Ventricular demand pacemakers are found in
patients who: are physically inactive, regardless of age, and therefore do not
require rate variability; have chronic atrial fibrillation or flutter, or giant,
silent atria; or have mental incapacity or terminal illnesses that make
dual-chambered pacing impractical. Another type of unit, atrioventricular
sequential pacemakers (DVI), is capable of pacing in both the atrium and
ventricle, senses only in the ventricle, and is inhibited by ventricular events.
Most AV sequential pacemakers are noncommitted. After a brief blanking period of
30 to 50 milliseconds following an atrial stimulus, sensing is continuous during
the AV interval. Therefore, noncommitted DVI pacing systems may pace atrium and
ventricle both, or atrium only, or be totally inhibited, depending on where the
R wave is detected with respect to the pulse generator's timing cycle. The ECG
in a DVI pacemaker shows a sharp spike before each P wave on paced atrial beats
and before each QRS on paced ventricular beats. The atrial and ventricular
spikes are separated by a present or programmable AV interval. Patients who have
a sick sinus syndrome accompanied by AV nodal or His-Purkinje disease or an AV
block with abnormal sinus node function and lack of ability to increase atrial
rate with exercise typically benefit from these pacemakers. They are also useful
in patients who have developed pacemaker syndrome with single-chambered
ventricular demand units, since the normal atrioventricular relationship is then
restored. A third, more commonly used type of pacemaker is the DDD pacemaker. A
DDD pacemaker can sense intrinsic activity in the atrium and ventricle, pace
either or both chambers when not inhibited by native activity, and thereby
maintain atrioventricular synchrony over a wide range of heart rates. DDD units
are noncommitted employing an atrial blanking period following atrial stimuli to
avoid sensing of such events on the ventricular channel. All such pacemakers
have upper rate characteristics and blocking modes to prevent 1:1 conduction
during atrial arrhythmias such as flutter and fibrillation. Virtually all such
devices are extensively programmable, and most have the ability to telemeter
both programmed and real-time parameters. One of the major initial problems
encountered with DDD pacing is pacemaker-mediated tachycardia, which is where
the pacemaker acts as one limb of a re-entrant circuit.
However, this has been
solved by the ability to program the interval at which atrial sensing resumes
after a ventricular sensed or paced event. Normally, this device sequentially
paces both the atrium and ventricle when atrial activity falls below the preset
base rate and atrial pacing is not followed by a ventricular event. When the
patient's intrinsic atrial activity exceeds the base rate, and if a spontaneous
QRS does not occur within the programmed AV interval, the pacemaker switches to
an atrial sensing-ventricular pacing mode. In this case, the ECG shows a P wave
that is followed by a sharp spike and a paced QRS. Sensed ventricular events
inhibit both atrial and ventricular output and reset the atrial escape interval.
The DDD pacemakers are found in patients who possess: AV block with or without
sinus node dysfunction; or moderate sick sinus syndrome and AV nodal or
His-Purkinje disease, with at least some ability to increase atrial rate with
exercise. Surgical implantation of cardiac pacemakers has dramatically improved
over the years. During the late 1950's and early 1960's when artificial pacing
was first being implemented, patients with severe Stokes-Adams attacks received
some of the first battery operated pacemakers developed by William M. Chardack,
chief of thoracic surgery at the Veterans Administration hospital and his
colleague Wilson Greatbatch. Physicians who implanted pacemakers in these
patients reported numerous serious failures that required new operation: broken
or dislodged leads, premature battery depletion, and leakage of body fluids into
the pulse generator. Yet despite the problems, pacemakers proved effective at
giving people months or years of life that they would not otherwise have
enjoyed. The operative procedure during this particular era was carried out
under general anesthesia with an endotracheal tube in place. Patients undergoing
surgery were under the control of an external pacemaker with a cardiac electrode
catheter passing through the right saphenous vein. Electrocardiographic leads
were attached to the arms and legs, and a continuous ECG was displayed on an
oscilloscope. Two incisions were made: a six-inch incision near the umbilicus
(naval) and a left sub mammary incision. A twin lead was passed up a
subcutaneous tunnel, which connects the chest and abdominal incisions to the
pericardium. The two electrodes were separated and implanted in the myocardium.
