Back to basics...

Basic anatomy of the heart

The heart is a 4-chambered muscle whose function is to pump blood throughout the body.

• The heart is really 2 "half hearts," the right heart and the left heart, which beat simultaneously.
• Each of these 2 sides has 2 chambers: a smaller upper chamber called the atrium (together, the 2 are called atria), and a larger lower chamber called the ventricle.
• Thus, the 4 chambers of the heart are called the right atrium, right ventricle, left atrium, and left ventricle.

Heart Function & the ECG

The cardiac cycle

Diastole Phase:

• Atrioventricular valves are open.
• The sinoatrial (SA) node, which starts cardiac conduction, contracts causing atrial contraction.
• The atria empty blood into the ventricles.
• Semilunar valves close preventing back flow into the atria.

Systole Phase:

• The ventricles contract.
• Atrioventricular valves close and semilunar valves open.
• Blood flows to either the pulmonary artery or aorta.

The heart normally beats between 60 and 100 times per minute, with many normal variations. For example, athletes at rest have slower heart rates than most people. This rate is set by a small collection of specialized heart cells called the sinoatrial (SA) or sinus node.

Located in the right atrium, the sinus node is the heart's "natural pacemaker."

• It has "automaticity," meaning it discharges all by itself without control from the brain.
• Two events occur with each discharge: (1) both atria contract, and (2) an electrical impulse travels through the atria to reach another area of the heart called the atrioventricular (AV) node, which lies in the wall between the 2 ventricles.
• The AV node serves as a relay point to further propagate the electrical impulse.
• From the AV node, an electrical wave travels to both ventricles, causing them to contract and pump blood.
• The normal delay between the contraction of the atria and of the ventricles is 0.12 to 0.20 seconds. This delay is perfectly timed to account for the physical passage of the blood from the atrium to the ventricle. Intervals shorter or longer than this range indicate possible problems.

The ECG thus detects the electrical activity of the heart!