channel 2, then channel 3 etc. The time lag between sampling of each channel is known as sampling skew. To reduce the sampling skew, some digital systems use burst mode sampling. This increases the speed between successive channels sampling in order to reduce the amount of sampling skew.
A third factor that affects the accuracy of digital EEG waveforms is the display. The accuracy of a monitor display depends on the number of points or pixels that are available. The number of pixels available is referred to as the screen resolution. Screen resolution is described in numbers that represent the pixels available in the horizontal and vertical axis.
A VGA display has a resolution of 640 x 480 pixels while a monitor with a Super VGA display will have a screen resolution of around 1024 x 768 pixels. A typical page of EEG contains 10 seconds of data. A digital EEG system, sampling at rates of 240 Hz will need to display 2400 samples horizontally for each recording channel. The highest screen resolutions available today do not have enough pixels to match the number of data samples. Systems that draw every other sample or every third sample in order to match the screen resolution will have the effect of reducing the sampling rate and displaying incomplete data. An accurate digital system will draw two data samples per screen pixel. This means that all data points can be displayed and sampling rates will not be decreased.
EEG signals that have been digitised can be manipulated to change the montage ‘on-line’ at the time of recording or ‘off-line’ after the recording is completed. This ‘remontaging’ is accomplished by recording all EEG channels with a common reference electrode. Regardless of the montage used to display the data while it is being recorded, data is stored into the computer memory in common reference mode. This allows the data to be displayed using different montages at a later time. Since digital systems store the analogue signal as numerical values, remontaging is a simple subtraction process which results in cancellation of the common reference.
An example is shown in the next figure. The reference electrode A1 is common to both channels on input 2. It has the identical value in each channel. Remontaging these two channels together into one new channel is by subtraction which mathematically will cancel the value at the reference electrode. The resulting channel will therefore display the potential difference between F3 (input) 1 and F4 (input 2).
Sampling at rates lower than this will mean that when the signal is converted back to analogue form, it will not resemble the original waveform A second factor that affects the accuracy of the waveform is sampling skew. Sampling skew occurs when all channels are not sampled simultaneously. Many digital EEG systems sample channel 1 first, then sample channel 2, then channel 3 etc. The time lag between sampling of each channel is known as sampling skew. To reduce the sampling skew, some digital systems use burst mode sampling. This increases the speed between successive channels sampling in order to reduce the amount of sampling skew.
A third factor that affects the accuracy of digital EEG waveforms is the display. The accuracy of a monitor display depends on the number of points or pixels that are available. The number of pixels available is referred to as the screen resolution. Screen resolution is described in numbers that represent the pixels available in the horizontal and vertical axis.
A VGA display has a resolution of 640 x 480 pixels while a monitor with a Super VGA display will have a screen resolution of around 1024 x 768 pixels. A typical page of EEG contains 10 seconds of data. A digital EEG system, sampling at rates of 240 Hz will need to display 2400 samples horizontally for each recording channel. The highest screen resolutions available today do not have enough pixels to match the number of data samples. Systems that draw every other sample or every third sample in order to match the screen resolution will have the effect of reducing the sampling rate and displaying incomplete data. An accurate digital system will draw two data samples per screen pixel. This means that all data points can be displayed and sampling rates will not be decreased.
EEG signals that have been digitised can be manipulated to change the montage ‘on-line’ at the time of recording or ‘off-line’ after the recording is completed. This ‘remontaging’ is accomplished by recording all EEG channels with a common reference electrode. Regardless of the montage used to display the data while it is being recorded, data is stored into the computer memory in common reference mode. This allows the data to be displayed using different montages at a later time. Since digital systems store the analogue signal as numerical values, remontaging is a simple subtraction process which results in cancellation of the common reference.
An example is shown in the next figure. The reference electrode A1 is common to both channels on input 2. It has the identical value in each channel. Remontaging these two channels together into one new channel is by subtraction which mathematically will cancel the value at the reference electrode. The resulting channel will therefore display the potential difference between F3 (input) 1 and F4 (input 2).
