Understanding and Management Of ECG

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