Laboratory 2.3 Intrinsic Conduction System of the Heart

Intrinsic conduction system of the heart

Comprised of specialized cardiac cells that depolarize to initiate and coordinate the electrical impulses that stimulate heartbeats and ensure synchronized contraction

Electrocardiography (EKG/ECG) purpose

Measures the electrical activity of the heart, allowing analysis of cardiac physiology using the conduction system

Sinoatrial (SA) node location

Located by the entrance of the superior vena cava in the right atrium

SA node as pacemaker

Considered the pacemaker because it spontaneously generates electrical impulses; without autonomic nervous system input, it sets a heart rate of 60-100 beats per minute in a resting adult

SA node impulse spread

The SA node's electrical impulses spread through the myocardium of the atria, causing them to contract, and the signal then reaches the AV node

Atrioventricular (AV) node location

Located above the right AV valve by the atrial septum

AV node function

The only electrical connection between the atria and the ventricles; delays sending its impulse, slowing the electrical signal before it passes to the ventricles

Purpose of AV node delay

Ensures the atria have enough time to fully contract, completely filling the ventricles before the lower chambers begin their contraction

Bundle of His

A pathway along the interventricular septum that carries the electrical impulse after the AV node; splits into right and left bundle branches extending along the septum to the apex of the heart

Purkinje fibers (conduction system)

Carry the impulse from the bundle branches and distribute the electrical impulse to the ventricular myocardium

EKG recording nature

The EKG does not record contractions or activity at specific nodes directly, but infers them from the stepwise depolarization and repolarization recorded as a series of waves corresponding to the cardiac cycle

Clinical use of EKG

A diagnostic tool healthcare professionals use to assess the heart's electrical activity and diagnose cardiac abnormalities such as arrhythmias, heart block, or myocardial infarction by analyzing wave pattern, frequency, and size

P wave

The first wave in the EKG recording, associated with atrial depolarization; typically a small, rounded wave

PR interval

Represents the period from the beginning of the P wave to the beginning of the QRS complex, when the electrical signal travels from the SA node to the AV node

QRS complex

A large complex representing depolarization (contraction) of the ventricles, comprised of three individual waves (Q, R, S); atrial repolarization also occurs during this time but is masked by the larger ventricular signal

ST segment

Connects the end of the QRS complex to the beginning of the T wave; represents the time between ventricular depolarization and repolarization

T wave

Associated with repolarization and relaxation of the ventricles; typically a smaller, rounded wave following the QRS complex

QT interval

Starts at the Q wave and ends after the T wave; extends through ventricular depolarization and repolarization during contraction and relaxation

Normal sinus rhythm

The correct pattern of EKG waves with a heart rate of 60-100 bpm

Junctional rhythm

Occurs when the SA node is not initiating depolarization and the AV node takes over; creates a slower heart rate (40-60 bpm) and the absence of the P wave

Heart block (general)

A delay or complete block in the conduction system

First-degree heart block

A delay in signal conduction where the signal still reaches the ventricles, characterized by a lengthened PR interval

Second-degree heart block

A block where there is a dropped beat or some impulses fail to reach the ventricles, resulting in more P waves than QRS complexes

Complete (third-degree) heart block

A block where the signal from the atria does not reach the ventricles at all; the atria and ventricles depolarize independently with different rhythms; a life-threatening condition

Ventricular fibrillation

Erratic electrical impulses that do not coordinate effective contraction to pump blood; life-threatening and requires immediate defibrillation

Defibrillation

An electrical shock given to the heart to restart coordinated electrical impulses, used to treat life-threatening arrhythmias like ventricular fibrillation

Cardiac arrhythmias (EKG diagnostic role)

Abnormal patterns of cardiac electrical activity; momentary arrhythmias are not inherently dangerous, but clinical problems arise when arrhythmias reduce the pumping efficiency of the heart

Premature atrial contractions (PACs)

Often occur in healthy individuals; the normal atrial rhythm is momentarily interrupted by a "surprise" atrial contraction; stress, caffeine, and various drugs may increase their incidence

Paroxysmal atrial tachycardia (PAT)

A premature atrial contraction triggers a flurry of atrial activity; the ventricles are still able to keep pace, and the heart rate jumps to about 180 beats per minute

Atrial fibrillation (AF)

Impulses move over the atrial surface at rates of perhaps 500 beats per minute instead of producing an organized contraction; the ventricular rate cannot follow the atrial rate and may remain within normal limits

Premature ventricular contractions (PVCs)

Occur when a Purkinje cell or ventricular myocardial cell (an ectopic pacemaker) depolarizes to threshold and triggers a premature contraction; single PVCs are common and not dangerous

Ventricular tachycardia (VT/V-tach)

Defined as four or more PVCs without intervening normal beats; multiple PVCs and VT may indicate serious cardiac problems

Ventricular fibrillation (VF) clinical significance

Responsible for the condition known as cardiac arrest; VF is rapidly fatal because the ventricles quiver and stop pumping blood