Tuesday, October 09, 2007
Analogue EEG instruments
Conventional analogue instruments consist of an amplifier, a galvanometer and a writing device. A galvanometer is a coil of wire inside a magnetic field. The output signal from the amplifier passes through the wire causing the coil to oscillate. A pen mounted on the galvanometer moves up and down each time the coil moves. The pen draws the trace onto paper moving below it.
The amplifier output is controlled by high and low frequency filters and sensitivity controls. The high and low frequency filter values will set the window within which the EEG activity is recorded. This is known as the bandwidth. The sensitivity controls the size of the activity displayed. For example a sensitivity of 10 µV/mm means that a signal with an amplitude of 100 µ V will produce a 1 cm vertical deflection.
The speed at which the paper moves on will also affect the appearance of the waveforms.
Thursday, October 04, 2007
EEG Montages
EEG machines use a differential amplifier to produce each channel or trace of activity. Each amplifier has two inputs. An electrode is connected to each of the inputs.
Differential amplifiers measure the voltage difference between the two signals at each of its inputs. The resulting signal is amplified and then displayed as a channel of EEG activity.The manner in which pairs of electrodes are connected to each amplifier of the EEG machine is called a montage. Each montage will use one of three standard recording derivations, common reference, average reference or bipolar.
Common reference derivation: Each amplifier records the difference between a scalp electrode and a reference electrode. The same reference electrode is used for all channels. Electrodes frequently used as the reference electrode are A1, A2, the ear electrodes, or A1 and A2 linked together.
Average reference derivation: Activity from all the electrodes are measured, summed together and averaged before being passed through a high value resistor. The resulting signal is then used as a reference electrode and connected to input 2 of each amplifier and is essentially inactive. All EEG systems will allow the user to choose which electrodes are to be included in this calculation.Bipolar derivation: These sequentially link electrodes together usually in straight lines from the front to the back of the head or transversely across the head. For example the first amplifier may have electrodes FP1 and F3 connected to it and the second amplifier F3 and C3 connected to it.
Electroencephalograph
Introduction
The electroencephalogram (EEG) is a recording of the electrical activity of the brain from the scalp. The first recordings were made by Hans Berger in 1929 although similar studies had been carried out in animals as early as 1870.
The waveforms recorded are thought to reflect the activity of the surface of the brain, the cortex. This activity is influenced by the electrical activity from the brain structures underneath the cortex.
The nerve cells in the brain produce signals that are called action potentials. These action potentials move from one cell to another across a gap called the synapse. Special chemicals called neurotransmitters help the signals to move across the gap. There are two types of neurotransmitters, one will help the action potential to move to the next cell, the other will stop it moving to another nerve cell.
The brain normally works hard to keep an equal amount of each of these neurotransmitters in the brain.
EEG activity is quite small, measured in microvolts (µV) with the main frequencies of interest up to approximately 30 Hertz (Hz).
Electrodes
Small metal discs called electrodes are placed on the scalp in special positions. These positions are identified by the recordist who measures the head using the International 10/20 System. This relies on taking measurements between certain fixed points on the head. The electrodes are then placed at points that are 10% and 20% of these distances.
Each electrode site is labelled with a letter and a number. The letter refers to the area of brain underlying the electrode e.g. F - Frontal lobe and T - Temporal lobe. Even numbers denote the right side of the head and odd numbers the left side of the head.
There is a great variety of electrodes that can be used. The majority are small discs of stainless steel, tin, gold or silver covered with a silver chloride coating. These normally have a lead attached. Alternative methods consist of a cap in which the electrodes are already imbedded.
pneumotachometer
A transducer for measuring expired air flow.
This measures the flow rate of gases during breathing. The breath is passed through a short tube (Fleisch tube) in which there is a fine mesh which presents a small resistance to the flow. The resulting pressure drop across the mesh is in proportion to the flow rate. The pressure drop is very small (e.g. 2 mmHg) and so the measuring circuit must be of high quality and produce very little drift with time. A differential pressure transducer is normally used.
