Anatomy and Physiology of the Heart

Anatomy of the Heart – The heart is situated between the lungs, at the center of the chest just under the rib cage. The heart is a muscular organ that is approximately the size of a human fist (see Figure 1.1).

The Cardiac Cycle – The cardiac cycle includes the stages of contraction and relaxation between the atria and ventricles that occur in one heart beat (see Figure 1.2).

Electrophysiology – The heart’s role in the cardiovascular system is to ensure the uninterrupted circulation of blood in the body. Blood contains oxygen and nutrients that must be distributed throughout the body. Blood also carries waste away from the tissues for excretion.

Electrocardiography: The Basics

Electrocardiography – Electrical signals from the heart can be measured from the surface of the skin by attaching electrodes from an electrocardiogram (ECG) machine to the skin (see Figure 2.1). An ECG grid displays the time versus amplitude of electrical vector projections, which represent the conduction system of the heart.

Approach to ECG Interpretation – Arrhythmias are often categorized by the origin of the pacemaker impulse. These pacemakers are sinus (sinoatrial node), atrial (atrioventricular node), junctional or ventricular. The usual pacemaker of the heart (see Part 1) is the sinoatrial node. Hence, the normal heart has a normal sinus rhythm. Normal ECG patterns vary from patient to patient, and can change in an instant due to certain conditions or illnesses. It is therefore important to use a systematic approach when interpreting ECGs.

Rhythms Originating from the Sinus Node

Sinus Node Rhythms – Normal sinus rhythm is the normal rhythmic pattern of the healthy heart (see Figure 3.1); it is not an arrhythmia.

Sinoatrial Node Dysfunction – The sinoatrial node comprises a group of pacemaker cells or P cells whose activities are integrated. Perinodal cells or T cells are responsible for transmitting the impulses generated from the sinoatrial node to the right atrium. Sinoatrial node dysfunction is the result of abnormal impulse generation from the pacemaker cells or abnormal conduction of electrical activity by the perinodal cells. This disease is usually acquired; it is rarely inherited. Its manifestation may be asymptomatic or highly symptomatic, as in the case of sick sinus syndrome.

Rhythms That Originate from the Atria

Rhythms That Originate from the Atria – As discussed in the earlier sections of this course, if the sinoatrial node fails to generate an impulse, a failsafe mechanism in the heart switches to the next fastest inherent rate. Usually, the atrial foci take over the sinoatrial node; the resulting groups of ECG anomalies are known as the atrial arrhythmias. Impulses generated from the atria travel towards the Purkinje fibers by way of the atrioventricular junction first. They then travel towards the ventricles.

Atrioventricular Junctional Rhythms

Atrioventricular Junctional Rhythms and Heart Block – In the lower region of the atrioventricular junction, where the atrioventricular node meets the bundle of His, there is a group of pacemaking cells responsible for the generation of junctional rhythms, or atrioventricular junctional rhythms. The impulse generated from the atrioventricular junction produces an unusual pattern of conduction. Since the impulse is generated in the middle of the heart, the atria and ventricles have a tendency to depolarize at almost the same time, because the impulse travels in two directions. This affects the impulse deflection on the ECG.

Atrioventricular Block – Atrioventricular (AV) blocks are conduction delays or a complete block of impulses from the atria into the ventricles. AV blocks can be caused by an increased vagal tone that occurs during sleep, athletic training or stimulation of the carotid sinus. Damage to the conduction system that is secondary to hereditary fibrosis or sclerosis of the cardiac skeleton is known as idiopathic progressive cardiac conduction disease. Ischemic heart disease causes 40% of AV blocks. AV blocks are also seen in cardiomyopathies, myocarditis, congenital heart diseases and familial diseases. A plasma potassium concentration above 6.3 mEq/L may also cause AV blocks, and they can be iatrogenic from medications such as verapamil, diltiazem, amiodarone and adenosine, or from cardiac surgeries and catheter ablations performed to treat arrhythmias.

Ventricular Rhythms

Ventricular Rhythms – The QRS complex corresponds to ventricular depolarization. Ventricular rhythms comprise impulses generated from the ventricles and have a QRS complex that is 120 milliseconds or longer. Other supraventricular arrhythmias may also prolong the QRS complex, but only if there is a disturbance in the conduction of supraventricular impulses to the ventricles.

Glossary

Glossary