Overview of Pacemakers
Pacemakers are battery-powered implantable devices that electrically stimulate the heart to contract and thus to pump blood throughout the body. Pacemakers consist of a pager-sized housing device that contains a battery, the electronic circuitry that runs the pacemaker, and one or two long thin wires that travel through a vein in the chest to the heart. Pacemakers are usually implanted in patients when the heart's own "spark plug" or electrical system no longer functions normally.
The Heart and the Heart's Electrical System
To explain why some patients require pacemakers and how these devices work, it is worth briefly discussing how the heart and its electrical system function. The heart is divided into right and left sides. Each side has a chamber that receives blood returning to the heart (called atria) and a muscular chamber that is responsible for pumping blood out of the heart (called ventricles).
Blood that has traveled to parts of the body and is now oxygen-poor returns to the heart and enters the right atrium. This blood is then pumped by contraction of the right atrium into the right ventricle. The right ventricle, in turn, contracts and pumps this blood to the lungs, where it absorbs oxygen. The oxygen-rich blood then returns from the lungs to the heart and enters the left atrium, where it is pumped into the left ventricle. The left ventricle is the most muscular of the heart's four chambers and serves as the main pumping chamber of the heart.
When the muscular tissue of the left ventricle contracts, blood is pumped into the aorta, the main artery of the body that supplies blood to other arteries. Oxygen-rich blood travels through the aorta to other arteries of the body and then, in turn, to the organs and tissues of the body, which require oxygen to function. Oxygen-poor blood returns from these organs and tissues through the veins back to the right atrium of the heart, and the cycle repeats.
The contractions of the different chambers of the heart are normally organized in a very specific mannera special type of electrical impulse travels through the heart and sets off contractions in the chambers as it passes through the chambers. Figure 2 shows the normal sequence by which this electrical impulse travels through the heart.
The heart's normal "spark plug" is an area of specialized heart tissue called the SA node, which is located in the right atrium. Each time this tissue "fires," an electrical impulse is generated that travels first through the right and left atria, signaling these chambers to contract and pump blood into the ventricles.
The impulse then travels down into another small patch of specialized heart tissue called the AV node, which is located between the atria and the ventricles. The electrical impulse is conducted through the AV node and then through specialized wire-like pathways into the ventricles, where it signals the ventricles to contract and to pump blood out into the lungs and throughout the body. This normal sequence of electrical activation of the chambers of the heart is called sinus rhythm. It occurs each time the heart beats, usually about 60 to 80 times every minute.