The advantage which this device has over the mechanical spirometer is that the patient under investigation can continue to breathe fresh air through the transducer while the measurements are taken. The volume in each breath and the cumulative volume can be found by electronic integration of the flow rate. Problems exist because inspired and expired volumes and gas mixtures are not the same, and so the pressure drop will not be the same for equal flow in each direction. Also, water vapour in the expired gases may condense on the mesh unless heating is applied, or some other anti-condensation measure is used.
The pneumotachograph may be used in lung function analysis, or during artificial ventilation of the lungs. For routine work in each of these applications simpler devices are normally used, such as the spirometer, dry gas meter, and the Wright's respirometer or Wright's respiration meter.
A transducer for measuring expired air flow.
This measures the flow rate of gases during breathing. The breath is passed through a short tube (Fleisch tube) in which there is a fine mesh which presents a small resistance to the flow. The resulting pressure drop across the mesh is in proportion to the flow rate. The pressure drop is very small (e.g. 2 mmHg) and so the measuring circuit must be of high quality and produce very little drift with time. A differential pressure transducer is normally used.
The advantage which this device has over the mechanical spirometer is that the patient under investigation can continue to breathe fresh air through the transducer while the measurements are taken. The volume in each breath and the cumulative volume can be found by electronic integration of the flow rate. Problems exist because inspired and expired volumes and gas mixtures are not the same, and so the pressure drop will not be the same for equal flow in each direction. Also, water vapour in the expired gases may condense on the mesh unless heating is applied, or some other anti-condensation measure is used.
The pneumotachograph may be used in lung function analysis, or during artificial ventilation of the lungs. For routine work in each of these applications simpler devices are normally used, such as the spirometer, dry gas meter, and the Wright's respirometer or Wright's respiration meter.
Medical ventilator
In medicine, mechanical ventilation is a method to mechanically assist or replace spontaneous breathing when patients cannot do so on their own, and must be done so after invasive intubation with an endotracheal or tracheostomy tube through which air is directly delivered (in contrast to noninvasive ventilation).
In its simplest form, a ventilator consists of a compressible air reservoir, air and oxygen supplies, a set of valves and tubes, and a disposable or reusable "patient circuit". The air reservoir is pneumatically compressed several times a minute to deliver room-air, or in most cases, an air/oxygen mixture to the patient. When overpressure is released, the patient will exhale passively due to the lungs' elasticity, the exhaled air being released usually through a one-way valve within the patient circuit. The oxygen content of the inspired gas can be set from 21 percent (ambient air) to 100 percent (pure oxygen). Pressure and flow characteristics can be set mechanically or electronically.
Ventilators may also be equipped with monitoring and alarm systems for patient-related parameters (e.g. pressure, volume, and flow) and ventilator function (e.g. air leakage, power failure, mechanical failure), backup batteries, oxygen tanks, and remote control. The pneumatic system is nowadays often replaced by a computer-controlled turbopump.
Modern ventilators are electronically controlled by a small embedded system to allow exact adaptation of pressure and flow characteristics to an individual patient's needs. Fine-tuned ventilator settings also serve to make ventilation more tolerable, and comfortable for the patient. In Germany, Canada, and the United States, respiratory therapists are responsible for tuning these settings.
The patient circuit usually consists of a set of three durable, yet lightweight plastic tubes, separated by function (e.g. inhaled air, patient pressure, exhaled air). In cases where long-term ventilator dependence may be required, the patient-end of the circuit will normally be connected to a tracheotomy canula, which is much more comfortable and practical for long-term care than is larynx or nasal intubation.
Wednesday, September 26, 2007
Spirometry
Watch the following animation on spirometry:
http://www.goldcopd.it/spieng_flash.html
Watch the following animation on spirometry:
http://www.goldcopd.it/spieng_flash.html
Spirometry testing
The spirometry test is performed using a device called a spirometer, which comes in several different varieties. Most spirometers display the following graphs:
a volume-time curve, showing volume (liters) along the Y-axis and time (seconds) along the X-axis
a flow-volume loop, which graphically depicts the rate of airflow on the Y-axis and the total volume inspired or expired on the X-axis
The most commonly used guidelines for spirometric testing and interpretation are set by the American Thoracic Society (ATS) and the European Respiratory Society (ERS).
