Understanding and Management Of ECG

Posted by e-Medical PPT Friday, August 5, 2011
Clinical uses of ECG
Gold standard for diagnosis of arrhythmias                                              
Often an independent marker of cardiac disease (anatomical, metabolic, ionic, or haemodynamic)                   
Sometimes the only indicator of pathological process

Limitations of ECG
It does not measure directly the cardiac electrical source or actual voltages               
It reflects electrical behavior of the myocardium, not the specialised conductive tissue, which is responsible for most arrhythmias                               
It is often difficult to identify a single cause for any single ECG abnormality

Cardiac Electrophysiology
Cardiac cellular electrical activity is governed by multiple transmembrane ion conductance changes
3 types of cardiac cells
Pacemaker cells     - SA node, AV node
Specialised conducting tissue-Purkinjie fibres
Cardiac myocytes

Cardiac Ion Channels
They are transmembrane proteins with specific conductive properties
They can be voltage-gated or ligand-gated, or time-dependent
They allow passive transfer of  Na+, K+, Ca2+, Cl- ions across cell membranes

Cardiac Ion Channels: Applications
Understanding of the cardiac action potential and specific pathologic conditions
e.g. Long QT syndrome                       
Therapeutic targets for antiarrhythmic drugs
e.g. Azimilide (blocks both components of delayed rectifier K current)

Mechanisms of Arrhythmias
Important to understand because treatment may be determined by its cause
1.Automaticity
Raising the resting membrane potential
Increasing phase 4 depolarization
Lowering the threshold potential
e.g. increased sympathetic tone, hypokalamia, myocardial ischaemia
2.Triggered activity   
from oscillations in membrane potential after an action potential
Early Afterdepolarization
Torsades de pointes induced by drugs
Delayed Afterdepolarization
Digitalis, Catecholamines
3.Re-entry
from slowed or blocked conduction
Re-entry circuits may involve nodal tissues or accessory pathways

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