Biomedical Engineering Theory And Practice/Biomedical Instrumentation/Electrocardiography

Biomedical Engineering Theory And Practice/Biomedical Instrumentation/Electrocardiography

This chapter ought to cowl the fundamentals of ECG methods from a design standpoint; not from a prognosis standpoint.

Physiological Background[edit]

An animation exhibiting how {the electrical} exercise of the guts is mirrored on the ECG sign

The perform of the guts is to contract rhythmically and pump blood to the lungs for oxygenation after which pump this oxygenated blood into the final circulation. This good rhythm is constantly maintained and signaled by the unfold {of electrical} indicators generated by the guts pacemaker, the sinoatrial (SA) node. [1] Detecting such electrical exercise of the guts will help establish many coronary heart problems. That is the primary idea behind utilizing an ECG (Electrocardiogram), tracing {the electrical} exercise of the guts.

By measuring and tracing the potential distinction between two factors on the outer floor of the physique we receive the best ECG chart. Two typical measuring factors between the left arm and the precise arm. By defining the 2 factors and establishing the traditional constructive course for measuring the voltage, we create what is named a “Lead”.

The primary section of the cardiac muscle activation is the stimulation of the precise and left atria by {an electrical} sign generated from the SA node. This section seems because the P-wave on the ECG chart. {The electrical} sign, initially generated by the SA node, then spreads by means of the Atrioventricual (AV) Junction, the bundle of His, and the Purkinje fibers to lastly attain and stimulate the ventricles. The unfold of {the electrical} sign by means of the ventricles causes ventricular contraction. The section of ventricular contraction seems because the attribute QRS advanced on the ECG chart. Lastly, with the relief of the 2 ventricles, a depolarization sign is generated and seems because the T-wave on the ECG chart.

Design of a Fundamental ECG Sign Acquisition Module[edit]

The acquisition of the ECG sign is a reasonably difficult process, because the case with many organic indicators. ECG voltage sign could be very low in magnitude (few millivolts) and has comparatively low frequency content material. The anticipated bandwidth of the sign usually begins from 0.01 Hz and extends to not more than 150 Hz.

One other problem in buying the ECG sign is the power-line interference that’s typically order of magnitudes higher than the unique ECG sign. All this means difficult necessities for the design of the sign acquisition module: It ought to have minimal loading impact, it ought to comprise an amplification stage to make the sign degree applicable for additional use, and it ought to comprise a filtration stage custom-made to take away the anticipated noise and power-line interference that usually corrupts the ECG.

On this part, we’re NOT going to think about the total design of the ECG sign; reasonably, we will deal with a minimal ECG acquisition module design that will simply work.

A VERY simplified block diagram of an ECG amplifier. The voltage of the precise arm (w.r.t proper leg) is subtracted from the voltage of the left arm (w.r.t proper leg) to get the ECG Lead I sign).

As mentioned within the physiological background part, an ECG sign is obtained because the voltage distinction between two factors on the pores and skin. This implies the necessity for some subtraction mechanism. The subtraction could possibly be completed utilizing an digital Distinction Amplifier. This amplifier principally subtracts and amplifies the distinction between two electrical factors. For the subtraction to work accurately, the voltage of each electrical factors must be measured with respect to a standard electrical reference. This widespread reference is usually chosen to be the precise leg of the affected person.

So, a simplified diagram of a easy ECG acquisition module could be as proven within the determine.

Sign Artifacts[edit]

Artifacts that corrupt the uncooked ECG sign have both physiological or non-physiological origin. Probably the most dominant artifact is the power-line interference which seems as an sinusoidal wave of frequency 50 Hz (or 60 Hz in USA). Different artifacts embody:[2]

  • Motion Artifacts because of affected person motion, and so forth…
  • Baseline Wander the place the ECG waveform baseline begins to float up and down in a sinusoidal sample following the affected person respiratory
  • EMG Interference the place muscle contraction indicators intervene with the ECG.
  • Electrode Contact Noise the place the electrodes usually are not tightly coupled to the affected person inflicting some distortion
  • Electrosurgical Unit (ESU) interference the place high-frequency indicators from the ESU utilized by surgeons throughout operation intervene with the ECG

Fundamental Sign Conditioning for the ECG[edit]

This part ought to talk about theoretical sign processing options to the above artifacts

Analog Implementations for the Sign Conditioning[edit]

Digital {Hardware} Implementations for the Sign Conditioning[edit]

Software program Implementations for the Sign Conditioning[edit]

Additional Studying[edit]

  • The thread “amplifying biomedical indicators: 150 uA with 16 bit decision?” has a number of op-amp recommendations, and mentions that “a superb, low-noise, low-cost, remoted EMG/EEG amplifier is among the most demanding analog electronics designs.”
  • Learn how to construct your personal ECG gadget
  • TI app notice “Biophysical Monitoring: Electrocardiogram (ECG) Entrance Finish” has a easy circuit: 390 KOhm resistors in-line with every lead — one finish touches affected person, the opposite finish straight linked to the instrumentation amp enter (or the right-leg drive amplifier output, which has no additional safety). The inst. amp has 2 safety diodes on every enter, on to +energy and -power. Additionally, 39 pF capacitor from every enter to analog GND, and 200 pF between the two inputs. The TI publication “Info for Medical Purposes” (2Q 2004) reprints that circuit, however leaves out the caps and the diodes.
  • Some folks use 420 Hz sampling charge, 10 bits/pattern.
  • “Excessive-Decision QRS Detection Algorithm for Sparsely Sampled ECG Recordings” by Timo Bragge et al. 2004 recommends: “the sampling frequency of the ECG must be no less than 500 Hz”
  • “Low-Energy, Low-Voltage IC Selections for ECG System Necessities” by Jon Firth and Paul Errico says “The multiplexed structure, primarily based on an outdated assumption that the converter is by far the most-expensive front-end part, is prevalent in at present’s electrophysiological measurement methods. Nonetheless, with the proliferation of sigma-delta converter architectures, converter-per-channel is now a power- and cost-competitive various”. It additionally offers a typical schematic for each architectures and suggests some elements.
  • Is there a Medical Electronics Discussion board?
  • Make: Do-it-yourself Electrocardiograph ([1] recommends pores and skin lotion or shampoo as a low-cost electrode gel)

References[edit]

Please cite all of the references you have got used. See the Native Handbook of Type for examples on citations.

  1. Medical Electrocardiography, a Simplified Strategy. Seventh Version; Ary L. Goldberger, Mosby-Elsevier, 2006
  2. Takla G, Petre JH, Doyle DJ, Horibe M, Gopakumaran B. The issue of artifacts in affected person monitor information throughout surgical procedure: a medical and methodological overview. Anesth Analg. 2006;103(5):1196-204


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