Procedure:
The basic FVC test varies slightly depending on the equipment used.
Generally, the patient is asked to take the deepest breath they can, and then exhale into the sensor as hard as possible, for as long as possible. It is sometimes directly followed by a rapid inhalation (inspiration), in particular when assessing possible upper airway obstruction. Sometimes, the test will be preceded by a period of quiet breathing in and out from the sensor (tidal volume), or the rapid breath in (forced inspiratory part) will come before the forced exhalation.
During the test, soft nose clips may be used to prevent air escaping through the nose. Filter mouthpieces may be used to prevent the spread of microorganisms, particularly for inspiratory maneuvers.
Limitations of test
The maneuver is highly dependent on patient cooperation and effort, and is normally repeated at least three times to ensure reproducibility. Since results are dependent on patient cooperation, FEV1 and FVC can only be underestimated, never overestimated.
Due to the patient cooperation required, spirometry can only be used on children old enough to comprehend and follow the instructions given (typically about 4-5 years old), and only on patients who are able to understand and follow instructions - thus, this test is not suitable for patients who are unconscious, heavily sedated, or have limitations that would interfere with vigorous respiratory efforts. Other types of lung function tests are available for infants and unconscious persons.
Related tests
Spirometry can also be part of a bronchial challenge test, used to determine bronchial hyperresponsiveness to either rigorous exercise, inhalation of cold/dry air, or with a pharmaceutical agent such as methacholine or histamine.
Sometimes, to assess the reversibility of a particular condition, a bronchodilator is administered before performing another round of tests for comparison. This is commonly referred to as a reversibility test, or a post bronchodilator test (Post BD), and is an important part in diagnosing asthma versus COPD.
The spirometry test is performed using a device called a spirometer, which comes in several different varieties. Most spirometers display the following graphs:
a volume-time curve, showing volume (liters) along the Y-axis and time (seconds) along the X-axis
a flow-volume loop, which graphically depicts the rate of airflow on the Y-axis and the total volume inspired or expired on the X-axis
The most commonly used guidelines for spirometric testing and interpretation are set by the American Thoracic Society (ATS) and the European Respiratory Society (ERS).
Procedure:
The basic FVC test varies slightly depending on the equipment used.
Generally, the patient is asked to take the deepest breath they can, and then exhale into the sensor as hard as possible, for as long as possible. It is sometimes directly followed by a rapid inhalation (inspiration), in particular when assessing possible upper airway obstruction. Sometimes, the test will be preceded by a period of quiet breathing in and out from the sensor (tidal volume), or the rapid breath in (forced inspiratory part) will come before the forced exhalation.
During the test, soft nose clips may be used to prevent air escaping through the nose. Filter mouthpieces may be used to prevent the spread of microorganisms, particularly for inspiratory maneuvers.
Limitations of test
The maneuver is highly dependent on patient cooperation and effort, and is normally repeated at least three times to ensure reproducibility. Since results are dependent on patient cooperation, FEV1 and FVC can only be underestimated, never overestimated.
Due to the patient cooperation required, spirometry can only be used on children old enough to comprehend and follow the instructions given (typically about 4-5 years old), and only on patients who are able to understand and follow instructions - thus, this test is not suitable for patients who are unconscious, heavily sedated, or have limitations that would interfere with vigorous respiratory efforts. Other types of lung function tests are available for infants and unconscious persons.
Related tests
Spirometry can also be part of a bronchial challenge test, used to determine bronchial hyperresponsiveness to either rigorous exercise, inhalation of cold/dry air, or with a pharmaceutical agent such as methacholine or histamine.
Sometimes, to assess the reversibility of a particular condition, a bronchodilator is administered before performing another round of tests for comparison. This is commonly referred to as a reversibility test, or a post bronchodilator test (Post BD), and is an important part in diagnosing asthma versus COPD.
