Modern pacemakers all have two functions. They listen to the heart's native electrical rhythm, and if the device doesn't sense any electrical activity within a certain time period, the device will stimulate the heart with a set amount of energy, measured in joules.
Advances in pacemaker function
When first invented, pacemakers controlled only the rate of speed at which the heart's two largest chambers, the ventricles, beat.
More recently, pacemakers which control not only the ventricles but the atria as well have become common. Timing the contractions of the atria to precede that of the ventricles improves the pumping efficiency of the heart and can be useful in congestive heart failure.
Another advancement in pacemaker technology is left ventricular pacing. A pacemaker wire is placed on the outer surface of the left ventricle, with the goal of more physiological pacing than what is available in standard pacemakers. This extra wire is implanted to improve symptoms in patients with severe heart failure.
Devices with pacemaker function
Sometimes devices resembling pacemakers, called ICDs (implantable cardioverter-defibrillators) are implanted. These devices have the ability to treat dangerously fast rhythm disturbances of the heart, either via pacing or defibrillation. Many of these can also treat slow heart rhythms the same way as pacemakers.
Indications for pacing
In most cases, the indication for permanent pacemaker placement is a slow heart rate (bradycardia) or a defect in the electrical conduction system of the heart (heart block) that causes the person symptoms. Typical symptoms that are associated with a slow heart rate include lightheadedness, poor exercise tolerance, and loss of consciousness.
Pacemakers can also be placed in patients that are at high risk for one of these slow heart rhythms. Rarely, in people that are prone to ventricular fibrillation, a slow rhythm in the heart can lead to a ventricular fibrillation. In these people, preventing the slow rhythm can prevent ventricular fibrillation.
Methods of pacing
External pacemakers can be used for initial stabilization of a patient, but implantation of a permanent pacemaker is usually required for most conditions. External cardiac pacing is typically performed by placing two pacing pads on the chest wall. Usually one pad is placed on the upper portion of the sternum, while the other is placed along the left axilla, near the bottom of the rib cage. When an electrical impulse goes from one pad to the other, it will travel through the tissues between them and stimulate the muscles between them, including cardiac muscle and the muscles of the chest wall. Stimulating any muscle, including the heart muscle, will make it contract. The stimulation of the muscles of the chest wall will frequently make those muscles twitch at the same rate as the pacemaker is set.
Pacing the heart via external pacing pads should not be relied upon for an extended period of time. If the person is conscious, he or she may feel discomfort due to the frequent stimulation of the muscles of the chest wall. Also, stimulation of the chest wall muscles does not necessarily mean that the heart is being stimulated as well.
Temporary internal pacing
An alternative to external pacing is the temporary internal pacing wire. This is a wire that is placed under sterile conditions via a central line. The distal tip of the wire is placed into either the right atrium or right ventricle. The proximal tip of the wire is attached to the pacemaker generator, outside of the body. Temporary internal pacing is often used as a bridge to permanent pacemaker placement. Under certain conditions, a person may require temporary pacing but would not require permanent pacing. In this case, a temporary pacing wire may be the optimal treatment option.
Permanent pacemaker placement
Placement of a permanent pacemaker involves placement of one or more pacing wires within the chambers of the heart. The distal tips of these wires are fixated to the muscle of the heart to prevent their accidental dislodgement. The proximal portions of these wires are screwed into the pacemaker generator. The pacemaker generator is a hermetically sealed device containing a power source and the computer logic for the pacemaker.
Most commonly, the generator is placed below the subcutaneous fat of the chest wall, superficial to the muscles and bones of the chest. However, the placement may vary on a case by case basis.