Pacemakers
Permanent pacemakers are implantable devices designed to alter the cardiac rhythm during pathologic states.
They are usually placed in the anterior chest wall, and have one or two wire leads which enter the subclavian vein,
travel to the right side of the heart, and are implanted into the wall of the right atrium and/or right ventricle. Newer
biventricular pacemakers, used in advanced heart failure, also have a lead to the left ventricle via the coronary
sinus. Each lead may serve to either pace a specific chamber, sense the intrinsic electrical activity of a specific
chamber, or both. Due to the wide variety of lead configurations and pacemaker modes, a system of codes has
been developed to easily describe each possible combination. Each pacemaker mode is assigned either 3 or 4
codes:
Position
1
2
3
4 (optional)
Function
Chambers Paced
Chambers
Sensed
O
A
V
D
Response to Sensed
Stimulus
O
T (triggered)
I (inhibited)
D (both triggered
and inhibited)
Rate Modulation?
O (none)
A (atrium)
V (ventricle)
D (both atrium
and ventricle)
O (non rate-responsive)
R (rate-responsive)
The term triggered means that an output pulse is created in response to a sensed event, whereas inhibited means
that the output pulse is blocked in response to a sensed stimulus and that the pacemaker then cycles for one or
more timing cycles. Pacemakers capable of rate modulation can increase their pacing rate in response to vibration,
minute ventilation, temperature, oxygen saturation, or other stimuli, independent of the heart’s intrinsic activity. A
fifth position is sometimes used to specify the absence or location of multisite pacing (i.e. biventricular pacing for
patients with end-stage left heart failure). Most modern pacemakers are capable of mode switching in response to
sustained atrial arrhythmias to prevent extreme ventricular response rates (i.e. DDD  VVI in the setting of atrial
flutter).
Index of Common Pacemaker Configurations
(Adapted from: Swanton RH. Cardiology, 5th ed. 2003.)
Key to Symbols:
VOO
Indication
Temporary mode sometimes used in
pacemaker-dependent patients during
surgery in order to prevent electrocautery devices from inhibiting the
pacing output.
Comments
Many pacemakers will revert to this mode if a magnet is placed over
them, to temporarily treat pacemaker-induced arrhythmias and other
pacemaker-related problems.
VVI
Indication
The combination of AV block and
chronic atrial arrhythmias (particularly
atrial fibrillation).
Comments
VVIR should be used in patients who exhibit chronotropic incompetence.
AAI
Indication
Sick sinus syndrome in the absence of
AV node disease or atrial fibrillation.
Comments
All patients needing AAI mode should be given rate-responsive
pacemakers (AAIR), even those with chronotropic competence, due to
the high likelihood that chronotropic incompetence will subsequently
develop.
VDD
Indication
AV block with intact sinus node
function (particularly useful in
congenital AV block).
Comments
This mode, most commonly engineered with a single-lead system, is
also known as atrially sensed ventricular inhibited pacing (ASVIP). If
atrial fibrillation occurs or excessive supraventricular bradycardia occurs,
the pacemaker usually will switch to VVI mode.
DDD
Indications
1. The combination of AV block and SSS.
2. Patients with LV dysfunction and LV
hypertrophy who need coordination of
atrial and ventricular contractions to
maintain adequate CO.
Comments
If atrial fibrillation occurs, the pacemaker usually will switch to VVI
mode. Ideally, all pacemakers in DDD mode should be rateresponsive (DDDR).
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Timing Cycles
A thorough understanding of pacemaker modes requires some discussion of the timed intervals which a pacemaker
senses, and which dictates its activity.
Timing Cycles Definitions
LR (Lower rate limit) – The longest period that a pacemaker will tolerate the absence of a QRS complex.
UR (Upper rate limit) – The shortest period between a QRS complex and a subsequent P wave, for which the
pacemaker’s ventricular lead will fire in response to the sensed P wave.
VRP (Ventricular refractory period) – The period of time after a sensed or paced QRS complex, during which
subsequent QRS complexes do not reset the timing cycle. This value is set by the pacemaker’s
programmer.
ARP (Atrial refractory period) – The period of time after a sensed of paced P wave, during which subsequent P
waves do not reset the timing cycle. This value is also set by the programmer.