A spirometer is an apparatus for measuring the volume of air inspired and expired by the lungs. It is a precision differential pressure transducer for the measurements of respiration flow rates. The spirometer records the amount of air and the rate of air that is breathed in and out over a specified time. The Spirometer and attached flow head function together as a pneumotachometer, with an output signal proportional to airflow. It was invented by John Hutchinson in 1846.
Labels: Spirometer
Wednesday, September 12, 2007
Strain Gauge Plethysmography
Strain Gauge Plethysmography is a standardized technique to measure flow and vascular resistance by using a special transducer (strain Gauge) placed on the forearm and use of two inflated blood pressure cuffs proximal and distal to the site for the temporary occlusion of either venous or arterial flow. This technique also has been used to assess endothelial dysfunction by measurements of blood flow volume and rate of flow into the forearm.
Strain Gauge Plethysmography is a standardized technique to measure flow and vascular resistance by using a special transducer (strain Gauge) placed on the forearm and use of two inflated blood pressure cuffs proximal and distal to the site for the temporary occlusion of either venous or arterial flow. This technique also has been used to assess endothelial dysfunction by measurements of blood flow volume and rate of flow into the forearm.
Labels: Strain Gauge Plethysmography
Optoelectronic plethysmography
Optoelectronic Plethysmography is a method to evaluate ventilation through an external measurement of the chest wall surface motion.
Photoplethsymograph
A photoplethysmograph(PPG) is an optically obtained plethysmograph, a volumetric measurement of an organ. A PPG is often obtained by using a pulse oximeter which illuminates the skin and measures changes in light absorption (Shelley and Shelley, 2001). A conventional pulse oximeter monitors the perfusion of blood to the dermis and subcutaneous tissue of the skin.
In theory the photoplethysmograph is simple - it measures the variation in amount of light passing through your finger caused by the pulsatile nature of blood flow. You will place a light source on one side of your finger, and a light sensitive resistor, a Cadmium Sulfide (CdS) cell, on the other side. By monitoring variations in resistance of the CdS cell you get an indication of blood flow in your finger.
The light passes through the finger and is attenuated a certain amount depending upon how much blood is in the finger. As the amount of light striking the CdS cell varies, so does its resistance. This changing resistance must be transduced, amplified and filtered. The output of the amplifier is sent to an A/D convertor, and finally to the PIC. The relative amount of fluid in the finger should be displayed on the four LEDs. Much fluid (little light) should have all LED's on. Little fluid (much light) should correspond to all LED's off.
Labels: Photoplethsymograph
Tuesday, September 11, 2007
Impedance Plethysmography
Impedance plethysmography, also called impedance test or blood flow or impedance phlebography, is a non-invasive test that uses electrical monitoring in the form of resistance (impedance) changes to measure blood flow in veins of the leg. Information from this test helps doctors detect deep vein thrombosis (blood clots or thrombophlebitis).
Purpose
Impedance plethysmography may be done in order to:
detect blood clots lodged in the deep veins of the leg
screen patients who are likely to have blood clots in the leg
detect the source of blood clots in the lungs (pulmonary emboli)
Accurate diagnosis of deep vein thrombosis (DVT) is critical because blood clots in the legs can lead to more serious problems. If a clot breaks loose from a leg vein, it may travel to the lungs and lodge in a blood vessel in the lungs. Blood clots are more likely to occur in people who have recently had leg injuries, surgery, cancer, or a long period of bed rest.
Precautions
Because this test is not invasive, it can be done on all patients and is easy to perform. However, the accuracy of the results is affected if the patient does not breathe normally or keep the leg muscles relaxed. Compression of the veins because of pelvic tumors or decreased blood flow, due to shock or any condition that reduces the amount of blood the heart pumps, may also change the test results. Both false-positives (e.g. when thrombi are non-occulsive) and false-negatives have been reported using this technique, which justifies repeated testing over a period of seven to ten days for patients with initial negative results. Success rates for this test have been estimated at anywhere from 65–66% to 92–98%.
