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A pacemaker (or "artificial pacemaker", so as not to be confused with
the heart's natural pacemaker) is a medical device designed to
regulate the beating of the heart. The purpose of an artificial
pacemaker is to stimulate the heart when either the heart's native
pacemaker is not fast enough or if there are blocks in the heart's
electrical conduction system preventing the propagation of electrical
impulses from the native pacemaker to the lower chambers of the heart,
known as the ventricles.
History of the mechanical pacemaker
The first pacemaker was designed and built by the Canadian
electrical engineer John Hopps in 1950, a substantial external device
it was somewhat crude and also painful for the patient in use. A number
of inventors, including Paul Zoll, made smaller but still bulky
devices in the following years. One of the first true implantable
pacemakers was completed in 1958 by the American Wilson Greatbatch.
All the early pacemakers utilized transistors.
The first pacemakers required wires (called leads) to be placed
surgically on the outer surface of the heart. In the mid 1960s, the
first transvenous leads were placed. This allowed the placement of
pacemakers without opening the thoracic cavity and therefore without
the use of general anesthesia.
The first American-made nuclear powered pacemaker was developed and
implanted at Newark Beth Israel Medical Center in Newark, New Jersey.
Basic pacemaker function
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.
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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.