27-11-2012, 04:26 PM
ECG signal and noise removal methods
History of the ECG
1856: Köllicker and Müller discovered that the heart muscle could produce electric activity.
Muirhead in London recorded the first electrocardiogram (ECG) in man in 1869 or 1870 with a siphon instrument
Waller in 1887 with a capillary electrometer.
Einthoven introduced "Le Télecardiograme" in 1906 by which a cable connected his instrument to a hospital one and a half kilometres away.
Rune Elmqvist developed the direct-writing inkjet recorder, first demonstrated at the Congress of Cardiology in Paris, 1950.
A Brief introduction to ECG
The electrocardiogram (ECG) is a time-varying signal reflecting the ionic current flow which causes the cardiac fibers to contract and subsequently relax. The surface ECG is obtained by recording the potential difference between two electrodes placed on the surface of the skin. A single normal cycle of the ECG represents the successive atrial depolarisation/repolarisation and ventricular depolarisation/repolarisation which occurs with every heart beat.
Simply put, the ECG (EKG) is a device that measures and records the electrical activity of the heart from electrodes placed on the skin in specific locations
Standardized Methods & Devices
Electrode Placement
Standardization improves accuracy of comparison ECGs
3 Lead and 12 Lead Placement are most common
Assure good conduction gel
Prep area with alcohol prep
Avoid
Bone
Large muscles or hairy areas
Limb vs. Chest placement Poor placement or preparation
Often results in artifact
Components of the ECG Complex
Q Wave
first negative deflection after P wave
depolarization of septum
not always seen
R Wave
first positive deflection following P or Q waves
subsequent positive deflections are R’, R”, etc
S Wave
Negative deflection following R wave
subsequent negative deflections are S’, S”, etc
may be part of QS complex
absent R wave in aberrant conduction
What the ECG is used for?
Screening test for coronary artery disease, cardiomyopathies, left ventricular hypertrophy
Preoperatively to rule out coronary artery disease
Can provide information in the presence of metabolic alterations such has hyper/hypo calcemia/kalemia etc.
With known heart disease, monitor progression of the disease
Discovery of heart disease; infarction, coronal insufficiency as well as myocardial, valvular and congenital heart disease
Evaluation of rhythm disorders
All in all, it is the basic cardiologic test and is widely applied in patients with suspected or known heart disease
What is Noise?
Noise is the part of a waveform that is not signal!
unwanted error in a waveform
Noise can be random (white) or have a statistical distribution
Noise can occur at random intervals (e.g., bumping of electrodes, power surges, floor impacts nearby) or regular (50/60 Hz line interference) or irregular intervals (e.g., ECG or EEG interference of EMGs).
Noise may have frequencies inside or outside the frequency range of the signal (if outside, filtering can effectively reduce the noise).
Digital Filtering
Used on data that have been sampled with fixed time intervals.
Types:
low-pass, high-pass, band-pass, band stop and notch (single or small band-stop filter, useful for AC interference)
Designs:
Butterworth (optimally flat in band pass), critically-damped, Chebyshev etc. (sharper cut offs), Generalized Cross-validation (GCV also called Woltring filter, 1986)
Problems:
Noise spikes alter a localized period in the signal
Phase distortion (usually phase-lags occur)
Does not reduce size of data file