Description
Using conductive jelly, the examiner strategically places two to four electrodes on the patient's calf (the four-electrode configuration yields a more uniform and precise current density and consequent measurement result). These electrodes are connected to an instrument called a plethysmograph, which records the changes in electrical resistance that occur during the test and produces a graph of the results.
The patient must lie down and raise one leg at a 30° angle so that the calf is above the level of the heart. The examiner then wraps a pressure cuff around the patient's thigh and inflates it to a pressure of 45–60 cm of water for 45 seconds. The plethysmograph records the electrical impedance changes that correspond to changes in the volume of blood in the vein at the time the pressure is exerted and again three seconds after the cuff is deflated. This procedure is repeated several times in both legs.
This test takes 30-45 minutes, costs an estimated $50-$100 (as of 2001), and results can be available within a few minutes.
Impedance plethysmography works by measuring the resistance to the transmission of electrical energy (impedance). This resistance is dependent upon the volume of blood flowing through the veins. By graphing the impedance, the doctor or technician can tell whether a clot is obstructing blood flow.
Preparation
Patients undergoing this test do not need to alter their diet, change their normal activities, or stop taking any medications. They will wear a surgical gown during the test and should be asked to urinate before the test starts. If keeping the legs elevated causes discomfort, mild pain medication can be given.
Aftercare
The patient may resume normal or postoperative activities after the test.
Limb plethysmography
Definition:
Limb plethysmography is a test that compares blood pressure in the legs and arms. It is usually done to check for blood flow blockages in the legs.
Alternative Names:
Plethysmography - limb
How the test is performed:
This test may be done in the health care provider's office or in a hospital. You will be asked to lie in a partially propped-up position.
Three blood pressure cuffs are wrapped snugly around your arm and leg. The health care provider inflates the cuffs and a machine called a plethysmograph measures the pulses from each cuff. The test records the maximum pressure produced when the heart contracts (systolic blood pressure).
Any differences between the pulses are noted. If there is there is a decrease in the pulse between the arm and leg, it may indicate a blockage.
When the test is complete, the blood pressure cuffs are removed.
How to prepare for the test:
Do not smoke for at least 30 minutes before the test. You will be asked to remove all clothing from the arm and leg being tested.
How the test will feel:
You should not have much discomfort with this test. You should only feel the pressure of the blood pressure cuff.
Why the test is performed:
The test is done to help rule out blocked arteries in the arms or legs.
Normal Values:
There should be less than 20 mmHg difference in the systolic blood pressure of the leg compared with that of the arm.
What abnormal results mean:
Abnormal results may be due to:
Arterial occlusive disease
Blood clots
Blood vessel changes due to diabetes
Injury to an artery
Other blood vessel disease (vascular disease)
Other conditions under which the test may be performed:
Deep venous thrombosis
What the risks are:
There are no risks.
Special considerations:
The test is not as accurate as an arteriography. Plethysmography may be done for very ill patients who cannot travel to the arteriography lab. This test can be used to screen for vascular disease or to follow up earlier abnormal tests.
Definition:
Limb plethysmography is a test that compares blood pressure in the legs and arms. It is usually done to check for blood flow blockages in the legs.
Alternative Names:
Plethysmography - limb
How the test is performed:
This test may be done in the health care provider's office or in a hospital. You will be asked to lie in a partially propped-up position.
Three blood pressure cuffs are wrapped snugly around your arm and leg. The health care provider inflates the cuffs and a machine called a plethysmograph measures the pulses from each cuff. The test records the maximum pressure produced when the heart contracts (systolic blood pressure).
Any differences between the pulses are noted. If there is there is a decrease in the pulse between the arm and leg, it may indicate a blockage.
When the test is complete, the blood pressure cuffs are removed.
How to prepare for the test:
Do not smoke for at least 30 minutes before the test. You will be asked to remove all clothing from the arm and leg being tested.
How the test will feel:
You should not have much discomfort with this test. You should only feel the pressure of the blood pressure cuff.
Why the test is performed:
The test is done to help rule out blocked arteries in the arms or legs.
Normal Values:
There should be less than 20 mmHg difference in the systolic blood pressure of the leg compared with that of the arm.
