Modern disposable electrodes are often constructed of silver, coated with silver chloride.
The fundamental frequency in the ECG is the pulse rate.
A transient averager or correlator is used to identify evoked EEG signals.
Changes in impedance can be detected as a change of a.c. voltage between the electrodes.
Needle electrodes should be avoided due to their poor electrical performance and increased risk of diathermy burns.
Using iron can lead to corrosion, creating irregular electrochemical potentials that reduce the signal-to-noise ratio.
Using circumferential electrodes around the neck and abdomen.
When absolute values are of less interest than relative changes.
Frequencies in excess of 10 kHz are used in impedance spirometry.
The latency of the evoked response is increased with deepening anaesthesia.
High input impedance minimizes current flow through the electrodes, preventing signal attenuation and ensuring accurate signal measurement.
By summating successive responses to an auditory stimulus, the evoked response can be extracted from the background EEG activity.
Monitoring changes in lung water.
High common mode rejection is essential to minimize the effect of unwanted noise and interference on the recorded biological signals.
Changes in impedance can be shown to have a reasonably linear approximation to the substance under examination, allowing for empirical calibration.
Fourier analysis is used to determine the fundamental frequency and the amplitudes of the harmonics.
Good amplifier design can help prevent Mams-frequency interference, but complete noise-free recording is unlikely in environments with a.c. potential sources.
A highly selective band-reject filter is usually incorporated to remove 50 Hz interference from the signal.
Interference can originate from electrostatic or electromagnetic induction from mains or radiofrequency sources, leading to noise in the recorded signals.
By surrounding each lead with a braided copper screen connected to the amplifier reference voltage.
A high input impedance is vital to lower the electrode-skin resistance, which improves the quality of biological signal detection.
The resistance of an electrode can be seen as an equivalent resistor, and any induced current must set up a potential difference across the electrode, which is then amplified.
The earth-loop effect occurs when a patient is connected to two electrical devices that are separately earthed, leading to interference due to different earth potentials.
The fetal ECG signal is mixed with a much larger maternal ECG signal, making isolation difficult.
About 0.5-2 mV.
The EEG signal rarely exceeds 200 µV in amplitude.
If the electrode impedance is high, a small induced current may create a significant potential difference across the impedance, leading to massive 50 Hz interference at the amplifier output.
RFI can enter through the mains distribution system mixed with 50 Hz current, directly from surgical diathermy, or through unsuppressed sparking contacts in electrical devices.
The active diathermy electrode acts as a radiotransmitting aerial, causing radiofrequency potentials to be detected by recording electrodes, which can obscure the ECG signal.
The fetal ECG can be recorded from abdominal skin electrodes on the mother, but a scalp electrode provides a clearer signal.
A reversible electrode, like silver-silver chloride (Ag-AgCl), does not alter its composition with current flow, maintaining a stable cell potential.
Mechanical movement can cause large potential changes due to alterations in the electrode-skin half-cell dimensions and changes in skin-electrode impedance.
Intracellular potentials can be recorded by inserting an extremely fine, saline-filled glass pipette directly into the cell.
The maternal ECG creates an interfering signal.
The foam pad ensures that mechanical distortion of the skin under the electrode does not significantly alter the electrode potential or resistance.
Polarization occurs when a signal applied to a polarized electrode distorts the signal due to the current affecting the cell potential.
The supply will work perfectly well, but the live-neutral current mismatch will generate a large magnetic field.
Single-cell action potentials can be sensed with either intracellular or extracellular electrodes.
The common mode rejection ratio (CMRR) for amplifiers detecting physiological potentials should exceed 1000:1.
Surrounding the circuitry with a copper or aluminum enclosure to induce opposing fields that cancel the original field.
EEG signals arise from largely asynchronous activity within the brain or simultaneous activity of many organized activities.
Movement artifacts can be greatly reduced by separating the electrode surface from the skin with a thick layer of electrolyte gel.
The electrical impedance depends on the constitution of the tissues and can change with variations in blood volume or thoracic contents.
