provide electrical stimuli to cause cardiac contraction
during periods when intrinsic cardiac electrical activity
is inappropriately slow or absent.
Pacing systems consist of a pulse generator and pacing
leads. With permanent systems, endocardial leads are
inserted transvenously and advanced to the right
ventricle and/or atrium where they are implanted into
the myocardial tissue. The pulse generator is placed
subcutaneously or submuscularly in the chest wall.
Pulse generators contain a battery as well as sensing,
timing and output circuits. The battery (most
commonly lithium-iodide) typically has a life span of 510 years.
Temporary systems use an external pulse generator
with leads placed either transcutaneously or
transvenously. Transcutaneous leads are the easiest
and most convenient to use for rapid application of
Pulse generators can be set to fixed-rate
(asynchronous) or demand (synchronous) modes. In
the asynchronous mode, impulses are produced at a
set rate independent of intrinsic cardiac activity. This
mode carries a small but inherent danger of producing
lethal dysrhythmias should the impulse coincide with
the vulnerable period of the T wave. In the
synchronous mode, the sensing circuit searches for an
intrinsic depolarization potential. If this is absent, a
pacing response is generated. This mode closely
mimics intrinsic myocardial electric activity.
On demand pacemakers detect spontaneous
ventricular activity and the output of the pacemaker is
either suppressed or discharged in order to make the
impulse fall within the safe period of the QRS complex.
Temporary pacemakersTemporary systems use an external pulse generator
with leads placed via transvenous or transcutaneous
approaches. Transcutaneous leads are the easiest to
use but transvenous leads are the most reliable.
For transvenous temporary pacing, catheters are
inserted through a central venous access and usually
require fluoroscopy for proper placement. Semifloating or flexible balloon-tipped catheters can be
used in emergencies since they can be positioned using
blood flow alone.
Transcutaneous external cardiac pacing is beneficial for
symptomatic bradycardia and as a consideration for
asystole. They also may be of some benefit for
overdrive pacing in treatment of certain tachycardias.
Most current models have fixed rate and synchronous
pacing. Pacing should generally be started in the
Types of pacemaker
• Permanent leads are either unipolar (where a
single contact is made with the heart) or bipolar.
• Unipolar systems (ventricular) are used in cases
where AV (atrio-ventricular) conduction is likely to
• When there is normal AV conduction and a SA
(sino-atrial) disorder, then the pacing wire is
situated in the right atrium.
Dual chamber pacemakers:
• Have pacing electrodes in both the right atrium
and the right ventricle.
• They allow maintenance of the physiological
relationship between atrial and ventricular
contraction and also allows the paced heart to
follow the increase in sinus rate that occurs during
Dual-site atrial pacing:
• Newer pacing systems have 2 atrial leads, one in
the right atrial appendage and the other either in
the coronary sinus or at the os of the coronary
• The ventricular lead is in the right ventricle, either
at the apex or at the outflow tract.
• This system has been proposed as a promising
treatment option for prevention of paroxysmal
• Pacemaker leads are placed in the right atrium,
right ventricle and left ventricle.
• Useful in the management of patients with heart
failure who have evidence of abnormal
interventricular conduction (most often evident as
left bundle branch block on ECG) which causes
deranged ventricular contraction or dyssynchrony.
Implantable cardioverter defibrillators (ICDs combined with internal defibrillator):
• Designed to directly treat a cardiac
• If a patient has a ventricular defibrillator and the
device senses a ventricular rate that exceeds the
programmed cut-off rate of the defibrillator, the
device performs cardioversion/defibrillation.
• Alternatively, the device, if so programmed, may
attempt to pace rapidly for a number of pulses,
usually around 10, to attempt pace-termination of
a ventricular tachycardia.
Pacemaker codes- It usually consists of three letters,
but some systems use four or five:
Letter 1: chamber that is paced (A=atria, V=ventricles,
Letter 2: chamber that is sensed (A=atria, V=ventricles,
D=dual chamber, 0=none)
Letter 3: response to a sensed event (T=triggered,
I=inhibited, D=dual - T and I, R=reverse)
Letter 4: rate responsive features; an activity sensor,
e.g. an accelerometer in the pulse generator, in single
or dual chamber pacemakers detects bodily movement
and increases the pacing rate according to a
programmable algorithm (R=rate responsive
Letter 5: Anti-tachycardia facilities.