What abnormal results mean:
Abnormal results may be due to:
Arterial occlusive disease
Blood clots
Blood vessel changes due to diabetes
Injury to an artery
Other blood vessel disease (vascular disease)
Other conditions under which the test may be performed:
Deep venous thrombosis
What the risks are:
There are no risks.
Special considerations:
The test is not as accurate as an arteriography. Plethysmography may be done for very ill patients who cannot travel to the arteriography lab. This test can be used to screen for vascular disease or to follow up earlier abnormal tests.
Plethysmography
Definition:
Plethysmography is a test used to measure changes in blood flow or air volume in different parts of the body. It may be done to check for blood clots in the arms and legs, or to measure how much air you can hold in your lungs.
A plethysmography test is performed by placing blood pressure cuffs on the extremities to measure the systolic pressure. The cuffs are then attached to a pulse volume recorder (plethysmograph) that displays each pulse wave. The test compares the systolic blood pressure of the lower extremity to the upper extremity, to help rule out disease that blocks the arteries in the extremities.
Sunday, August 26, 2007
Measuring The Blood Pressure
'Normal' or 'acceptable' blood pressure varies with age, state of health and clinical situation. At birth, a typical blood pressure is 80/50 mmHg. It rises steadily throughout childhood, so that in a young adult it might be 120/80 mmHg. As we get older, blood pressure continues to rise and a rule of thumb is that normal systolic pressure is age in years + 100. Blood pressure is lower in late pregnancy and during sleep.
From this, you can see that a systolic pressure of 160mmHg for an elderly man or 90 mmHg for a pregnant woman may be quite normal.
From this, you can see that a systolic pressure of 160mmHg for an elderly man or 90 mmHg for a pregnant woman may be quite normal.
Automatic non-invasive blood pressure measurement (Electronic Sphygmomanometer)
Automatic devices which essentially apply the same principle as the oscillotonometer have been produced (e.g. the 'Dinamap' made by Critikon). They require a supply of electricity. A single cuff is applied to the patients arm, and the machine inflates it to a level assumed to be greater than systolic pressure. The cuff is deflated gradually. A sensor then measures the tiny oscillations in the pressure of the cuff caused by the pulse. Systolic is taken to be when the pulsations start, mean pressure is when they are maximal, and diastolic is when they disappear. They can produce fairly accurate readings and free the hands of the anaesthetist for other tasks. There are important sources of inaccuracy, however. Such devices tend to over-read at low blood pressure, and under-read very high blood pressure. The cuff should be an appropriate size. The patient should be still during measurement. The technique relies heavily on a constant pulse volume, so in a patient with an irregular heart beat (especially atrial fibrillation) readings can be inaccurate. Sometimes an automatic blood pressure measuring device inflates and deflates repeatedly "hunting" without displaying the blood pressure successfully. If the pulse is palpated as the cuff is being inflated and deflated the blood pressure may be estimated by palpation and reading the cuff pressure on the display.
Automatic devices which essentially apply the same principle as the oscillotonometer have been produced (e.g. the 'Dinamap' made by Critikon). They require a supply of electricity. A single cuff is applied to the patients arm, and the machine inflates it to a level assumed to be greater than systolic pressure. The cuff is deflated gradually. A sensor then measures the tiny oscillations in the pressure of the cuff caused by the pulse. Systolic is taken to be when the pulsations start, mean pressure is when they are maximal, and diastolic is when they disappear. They can produce fairly accurate readings and free the hands of the anaesthetist for other tasks. There are important sources of inaccuracy, however. Such devices tend to over-read at low blood pressure, and under-read very high blood pressure. The cuff should be an appropriate size. The patient should be still during measurement. The technique relies heavily on a constant pulse volume, so in a patient with an irregular heart beat (especially atrial fibrillation) readings can be inaccurate. Sometimes an automatic blood pressure measuring device inflates and deflates repeatedly "hunting" without displaying the blood pressure successfully. If the pulse is palpated as the cuff is being inflated and deflated the blood pressure may be estimated by palpation and reading the cuff pressure on the display.
