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Introduction to KTU Notes

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Biopotential Electrodes and ECG Introduction

Bioelectric Potentials

• Bioelectric potentials are ionic potentials generated in the body due to ionic current flow. To utilize these potentials electronically, they must be converted into electronic signals using electrodes.

Electrode Theory

• The interface of metallic ions in a solution creates electrical potential known as electrode potential, influenced by the diffusion rates of ions across the metal. This creates equilibrium through the formation of charged layers at the interface.

Electrode Potential Measurement

• Measuring the potential of an electrode requires another metallic interface in solution. Electrode potentials are relative and often referenced to standard electrodes such as hydrogen electrodes.

• The Nernst Equation relates potential across a semi-permeable membrane and the concentration of ions, formulated as:

  [ E = -\frac{RT}{nF} \ln \frac{C1}{C2} ]

• Where:

  • R: Gas constant (8.315 x 10^7 cgs/mole/K)

  • F: Faraday constant (96500 Coulombs)

  • C1, C2: Ion concentrations on either side of the membrane.

Nernst Equation Variables

• The variables in the Nernst equation include:

  • n: Valence of the ion.

  • f1, f2: Activity coefficients on both sides.

• The activity of the ion is the product of ion concentration and its activity coefficient, significant when calculating electrode potential across membranes.

Types of Biopotential Electrodes

Microelectrodes

• Used to measure bioelectric potentials very close to or within individual cells. They have small tips (0.5 to 5 um) to allow penetration without damaging cells.

Surface Electrodes

• Used externally to measure ECG, EEG, and EMG from the skin surface.

Needle Electrodes

• Inserted into the skin for localized measurement of EEG or EMG potentials.

Metal and Glass Micropipette Electrodes

Metal Microelectrodes

• Formed by electrolytically etching a tungsten or stainless-steel wire. They feature an insulating coating, and the metal-ion interface occurs at the tip.

Glass Tip Micropipette Electrodes

• These consist of a glass micropipette filled with an electrolyte. This type has two ionic interfaces: one inside with the metal wire and one with the cellular fluids outside.

Skin Surface Electrodes

• Designed to obtain bioelectric potentials from the skin, used for measuring ECG and EEG potentials. Earlier versions involved immersion electrodes that presented issues with positioning and electrolyte spillage.

: Floating Electrodes

• Floating electrodes prevent motion artifacts by avoiding direct metal contact with skin, using an electrolyte bridge instead. They are used with dual-sided adhesive for easy application.

: Needle Electrodes

• Needle electrodes are used in clinical settings for accessing deep tissue potentials. Two types include monopolar (single) and concentric (dual) needle electrodes.

Biochemical Transducers

Measuring Ion Concentrations

• Utilizes an indicator electrode sensitive to the target substance and a reference electrode that is less sensitive.

Reference Electrodes

• Hydrogen electrode provides a reference with a potential of zero volts but is not always stable. Alternatives include silver/silver chloride and calomel electrodes.

: pH Electrodes

• pH measures hydrogen ion concentration in solutions, with values indicating acidity or alkalinity. A glass electrode selectively measures hydrogen ions can be combined with a reference electrode for accurate pH measurements.

: Blood Gas Electrodes

• Measure partial pressures of gases like O2 and CO2 in blood. The electrode comprises a platinum wire for oxygen measurement and a similar setup for CO2 using pH measurement.

: Biomedical Transducers

Types of Transducers

• They include:

  • Force or Pressure: Piezoelectric and strain gauges.

  • Displacement/Acceleration: Variable resistance and inductance transducers.

  • Temperature: Thermocouples and thermistors.

  • Light: Photovoltaic and photoresistive devices.

: Electrocardiac System of the Heart

Cardiac Conduction System

• The heart initiates its electrical excitement through the sinoatrial (SA) node, located at the right atrium junction. The impulse spreads, triggering contraction and coordination with the atrioventricular (AV) node for rhythm.

: Electrocardiogram (ECG)

• The ECG exhibits rhythmic depolarization/repolarization of the myocardium through graphical representation. The main components include:

  • P wave: Atrial depolarization

  • QRS Complex: Ventricular depolarization

  • T wave: Ventricular repolarization

: ECG Leads and Electrodes

• Leads connect electrodes placed on patients to record ECG. Twelve leads are used to capture detailed ECG data. The standard color code helps identify electrode placement.

: Einthoven Triangle

• The triangle is a geometric representation of electrical axes in the heart based on limb lead positions. Lead configurations measured by the ECG provide a detailed understanding of heart activity.

: Instrumentation in ECG Machines

• Output from electrodes passes through a series of amplifiers, leading to chart recording. The preamplifier handles small signals while the feedback system ensures accuracy.

: Instrumentation Amplifiers

• These help overcome limitations of standard differential amplifiers. They provide high input impedance and excellent common-mode rejection ratio (CMRR), suitable for precise biomedical applications.

: Isolation Amplifiers

• Vital for ensuring patient safety by breaking conductivity between input and output. They can be transformer, optical, or capacitive to suit different applications.

: Chopper Amplifiers

• Used to minimize noise and drift in biopotential recordings. It samples data, amplifies it, and then converts it back to a usable DC signal, enhancing measurement quality.

: Question Bank for Biomedical Instrumentation

• Lists various questions focusing on key concepts such as Nernst equation, electrode types, ECG principles, instrumentation amplifiers, and more for student exam preparation.

: Assignment Questions for Further Study

• Additional assignments emphasize topics like the electro conduction system of the heart and the Nernst equation application in electrode theory