Cardiac Conduction System Flashcards
What does conduction begin with?
Conduction begins with the sinoatrial (SA) node (also called the sinus node)
Where is the sinoatrial node located?
Located close to the surface of the right atrium near its junction with the superior vena cava.
What is the sinoatrial nodes’ responsibility/role?
- The SA node is the heart’s primary pacemaker. It can spontaneously and rhythmically generate electrical impulses at a rate of 60 to 100 beats/min and therefore has the greatest degree of automaticity (pacing function). The SA node is richly supplied by the sympathetic and parasympathetic nervous systems, which increase and decrease the rate of discharge of the sinus node, respectively. This process results in changes in the heart rate.
- Impulses from the sinus node move directly through atrial muscle and lead to atrial depolarization, which is reflected in a P wave on the electrocardiogram (ECG). Atrial muscle contraction should follow. Within the atrial muscle are slow and fast conduction pathways leading to the atrioventricular (AV) node.
What does the atrioventricular junction consist of?
The atrioventricular (AV) junction consists of a transitional cell zone, the AV node itself, and the bundle of His.
Where is the atrioventricular node located?
The AV node lies just beneath the right atrial endocardium, between the tricuspid valve and the ostium of the coronary sinus.
What is the role/responsibility of the atrioventricular node?
Here T-cells (transitional cells) cause impulses to slow down or be delayed in the AV node before proceeding to the ventricles. This delay is reflected in the PR segment on the ECG. This slow conduction provides a short delay, allowing the atria to contract and the ventricles to fill. The contraction is known as atrial kick and contributes additional blood volume for a greater cardiac output. The AV node is also controlled by both the sympathetic and parasympathetic nervous systems. The bundle of His connects with the distal portion of the AV node and continues through the interventricular septum.
Where is the ‘bundle of his’ located?
The bundle of His extends as a right bundle branch down the right side of the interventricular septum to the apex of the right ventricle. On the left side, it extends as a left bundle branch, which further divides.
At the ends of both the right and the left bundle branch systems are the Purkinje fibers.
Where are the purkinje fibers located?
These fibers are an interweaving network located on the endocardial surface of both ventricles, from apex to base.
What is the role/responsibility of the purkinje fibers?
The fibers partially penetrate into the myocardium. Purkinje cells make up the bundle of His, bundle branches, and terminal Purkinje fibers. These cells are responsible for the rapid conduction of electrical impulses throughout the ventricles, leading to ventricular depolarization and the subsequent ventricular muscle contraction. A few nodal cells in the ventricles also occasionally demonstrate automaticity, giving rise to ventricular beats or rhythms.
What are the electrophysiologic properties?
The electrophysiologic properties of these cells regulate heart rate and rhythm and possess unique properties: automaticity, excitability, conductivity, and contractility.
What is automaticity?
Automaticity (pacing function) is the ability of cardiac cells to generate an electrical impulse spontaneously and repetitively. Normally only the sinoatrial (SA) node can generate an electrical impulse. However, under certain conditions, such as myocardial ischemia (decreased blood flow), electrolyte imbalance, hypoxia, drug toxicity, and infarction (cell death), any cardiac cell may produce electrical impulses independently and create dysrhythmias. Disturbances in automaticity may involve either an increase or a decrease in pacing function.
What is excitability?
Excitability is the ability of nonpacemaker heart cells to respond to an electrical impulse that begins in pacemaker cells.
What is depolarization?
Depolarization occurs when the normally negatively charged cells within the heart muscle develop a positive charge.
What is conductivity?
Conductivity is the ability to send an electrical stimulus from cell membrane to cell membrane. As a result, excitable cells depolarize in rapid succession from cell to cell until all cells have depolarized. The wave of depolarization causes the deflections in the ECG waveforms that are recognized as the P wave and the QRS complex. Disturbances in conduction result when conduction is too rapid or too slow, when the pathway is totally blocked, or when the electrical impulse travels an abnormal pathway.
What is contractility?
Contractility is the ability of atrial and ventricular muscle cells to shorten their fiber length in response to electrical stimulation, causing sufficient pressure to push blood forward through the heart. In other words, contractility is the mechanical activity of the heart.
