CARDIAC
PACING
When patients are symptomatic because of symptomatic
bradycardia (e.g., syncope, dizziness, lethargy), a cardiac pacemaker can
effectively decrease or eliminate symptoms because it treats the
pathophysiologic problem: slow heart rate, which can result from sinus node dysfunction
or A-V block. Some devices pace only the atrium, some only the right ventricle,
and some pace both the atrium and the ventricle sequentially. Some pacemakers
also are combined with an implantable cardioverter-defibrillator (ICD), and
others can improve cardiac synchronization (e.g., BiV pacemakers).
CURRENT TRANSVENOUS PACEMAKERS
A modern
pacemaker generator (usually a lithium battery) is placed subcutaneously under
the clavicle. This generator is sealed so as not to imbibe body fluids and can
deliver electrical impulses to the electrode leads within the right atrial
appendage and right ventricle and to the left ventricle by way of a coronary
sinus electrode lead (BiV pacing). The pacemaker generator is immunologically
inert.
The pacemaker
implant procedure is performed under fluoroscopy, usually by a trained
cardiologist or cardiac surgeon. Most often, percutaneous access to the left
subclavian lead is used to pass the electrode leads into the heart, but the
right subclavian vein can be used when the left is not available. Fluoroscopy
confirms the positioning of the pacing leads in the right atrial or right
ventricular chambers and of the left ventricular epicardial lead when BiV
pacing is employed.
Pacemakers can
be of three types and are used to pace a single or multiple chambers of the
heart. A single-chamber pacemaker involves the placement of a single
lead into an atrium or ventricle, which then can sense and pace either the
atrium or the ventricle. The most common reason for pacing only the atrium is
dysfunction of the sinus node, such as sick sinus syndrome. The most frequent
reason for pacing only the right ventricle is atrial fibrillation.
In contrast, a dual-chamber
pacemaker consists of two leads inserted into the heart (see Plate 2-29). One lead can be inserted into the right
atrial appendage and fixated in that position, pacing the atrium. Another
pacing lead can be inserted into the right ventricle and fixated in that
position. When activated and functioning, these two pacing leads pace the
atrium and ventricle sequentially, closely simulating the natural conduction
system of the heart. This type of pacing is the most common type currently
used.
A rate-responsive
pacemaker is usually a dual-chamber pacemaker (can be single chamber) that
responds to increased demand for an increased heart rate. The patient’s
increased physical activity (exercise) increases pectoral muscle activity,
which is sensed by the pacemaker generator. Once this physical activity is
detected, the pacemaker increases the rate of electrical impulses and increases
heart rate to meet the patient’s physiologic demands for increased cardiac
output. Rate- responsive pacemakers are often used for patients with
symptomatic bradycardia.
In contrast to
standard two-lead A-V sequential pacing, biventricular (BiV) pacing consists of
three leads: RA appendage lead, RV lead, and a lead introduced into the
coronary sinus and advanced to a lateral vein on the epicardial surface of the
LV free wall. The position of the epicardial lead corresponds to the position
of an obtuse marginal artery. This lead system, in addition to A-V pacing,
enables pacing of the LV free wall (see Plate 2-29).
Cardiac
resynchronization therapy (CRT) uses BiV pacing because the lead system paces
both the septal LV wall and the lateral LV wall. When viable tissue is present
in these areas, the left ventricle resynchronizes the contraction of a heart
whose opposing walls do not contract in synchrony. Dyssynchrony frequently
occurs in patients with systolic heart failure, many with QRS duration longer
than 120 msec, which qualifies a patient for CRT. These patients are at high
risk for sudden cardiac death because many have an LV ejection fraction of 35%
or less. The BiV lead system can be combined with an ICD to prevent sudden
cardiac death from ventricular tachycardia or ventricular fibrillation, a
common companion to severe heart failure.
