Biomedical Engineering Theory AndPractice/Biomedical

Instrumentation/Electrocardiography

This chapter should cover the basics of ECG systems froma design point of view; not from a diagnosis point of view.

1 Physiological Background

P

Q

R

S

TST

SegmentPRSegment

PR Interval

QT Interval

QRS Complex

Typical ECG signal

The function of the heart is to contract rhythmically andpump blood to the lungs for oxygenation and then pumpthis oxygenated blood into the general circulation. Thisperfect rhythm is continuously maintained and signaledby the spread of electrical signals generated by the heartpacemaker, the sinoatrial (SA) node. [1] Detecting suchelectrical activity of the heart can help identifymany heartdisorders. This is the main concept behind using an ECG(Electrocardiogram), tracing the electrical activity of theheart.By measuring and tracing the potential dierence be-tween two points on the outer surface of the body weobtain the simplest ECG chart. Two typical measuringpoints between the left arm and the right arm. By den-ing the two points and setting up the conventional posi-tive direction for measuring the voltage, we create whatis called a Lead.The rst phase of the cardiac muscle activation is thestimulation of the right and left atria by an electrical sig-

An animation showing how the electrical activity of the heart isreected on the ECG signal

nal generated from the SA node. This phase appears asthe P-wave on the ECG chart. The electrical signal, orig-inally generated by the SA node, then spreads throughthe Atrioventricual (AV) Junction, the bundle of His, andthe Purkinje bers to nally reach and stimulate the ven-tricles. The spread of the electrical signal through theventricles causes ventricular contraction. The phase ofventricular contraction appears as the characteristic QRScomplex on the ECG chart. Finally, with the relaxationof the two ventricles, a depolarization signal is generatedand appears as the T-wave on the ECG chart.

2 Design of a Basic ECGSignal Ac-quisition Module

The acquisition of the ECG signal is a rather challeng-ing task, as the case with many biological signals. ECGvoltage signal is very low in magnitude (few millivolts)and has relatively low frequency content. The expectedbandwidth of the signal typically begins from 0.01 Hz and

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2 8 FURTHER READING

extends to no more than 150 Hz.Another challenge in acquiring the ECG signal is thepower-line interference that is often order of magnitudesgreater than the original ECG signal. All this suggestschallenging requirements for the design of the signal ac-quisition module: It should have minimum loading eect,it should contain an amplication stage to make the signallevel appropriate for further use, and it should contain altration stage customized to remove the expected noiseand power-line interference that often corrupts the ECG.In this section, we are NOT going to consider the full de-sign of the ECG signal; rather, we shall focus on aminimalECG acquisition module design that would just work.

Patient Dierence AmplierAVERY simplied block diagram of an ECG amplier. The volt-age of the right arm (w.r.t right leg) is subtracted from the voltageof the left arm (w.r.t right leg) to get the ECG Lead I signal).

As discussed in the physiological background section, anECG signal is obtained as the voltage dierence betweentwo points on the skin. This suggests the need for somesubtraction mechanism. The subtraction could be doneusing an electronic Dierence Amplier. This amplierbasically subtracts and amplies the dierence betweentwo electrical points. For the subtraction to work cor-rectly, the voltage of both electrical points should be mea-sured with respect to a common electrical reference. Thiscommon reference is typically chosen to be the right legof the patient.So, a simplied diagram of a simple ECG acquisitionmodule would be as shown in the gure.

3 Signal ArtifactsArtifacts that corrupt the raw ECG signal have eitherphysiological or non-physiological origin. The most dom-inant artifact is the power-line interference which ap-pears as an sinusoidal wave of frequency 50 Hz (or 60 Hzin USA). Other artifacts include:[2]

Movement Artifacts due to patient movement,etc...

Baseline Wander where the ECG waveform base-line starts to drift up and down in a sinusoidal patternfollowing the patient breathing

EMG Interference where muscle contraction sig-nals interfere with the ECG.

Electrode Contact Noise where the electrodes arenot tightly coupled to the patient causing some dis-tortion

Electrosurgical Unit (ESU) interference wherehigh-frequency signals from the ESU used by sur-geons during operation interfere with the ECG

4 Basic Signal Conditioning for theECG

This section should discuss theoretical signal processingsolutions to the above artifacts

5 Analog Implementations for theSignal Conditioning

6 Digital Hardware Implementa-tions for the Signal Conditioning

7 Software Implementations forthe Signal Conditioning

8 Further Reading Programmable Chip EEG the OpenEEG Wiki

The thread amplifying biomedical signals: 150 uAwith 16 bit resolution?" has several op-amp sugges-tions, and mentions that a good, low-noise, low-cost, isolated EMG/EEG amplier is one of themost demanding analog electronics designs.

How to build your own ECG device TI app note Biophysical Monitoring: Electrocar-diogram (ECG) Front End has a simple circuit: 390KOhm resistors in-line with each lead -- one endtouches patient, the other end directly connected tothe instrumentation amp input (or the right-leg driveamplier output, which has no further protection).The inst. amp has 2 protection diodes on each in-put, directly to +power and -power. Also, 39 pF ca-pacitor from each input to analog GND, and 200 pFbetween the 2 inputs. The TI publication Informa-tion for Medical Applications (2Q 2004) reprintsthat circuit, but leaves out the caps and the diodes.

Some people use 420 Hz sampling rate, 10bits/sample.

3 High-Resolution QRS Detection Algorithm forSparsely Sampled ECG Recordings by TimoBragge et al. 2004 recommends: the sampling fre-quency of the ECG should be at least 500 Hz

Low-Power, Low-Voltage IC Choices for ECGSystem Requirements by Jon Firth and Paul Erricosays The multiplexed architecture, based on anold assumption that the converter is by far themost-expensive front-end component, is prevalent intodays electrophysiological measurement systems.However, with the proliferation of sigma-delta con-verter architectures, converter-per-channel is nowa power- and cost-competitive alternative. It alsogives a typical schematic for both architectures andsuggests some parts.

Is there a Medical Electronics Forum? Make: Homemade Electrocardiograph ( recom-mends skin lotion or shampoo as a low-cost elec-trode gel)

9 ReferencesPlease cite all the references you have used. See the LocalManual of Style for examples on citations.

[1] Clinical Electrocardiography, a Simplied Approach.Seventh Edition; Ary L. Goldberger, Mosby-Elsevier,2006

[2] Takla G, Petre JH, Doyle DJ, Horibe M, GopakumaranB. The problem of artifacts in patient monitor data dur-ing surgery: a clinical and methodological review. AnesthAnalg. 2006;103(5):1196-204

4 10 TEXT AND IMAGE SOURCES, CONTRIBUTORS, AND LICENSES

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Physiological Background Design of a Basic ECG Signal Acquisition Module Signal Artifacts Basic Signal Conditioning for the ECG Analog Implementations for the Signal Conditioning Digital Hardware Implementations for the Signal Conditioning Software Implementations for the Signal Conditioning Further Reading References Text and image sources, contributors, and licensesTextImagesContent license