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Learning Modules The Conduction System ECG Basics How to Analyze a Rhythm Normal Sinus Rhythm Heart Arrhythmias Advanced 12-Lead Interpretation
Generating rhythmical electrical impulses to cause rhythmical contraction of the heart muscle (2) Conducting these impulses rapidly through the heart The Conduction System
Depolarization and Impulse Conduction Heart is autorhythmic Depolarization begins in sinoatrial (SA) node Spread through atrial myocardium Delay in atrioventricular (AV) node
Depolarization and Impulse Conduction Spread from atrioventricular (AV) node AV bundle Bundle branches Purkinje fibers
Specialized Excitatory and Conductive System of the Heart Sinoatrial node Internodal pathways AV node A-V Bundle (Bundle of His) Bundle Branches Purkinje fibers
Sinoatrial (also called sinus or S-A node, in which the normal rhythmical impulse is generated Internodal pathwaysthat conduct the impulse from the sinus node to the atrioventricular (A-V) node A-V node, in which the impulse from the atria is delayed before passing into the ventricles
A-V bundle, which conducts the impulse from the atria into the ventricles Left and right bundle branches of Purkinje fibers, which conduct the cardiac impulse to all parts of the ventricles
The impulse reaches the A-V node about 0.03 second after its origin in the sinus node Then there is a delay of another 0.09 second in the A-V node A final delay of another 0.04 second occurs mainly in penetrating of the A-V bundle
The total delay in the A-V nodal and A-V bundle system is about 0.13 second. This, in addition to the initial conduction delay of 0.03 second from the sinus node to the A-V node This makes a total delay of 0.16 second before the excitatory signal finally reaches the contracting muscle of the ventricles
The total time for transmission of the cardiac impulse from the initial bundle branches to the last of the ventricular muscle fibers in the normal heart is about 0.06 second.
The Spread of the Cardiac Impulse Through the Heart
The impulse spreads through the atria but is delayed more than 0.1 second in the A-V nodal region before appearing in the ventricular septal A-V bundle
Once it has entered this bundle, it spreads very rapidly through the Purkinje fibers to the entire endocardial surfaces of the ventricles
Then the impulse spreads through the ventricular muscle to the epicardial surfaces
Pacemakers of the Heart SA Node - Dominant pacemaker with an intrinsic rate of 60 - 100 beats/minute. AV Node - Back-up pacemaker with an intrinsic rate of 40 - 60 beats/minute. Purkinje fibers- Back-up pacemaker with an intrinsic rate of 20 - 45 bpm.
Atrial Muscle (Nodal) Action Potential Outward K+ current (repolarization) Inward Na+ and Ca++ ions (depolarization) Threshold Slow spontaneous Inward Na+ ions Automaticity - a pacemaker cell’s ability to spontaneously depolarize, reach threshold, and propagate an AP
Self-Excitation of Sinus Nodal Fibers Sodium ions from outside the fibers tend to leak to the inside Influx of positively charged sodium ions causes a slow rise in the resting membrane potential in the positive direction Sodium- Calcium channels become activated and causing the action potential Influx of sodium and calcium ions then ceases while large quantities of positive potassium ions diffuse out of the fiber
Normal Electrocardiogram The electrocardiogram (EKG) is a representation of the electrical events of the cardiac cycle The electrical potentials generated by the current can be recorded; the recording is known as an electrocardiogram It is most valuable diagnostic tools for the recognition of large variety of cardiac disorders