A pacemaker is a small device that sends small electrical impulses to the heart muscle to maintain a suitable heart rate or to stimulate the lower chambers of the heart (ventricles). A pacemaker may also be used to treat fainting spells and congestive heart failure.
Pacemakers consist of a pager-sized housing device, which contains a battery and electronic circuitry, along with one or two long thin electrical wires that travel from the pacemaker-housing device to the heart. The housing device is implanted below the skin in the shoulder area. The thin wires, which can conduct electrical impulses, are then threaded from the housing device through a vein that runs in the chest, on to the heart. In some patients, only one of these long, thin electrical wires, called leads, is implanted into one of the chambers of the heart. Most patients who receive pacemakers will have two leads implanted, one going to the right atrium of the heart and one going to the right ventricle.
The pacemaker and leads can be programmed in various and often complex ways to analyze the heartbeat and then to decide if the pacemaker should electrically stimulate the heart to contract.
In the most common manner that pacemakers are now programmed, the electrical leads that are implanted in the right atrium and/or right ventricle can perform two functions. They can serve as sensors, detecting if electrical impulses generated by the SA node have occurred and if such electrical impulses have been conducted by the AV node down into the ventricle. These same electrical leads can also be used to transmit an electrical impulse from the pacemaker’s battery down into the right atrium and/or right ventricle. If the lead implanted into the right atrium does not detect that the SA node has fired and created an electrical impulse, the pacemaker itself will send an electrical impulse to the right atrium, taking over the function of being the heart’s spark plug.
If the lead implanted into the right ventricle does not detect that an electrical impulse has made it through the AV node down into the ventricle, the pacemaker will generate an electrical impulse that is conducted via the electrical lead in the right ventricle to the ventricles. In this manner, the pacemaker can supervise the heart and ensure that it continues to contract at a heart rate adequate to pump sufficient blood throughout the body.