Biomedical Instrumentation: Diagnosis and Therapy

Biomedical Instrumentation: Diagnosis and Therapy

John G. Webster (Department of Biomedical Engineering, University of Wisconsin, Madison, WI, USA)
DOI: 10.4018/IJSBBT.2015010102
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Abstract

This paper covers the measurement of biopotentials for diagnosis: the electrical voltages that can be measured from electrodes placed on the skin or within the body. Biopotentials include: the electrocardiogram (ECG), electroencephalogram (EEG), electrocortogram (ECoG), electromyogram (EMG), electroneurogram (ENG), electrogastrogram (EGG), action potential (AP), electroretinogram (ERG), electro-oculogram (EOG). This paper also covers skin conductance, pulse oximeters, urology, wearable systems and important therapeutic devices such as: the artificial cardiac pacemaker, defibrillator, cochlear implant, hemodialysis, lithotripsy, ventilator, anesthesia machine, heart-lung machine, infant incubator, infusion pumps, electrosurgery, tissue ablation, and medical imaging. It concludes by covering electrical safety. It provides future subjects for research such as a blood glucose sensor and a permanently implanted intracranial pressure sensor.
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Electrocardiographic Amplifier

The high fat, low roughage diet of the western world promotes development of plaque, which deposits inside arteries and eventually plugs them. This can cause a heart attack, stroke, or kidney failure. When a subject experiences chest pain, the first measurement is to take an electrocardiogram (ECG) to determine damage to the heart. Electrodes are applied to the chest. The best electrodes are formed of silver/silver chloride (Ag/AgCl) because the AgCl salt coating the Ag electrodes is very stable, with the voltage changing little in response to movement or electric current flow. Placing a conductive gel between the electrode and the skin promotes good electrical contact. The gel may also be an adhesive or may be surrounded by an adhesive to mechanically stabilize the electrode on the skin. The electrodes are placed above and below the heart to continuously monitor the maximum voltage of about 1 mV. Different waves occur: atrial contraction causes the P wave, ventricular contraction causes the QRS waves, ventricular relaxation causes the T wave. Ten electrodes may be placed surrounding the heart to yield a one-time 12-lead diagnostic ECG. This yields views of the cardiac excitation from different angles. The ECG yields information on rhythm, such as bradycardia (slow rhythm), tachycardia (fast rhythm), total heart block (no excitation from the atria to the ventricles), atrial fibrillation (chaotic atrium), and ventricular fibrillation (chaotic ventricles). S–T depression also yields information on cardiac damage.

Figure 1 shows an ECG amplifier which has a high gain to boost the 1 mV ECG to about 1 V. The LT1920 instrumentation amplifier has high input impedance. The high input impedance is required to prevent attenuated loading because the skin may have a resistance of about 1 MΩ. The amplifier also has high common mode rejection ratio (CMRR = Gd/Gc) where Gd is the differential gain and Gc is the common mode gain. High CMRR minimizes the power line interference from appearing on the ECG. The amplifier gain is G1 = (49.4 kΩ/RG) + 1 = (49.4 kΩ/3 kΩ) + 1 = 17.5. The Association for the Advancement of Medical Instrumentation (AAMI) specification requires that the amplifier operate with differential 0.3 V dc, so the gain must be kept low (Neuman, 2010a, p. 252).

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