Pacemakers and Implantable Cardioverter-Defibrillators

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11Fast & Easy ECGs, 2EFast & Easy ECGs, 2E

Pacemakers and Implantable Cardioverter-Defibrillators

Fast & Easy ECGs, 2nd E – A Self-Paced Learning Program

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Artificial Pacemakers

• Medical devices used to generate and deliver electrical impulses to the myocardium to stimulate a normal heartbeat

• Some are external to the body and provide temporary treatment, others are permanently implanted in the chest

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Artificial Pacemakers

• One type of temporary pacemaker is the transcutaneous pacemaker– It delivers electrical impulses through lead wires

to electrode pads that are applied to the surface of the patient’s chest

• Permanent pacemakers are implanted in a surgically created pocket beneath the skin in the patient’s chest wall just below the clavicle

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Permanent Pacemakers • Consist of:– A generator – One or more lead

wires – A power source

(often a lithium battery)

– Logic circuits that detect cardiac electrical activity and determine the appropriate response

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Permanent Pacemakers• May be used to:

– Symptomatic bradycardia– Sick sinus syndrome– Atrial fibrillation with bradycardia– 3rd-degree (complete) AV heart block– Symptomatic 2nd-degree AV heart block, particularly type II– The sudden development of various combinations of AV

heart block and bundle branch block in patients experiencing acute MI

– Recurrent tachycardias that can be overdriven and thereby terminated by pacemaker activity

– Synchronization of the heart beat in heart failure (cardiac resynchronization therapy)

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Pacemaker Electrodes

• Are either positioned in the atrium or ventricle alone (single-chamber pacemakers) or, more often, in both chambers (dual-chamber pacemakers or AV sequential pacemakers)

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Permanent Pacemakers

• Are programmable – Receive and transmit data/programming

instructions through the skin using electromagnetic waves

• Adjustments can be made to:– Output– Sensitivity– Refractory period– Rate adaption

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Pacing Modes• Single-chamber – One pacing lead is inserted into either the right atrium or

right ventricle but not both • Dual-chamber– Electrodes are placed into two chambers of the heart – One lead paces the atrium while the other paces the

ventricle – By assisting the heart in coordinating the function

between the atria and ventricles, this type of pacemaker acts similarly to how the heart naturally paces itself

– Also referred to as an AV sequential pacemaker– Most can be programmed to a single chamber mode,

which can be useful if the atrial lead wire fails

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Pacing Modes

• Fixed-rate– Paces the heart at a single, preset rate

• Rate-responsive– Has sensors that identify increases or decreases in the

patient’s physical activity and automatically adjusts base pacing rate to meet the body’s metabolic needs

– Can boost the heart rate in response to motion or increased respirations for those patients whose body cannot appropriately increase the heart rate during activity

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Pacing Modes

• Demand– Most common type used – Fires only when the patient’s intrinsic heart rate

falls below a given threshold level• i.e., if the pacemaker is set at 60 beats per minute it

remains inactive until there is a pause between beats that translates into a rate below 60, then the pacemaker fires

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Coding System

• Pacemaker mode and function described by a five letter coding system– in practice, only three to four are commonly used

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Coding System• First letter represents the heart chamber being paced.

This letter may be – O = none – A = atrium – V = ventricle– D = dual (ventricle and atrium)

• Second letter represents the chamber of the heart being sensed by the pacemaker. This letter may be – O = none – A = atrium – V = ventricle– D = dual (ventricle and atrium)

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Coding System• Third letter indicates how the pacemaker

generator responds to sensing. This letter may be – O = none – T = triggers pacing– I = inhibits pacing– D = dual (triggers and inhibits pacing)

• Fourth letter has to do with adjustment of the pacing rate in response to exercise – If pacemaker is rate responsive, it is denoted with the

letter “R” – If there is none, it is denoted as “O”

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Coding System

• The fifth letter indicates multisite pacing. This letter may be – O = none – A = atrium– V = ventricle– D = dual (ventricle and atrium)

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Coding System Examples

• VOO – In this mode, the ventricle is paced and there is no

sensing function• AAI– Pacemaker paces and senses in the atrium – When it senses atrial activity, pac ing is inhibited

• VVI– Ventricle is paced and sensed– If spontaneous cardiac output is detected, then the

device is inhibited

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Coding System Examples

• VDD– Here the pacemaker paces the ventricle and

senses both the atrium and ventricle– On sens ing intrinsic atrial activity, the pacemaker

triggers ventricular pac ing; on sensing ventricular activity, the pacemaker inhibits pacing

– It is also known as a P-synchronous pacer

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Coding System Examples• DVI– Pacemaker can pace in the atrium, the ventricle, or both– Sensing takes place only in the ventricle– When the pacemaker senses intrinsic ventricular activity,

it inhibits pacing • DDD – Pacemaker paces and senses in the atrium, the ventricle,

or both – On sensing ac tivity in either chamber, the pace maker

inhibits pacing in that chamber– Or, on sensing atrial ac tivity, the pacemaker may trigger

ventricular pacing

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Cardiac Resynchronization Therapy (CRT)

• Used to resynchronize a heart that does not beat in synchrony, a common problem in patients with heart failure

• Employs three leads: – one is placed in right atrium – one is located in right ventricle – last one is inserted through the coronary sinus to

pace the free wall of the left ventricle– These three wires are connected to a CRT generator

and programmed so that the two ventricular wires are activated simultaneously

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Unipolar and Bipolar Systems

• Unipolar– positive electrode is positioned in the heart tissue

and the negative electrode is connected to the pulse generator

– produces tall pacing spikes on the ECG

• In a bipolar system,– electrodes are only millimeters apart in the

cardiac tissue– produces short pacemaker spikes

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ECG Features of a Pacemaker

• Depending on how many chambers are paced, the firing of a pacemaker produces one or two narrow pacemaker spikes on the ECG

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ECG Features of a Pacemaker

• A paced ECG complex shows two features: (a) a narrow “pacing spike,” which reflects the impulse depolarizing the paced chamber and (b) a P wave or QRS complex that immediately follows the pacing spike

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Pacemaker Failure

• Pacemakers may not work properly for a number of reasons, including a failure to capture, a failure to pace, a failure to sense, oversensing, and pacemaker-mediated tachycardia

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Pacemaker Failure

• Failure to capture is seen as the presence of pacemaker spikes that are not followed by a P wave or broad QRS complex

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Pacemaker Failure

• Failure of the pacemaker to sense is seen as the presence of ECG pacemaker spikes that fall where they shouldn’t

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Pacemaker Failure

• Oversensing is seen as an absence of pacemaker spikes in the presence of a heart rate that is slower than the rate set for the pacemaker

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Pacemaker Failure

• Pacemaker-mediated tachycardia is seen as a fast heart rate with a pacemaker spike preceding each QRS complex on EGG.

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Implantable Cardioverter-Defibrillator (ICD)

• Is implanted in patients who are at risk of sudden cardiac death due to ventricular fibrillation and ventricular tachycardia

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Implantable Cardioverter-Defibrillator

• Is programmed to detect cardiac dysrhythmias and correct them by delivering paced beats, cardioversion, or defibrillation

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Therapies Provided by the ICD

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Practice Makes Perfect

• Analyze this ECG tracing

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Pacemakers and Implantable Cardioverter-Defibrillators