AVI (Atrioventricular interval) – The period of time between the onset of a P wave and the onset of a subsequent
QRS complex (either paced or intrinsically conducted)
PV (P wave – ventricular interval) – The period of time between the onset of an intrinsic P wave and a subsequent
ventrticular pacing impulse.
VA (Ventriculoatrial interval) – Equals to the LR – AVI.
PVARP (Post-ventricular atrial refractory period) – The period after the onset of a QRS complex (either sensed or
paced) during which a sensed atrial impulse does not reset the timing cycle. The PVARP is a significant
factor in determining the upper rate limit, thus preventing pacemaker-induced tachycardia.
TARP (Total atrial refractory period) – Equal to the AVI + PVARP.
VAI (Ventriculoatrial interval) – The period of time between the onset of a QRS (either paced or sensed) and the
onset of a subsequent P wave.
VVI
If the LR time expires
without sensing a
ventricular impulse, the
single ventricular lead
fires. The LR time is
reset with each QRS
complex that is either
sensed or paced, and
which does not occur
during the VRP.
AAI
If the LR time expires
without sensing an atrial
impulse, the single atrial
fires. The LR time is
reset with each P wave
that is sensed or paced,
and which does not
occur during the ARP.
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VDD
A sensed P wave
initiates the AV interval.
If the AVI expires without
a sensed QRS complex,
a ventricular pacing
impulse is delivered. If
the LR time expires
without sensing a
ventricular impulse, a
ventricular pacing
impulse is also
delivered.
DDD
A sensed or paced P
wave initiates the AV
interval. If the AVI
expires without a sensed
QRS complex, a
ventricular pacing
impulse is delivered. If
the LR time expires
without sensing a
ventricular impulse, a
ventricular pacing
impulse is also
delivered. If the VA
interval expires, an atrial
pacing impulse is
delivered.
Above diagrams taken from: Braunwald's Heart Disease: A Textbook of Cardiovascular Medicine, 7th ed., 2005.
Summary of Arrhythmia-Related Indications for Pacemaker Placement
Class I
1. Sinus node dysfunction with documented symptomatic bradycardia
2. Symptomatic chronotropic incompetence (failure to increase HR with exercise or increased metabolic
demand)
3. 3° and advanced 2° AV block at any anatomic level, associated with any of the following:
Symptomatic bradycardia
Arrhythmias that require drugs resulting in symptomatic bradycardia
Sinus pauses > 3 seconds
Asymptomatic escape rate < 40bpm while awake
Post-op AV block, not expected to resolve after surgery
Neuromuscular disease with AV block, regardless of presence or absence of symptoms (there may
be an unpredictable progression of AV conduction disease)
4. 2° AV block, regardless of type or site of block, with associated symptomatic bradycardia
5. Type II 2° AV block with wide QRS, regardless of symptoms or escape rate
Class IIa
1. Syncope of unexplained origin when major abnormalities of sinus node function are discovered or
provoked during EP studies.
2. Asymptomatic 3° AV block with an awake ventricular rate > 40bpm
3. Asymptomatic type II 2° AV block
4. Asymptomatic type I 2° AV block at intra or infra-His levels
5. 1° or 2° AV block with symptoms suggestive of pacemaker syndrome
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Problems with Pacemakers
(Rhythm strips from: Braunwald's Heart Disease: A Textbook of Cardiovascular Medicine, 7th ed., 2005
Failure to Capture
Causes:
Threshold rise (electrolytes,
drugs)
Lead dislodgement
Lead fracture
RV infarct
In the above rhythm strip, the second pacing impulse fails to lead to ventricular
activity for unclear reasons, and is subsequently followed by a failure to pace.
Failure to Pace
Causes:
Oversensing
Battery failure
Crosstalk
Internal insulation failure
Conductor coil fracture
In the second beat, the atrial pacing impulse is not followed by a ventricular
impulse, likely due to crosstalk, in which the ventricular lead senses the atrial
spike and interrupts it as intrinsic ventricular activity.
Failure to pace due to oversensing of myopotentials leading to a pause in
cardiac rhythm.
Failure to Sense
Causes:
Undersensing (False
Inhibition)
Lead Fracture
PVC
Despite intrinsic ventricular activity, the pacemaker continues ventricular pacing
uninterrupted, likely as a result of undersensing.