Indications for pacing:
Indications for a temporary pacemaker:
1)Acute myocardial infarction with:
• Symptomatic bradycardia (sinus bradycardia with
hypotension and type I 2nd degree AV block with
hypotension not responsive to atropine)
• Bilateral bundle branch block (alternating BBB or
RBBB with alternating LAHB/LPHB)
• New or indeterminate age bifascicular block with
first degree AV block
• Mobitz type II second degree AV block
• After an anterior myocardial infarction, a
pacemaker may be used to prevent bi- or trifascicular block, second or third degree AV block. A
pacemaker is only indicated in an inferior
myocardial infarction if these conduction
disturbances are present
2)Bradycardia not associated with acute myocardial
• 2nd or 3rd degree AV block with haemodynamic
compromise or syncope at rest
• Ventricular tachyarrhythmias secondary to
3)Suppression of drug-resistant ventricular
tachyarrhythmia or supraventricular tachycardia
4)Drug overdose, e.g. digoxin, b-blockers, verapamil
1)Support for procedures that may promote
2)General anaesthesia with:
• 2nd or 3rd degree AV block
• Intermittent AV block
• 1st degree AV block with bifascicular block
• 1st degree AV block and LBBB
• Aortic surgery
• Tricuspid surgery
• Ventricular septal defect closure
• Ostium primum repair
4)Rarely considered for coronary angioplasty
(usually to right coronary artery) but may be
required for angioplasty-induced bradycardia
Indications for a permanent pacemaker
• Persisting symptomatic bradycardia.
• Complete AV block (Stokes-Adams attacks,
asymptomatic, congenital), Mobitz type II AV
block, persistent AV block post anterior MI.
• Pacemakers may have a role in the suppression of
• Prevention of atrial fibrillation.
• Pacemakers have a role in the management of
some patients with dilated cardiomyopathy or
hypertrophic obstructive cardiomyopathy.
1)Failure to output: no pacing spike is present
despite an indication to pace. This may be due to
battery failure, lead fracture, a break in lead
insulation, oversensing (inhibiting pacer output),
poor lead connection at the takeoff from the pacer,
and "cross-talk" (i.e. a phenomenon seen when
atrial output is sensed by a ventricular lead in a
2)Failure to capture: pacing spike is not followed by
either an atrial or a ventricular complex. This may
be due to lead fracture, lead dislodgement, a break
in lead insulation, an elevated pacing threshold,
myocardial infarction at the lead tip, certain drugs
(e.g. flecainide), metabolic abnormalities (e.g.
hyperkalaemia, acidosis, alkalosis), cardiac
perforation, poor lead connection at the takeoff
from the generator, and improper amplitude or
pulse width settings.
3)Oversensing: pacer incorrectly senses electrical
activity and is inhibited from correctly pacing. This
may be due to muscular activity, particularly
oversensing of the diaphragm or pectoralis
muscles, electromagnetic interference, or lead
4)Undersensing: pacer incorrectly misses intrinsic
depolarisation and paces despite intrinsic activity.
This may be due to poor lead positioning, lead
dislodgment, magnet application, low battery
states, or myocardial infarction.
6)Pacemaker syndrome is a phenomenon where a
patient feels symptomatically worse after
pacemaker placement and presents with
progressively worsening symptoms of congestive
heart failure (CHF). This is mainly due to the loss of
atrioventricular synchrony whereby the pathway is
reversed and now has a ventricular origin. The
atrial contribution to the preload is lost and cardiac
output as well as blood pressure fall.
7)Twiddler's syndrome: Some patients will
persistently disturb and manipulate the pacemaker
generator resulting in malfunction. A chest
radiograph may reveal twisting or coiling, or lead
fracture, dislodgement, or migration. This situation
will require surgical correction with further patient
education and counseling.
• Pneumothorax (may require chest drain)
• Air embolism
• Erosion of the pacer through the skin (rare requires pacer replacement and
• Haematomas (may require drainage)
• Lead dislodgment - usually occurs within 2 days
following implantation of a permanent pacer and
may be seen on chest radiography (if the lead is
floating freely in the ventricle, malignant
arrhythmias may develop)
• Venous thrombosis - rare and usually presents as
unilateral arm oedema