In modern devices, all circuits making contact with the patient must be isolated from ground to prevent earth loops, which are now uncommon.
They are no longer equal and self-cancelling, leading to electromagnetic interference.
A high impedance amplifier is needed to avoid signal distortion.
The input impedance must exceed 5 MΩ to avoid problems in detecting physiological potentials.
The signal amplitude depends on factors such as the mass of synchronously depolarizing tissue and the coherence of the depolarization process.
An earth current sets up a potential difference across a small resistance, which appears across the patient and is detected as interference by the amplifier.
Low input impedance can cause current to flow between the electrode and amplifier, leading to attenuation of the signal due to the potential divider effect.
Absolute calibration is difficult because it depends on the position of the electrodes, the frequency or current employed, and the tissue structure of the individual.
Good design, the use of high-grade components, and careful screening.
The standard frequency response for ECG machines is from 0.14 up to 50 Hz, allowing for 30 dB attenuation at 0.05 and 100 Hz.
The mains potential, being sinusoidal, causes the patient's surface potential to fluctuate at 50 Hz, creating spatial variations depending on the patient's surroundings.
Needle electrodes can be inserted into muscles to record muscle action potentials (EMG) and determine conduction velocities in nerves.
EMG electrodes can be placed directly adjacent to or even in the muscle itself, while EEG electrodes are separated from the brain by a poorly conducting box.
It is advisable to e-grease the skin with ether before applying an electrode to reduce resistance and ensure satisfactory adhesion.
The signal-to-noise ratio should be high.
Noise arising from the patient or his surroundings.
A 1-2 mA current can be used safely and without discomfort at high frequencies.
An equal and opposite charge develops on the uncharged body, which can affect the potential readings if the uncharged body is a patient near electrical sources.
By recording the maternal ECG from chest leads and using careful waveform matching to subtract it from the abdominal signal.
The second electrode is usually attached to the inside surface of the mother's thigh.
An a.c. current generates a magnetic field that is constantly growing and collapsing, inducing a current in nearby conductors at the same frequency.
With random depolarization and non-coherent propagation, the potentials are averaged, resulting in small amplitude voltages detected by chest wall electrodes.
A long time constant (>2 s) provides optimal waveform reproduction but may cause baseline instability, while a short time constant (<0.5 s) ensures good baseline stability at the expense of waveform fidelity.
It is undesirable for safety reasons and it intensifies the potential gradients on the patient’s surface.
The filter is used to block unwanted radiofrequency signals before the ECG signal enters the isolated input circuit.
A scalp electrode, which is a stainless steel spiral, is used for a clearer fetal ECG signal.
High CMRR means that the amplifier strongly attenuates common signals while amplifying the difference between the two signals.
Extracellular recording is used to measure potentials from outside the cell, often with needle or wire electrodes.
Synchronous depolarization of a large number of cells leads to the formation of an electrical field, allowing potential changes to be detected at distant points.
An ECG amplifier should be of the differential type with high input impedance and high CMRR.
Direct current amplifiers are unsuitable for ECG recording because they are subject to slowly fluctuating offset potentials that can distort the trace.
Keeping the signal leads as short as possible to reduce interference.
The electrodes detect the potential gradient across the tissue, reflecting the impedance.
It can lead to gross interference due to larger EMG signals.
For every live cable carrying current to a device, there is a neutral cable carrying an exactly equal current away from it, often twisted together.
It presents insuperable problems, especially with spark-gap and valve-modulated generators, causing large radiofrequency signals to saturate the input stages.
It induces an electromotive force (e.m.f.) in all the wires within its vicinity, especially if the wires are coiled.
They are readily penetrated by electromagnetic fields, making electromagnetic interference a major problem.
An electrochemical half-cell is produced, generating an electromotive force (e.m.f.).
A differential amplifier with high common mode rejection.
Stray capacitance can introduce 50 Hz interference into the recorded signal, complicating the measurement of biological potentials.
Enclosing the entire patient in an iron box or room along with the low-level stages of signal detection and amplification.
The main deflection in the ECG signal is caused by the depolarization of the large ventricular tissue mass.