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Lead-related complications can be distinguished intraoperatively based upon the following:
Wire fracture
Insulation break
Lead dislodgement
Lead exit block
Voltage Threshold
High
Low
High
High
Current Threshold
Variable
High
High
High
Lead Impedence
High
Low
Normal
Normal
Pacemaker-Mediated Tachycardia (also known as pacemaker-reentry tachycardia or endless-loop
tachycardia)
Frequently, during normal AV synchronous pacing a PVC can occur that results in retrograde ventriculoatrial
conduction and a retrograde P wave. If this retrograde conduction is sensed by the atrial lead, the AVI can be
initiated, leading to a paced ventricular complex, which can again result in a retrograde P wave, forming a type of
reentrant circuit and a tachyarrhythmia. This potential arrhythmia is best prevented by choosing a PVARP long
enough to prevent the sensing of retrograde P waves.
Transvenous Pacemakers
Transvenous pacemakers are only one form of temporary cardiac pacing, but are the form most likely to be
encountered outside the cardiac surgery setting. (Other forms include epicardial, transesophageal, and
transcutaneous pacing.) Transvenous pacers are typically placed under fluoroscopic guidance, but can be also
placed via a balloon-tipped catheter via the right internal jugular or left subclavian veins. Although some SwanGanz catheters are equipped with pacing ability, they are less stable than other endocardial leads, and should be
avoided in patients who are pacemaker dependent.
Situations in which a temporary pacemaker (transvenous or epicardial) may be preferred over a permanent
implantable pacemaker:
▪ Injury to the conduction system during CABG (frequently improves over time)
▪ Lyme disease
▪ Temporary SA node injury following heart transplantation
▪ Cardiac trauma
▪ Reverisble toxic/metabolic conditions (hyperkalemia, digoxin toxicity, beta blocker overdose)
▪ To establish the possible benefit from a permanent pacemaker in a patient with sinus
bradycardia or AV block when the cause of symptoms is unclear
▪ Symptomatic bradycardia or new bifascicular block during an acute MI
▪ Patient with an indication for a permanent pacemaker, but also with an active systemic infection
Transvenous pacemakers typically consist of a single ventricular lead, which both senses and paces, however
temporary dual chamber AV sequential pacing all exists. Serious complication from transvenous pacemakers are
generally rare, but include:
Local bleeding
Pneumothorax
Thrombophlebitis
Air embolism
Infection
Cardiac perforation and subsequent
tamponade
Induction of ventricular arrhythmias
Catheter knotting
Subdiaphragmatic stimulation
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Procedures for basic single chamber pacing:
1. Determine the Pacing Mode
This is determined by the position of the single lead (atrial vs.
ventricular). The pacemaker can also be placed into demand
(synchronous) mode or asynchronous mode, which can
usually be done by adjusting the sensitivity dial. In demand
mode, the pacing output is inhibited by sensed intrinsic
electrical activity, whereas in asynchronous mode, the pacing
output is constant regardless of intrinsic activity.
(Asynchronous mode is best suited for patients whose
intrinsic HR is constantly below the paced rate.)
2. Determine the Sensing Potential
Turn the rate dial to at least 10bpm below the patient’s
intrinsic HR. Then set the output to the minimal setting (to
prevent competitive pacing). Next, turn the sensitivity dial to
the most sensitive setting. Slowly decrease the sensitivity
until the pace indicator begins to flash. (At minimum output,
there should be no actual pacing of the cardiac rhythm.)
Increase the sensitivity until the sense indicator begins to
flash again. The millivolt value at which this occurs is the
sensing threshold. The sensitivity dial should then be set at
twice this sensitivity (to ensure a margin of safety). For
example, if the threshold is 6mV, the sensitivity dial should be
set at 3mV. Lastly, reset the rate and output dials to their
original values.
3. Determine the Stimulation Threshold
Turn the rate dial to 10bpm above the patient’s intrinsic HR.
Verify 1:1 capture on the ECG. Gradually decrease the
pacing output until 1:1 capture is lost. Then slowly increase
the output until 1:1 capture is regained. This value is the
stimulation threshold. The output dial should then be set at
twice the threshold value.
4. Select the Appropriate Pacing Rate
This value will depend upon the specific clinical situation.
A typical transvenous pacemaker
(Medtronic 5348 pictured)
Some transcutaneous pacemakers are equipped with the ability to deliver rapid atrial pacing for termination of
some atrial arrhythmias, however, this should be done under the supervision of an experienced cardiologist only.
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