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The sensor thinks it saw a ventricular beat where the arrows were. The star is where the pacer

should have shocked, had the sensor not oversensed.The minimal amount of electrical energy required to cause depolarization is termed the capturethreshold, and it is a function of the electrode-tissue interface. Pacers are programmed to deliver

energy above that threshold and this output is seen as a spike on the EKG, termed a pacing

stimulus artifact. A failure to capture is when this energy is not sufficient enough to causedepolarization. What does this look like on EKG?

Failure to capture occurs when a pacing stimulus artifact is not followed by a P wave or QRS

complex (depending on which chamber is being paced), and occurs as a result of either

inappropriately low output or poor electrode-tissue interface from lead dislodgement

How to program? Easier than your VCR: there are only a few settings - lower rate limit, AV

interval, and upper rate limit . The lower rate limit is the lowest bpm the pacemaker will allow

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The upper rate limit is a programmed rate above which the pacemaker will not pace. In patientswith sinus or atrial tachycardia that exceeds the upper rate limit, the pacemaker will pace the

ventricle only as fast as the upper rate limit. This may result in atrial events that are not followed

by ventricular pacing, and may lead to misinterpreting pacemaker function from the surfaceelectrogram. For exame, if a patient has Mobitz type II with a 2:1 atria to ventricular conduction,

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INDICATIONS:

The indications for permanent pacemaker implantation relate primarily to bradycardia caused bysinus and/or AV node dysfunction. In general, permanent pacemakers should be considered in

patients with sinus node dysfunction only when the patient is symptomatic as a result of

bradycardia. Sinus node dysfunction that manifests as symptomatic sinus bradycardia, sinus pauses, or chronotropic incompetence (inability of the sinus node to adequately accelerate in

response to physical activity) are class I indications for pacing. Additionally, when drugs (ie, beta-blockers) that may result in symptomatic sinus bradycardia are required for other

indications, pacing should be considered.

The indications for cardiac pacing in patients with AV node disease are more varied. Similar to

sinus node dysfunction, pacing is generally used in symptomatic patients. However, in the case

of AV node disease, permanent pacemakers are required in certain conditions even inasymptomatic patients. For instance, patients with third-degree AV block or type 2 second-

degree AV block (Mobitz 2) should be considered for pacing regardless of symptoms. Even in

patients with type 1 second-degree AV block (Mobitz 1 or Wenkebach) pacing should be used if

the patient is symptomatic. Occasionally, evaluation of the AV conduction system duringelectrophysiologic study is required to identify patients with advanced AV node disease. In this

situation, certain criteria exist to suggest the need for pacing.

Slow AV conduction during atrial fibrillation is a common indication for permanent pacing. This

may occur as a result of native AV nodal disease, medications that slow AV conduction but arerequired for other indications, or iatrogenic ablation of the AV node for rapid ventricular rates.

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With the exception of patients who have undergone AV nodal ablation, pacing should be used

only in patients who have symptoms from slow ventricular rates. However, in patients with atrial

fibrillation and pauses of five seconds or longer, pacing is indicated even in the absence ofsymptoms. To summarize:

- Symptomatic bradycardia

- Consider when drugs ie bblockers required that cause symptomatic bradycardia- AV node dz (blocks): mobitz regardless of sx, symptomatic wenkebach, slow afib or afib

with >5 pauses in absence of sx

More like prevention of SCD from VT: ICDs were initially designed for patients who survived

cardiac arrest from sustained ventricular tachycardia (VT) or ventricular fibrillation (VF).

However, in the past two decades, indications for ICDs have expanded significantly. Multiple

trials have established their superior mortality benefit when compared to antiarrhythmic

medications for primary prevention of sudden cardiac arrest. Their current use can be divided

into primary prevention of sudden cardiac arrest or secondary prevention for patients who have

survived cardiac arrest from VT or VF.

The use of ICDs for primary prevention of sudden death is indicated in patients with ischemic or

nonischemic cardiomyopathy, ejection fraction less than or equal to 35%, and New York HeartAssociation (NYHA) class II-III heart failure symptoms. Patients with ejection fraction less than

or equal to 30% and class I symptoms may also be candidates for ICD implantation. When

properly selected, these patients experience a 25-30% relative reduction in mortality whenimplanted with an ICD. Primary prevention is the largest indication for ICDs and remains an

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important way for hospitalists to intervene on long-term patient outcome. When hospitalists

encounter these patients, inpatient cardiology or electrophysiology consult can help determine if

an ICD is appropriate.

Patients who have survived cardiac arrest from VT or VF should undergo ICD implantationregardless of the presence or absence of structural heart disease, unless a clearly reversible cause

of the arrest can be identified and corrected. In patients with moderately reduced ejection

fraction (less than or equal to 40%), nonsustained VT, and inducible VT or VF duringelectrophysiologic study, ICD implantation is appropriate.

There are several considerations when timing ICD implantation. For patients with newly

diagnosed nonischemic cardiomyopathy, the exact timing of ICD implantation varies. An ICD is

indicated when the cardiomyopathy is nonreversible and patients are receiving appropriate heartfailure medications. In patients with ischemic cardiomyopathy from myocardial infarction, ICDs

should not be implanted within the first 40 days after the infarct. This recommendation is based

on data that suggests no overall mortality benefit from ICDs implanted within this peri-infarct period.

Other patient populations who may benefit from ICD therapy in certain situations include those

with hypertrophic cardiomyopathy, long QT syndrome, Brugada syndrome, and cardiac sarcoid.

When should patients be referred for cardiac resynchronization therapy (CRT)? Patients with

advanced heart failure and a severely reduced ejection fraction often have intraventricular

conduction disturbances that result in mechanical "dyssynchrony" of the left ventricle. Thisdyssynchrony leads to inefficient ventricular contraction and increased maladaptive ventricular

remodeling. Prolongation of the QRS duration (in particular, left bundle-branch block) is often

used as an electrical surrogate for mechanical dyssynchrony. Many of these patients benefit from

biventricular pacing (otherwise known as cardiac resynchronization therapy or CRT). Cardiac

resynchronization is obtained by positioning a lead in the coronary sinus to pace the lateral wall

of the left ventricle, in addition to the standard right ventricular lead. A CRT device can be

programmed to pace the right and left ventricles at specific times to maximize cardiac output.

The ideal candidates for cardiac resynchronization include patients in sinus rhythm with ejectionfractions <35%, NYHA class III symptoms, and left bundle-branch block with a QRS >120

msec. When patients are properly selected for this therapy, they experience an approximate 30%reduction in heart failure hospitalizations and mortality. Roughly two-thirds of patients who

undergo CRT implantation will receive clinical benefit from this therapy.

Candidates for cardiac resynchronization therapy (CRT) include patients in sinus rhythm

with ejection fractions <35%, NYHA class III symptoms, and left bundle-branch block

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with a QRS >120 msec. When patients are properly selected for this therapy, they

experience approximately a 30% reduction in heart failure hospitalizations and mortality.

Chronic right ventricular apical pacing may worsen heart failure symptoms and ejection fraction.

Thus, another patient population that may benefit from CRT is patients with decreased ejection

fraction and a need for chronic ventricular pacing. Cardiac resynchronization in these patients

may reduce maladaptive left ventricular remodeling and preserve ejection fraction. One common

example of CRT used in this setting is a patient with decreased ejection fraction and atrial

arrhythmias who undergoes ablation of the AV node for control of rapid ventricular rates.

HOW DO AICDS work

ICDs are used to treat ventricular tachyarrhythmias. To do this, the device must recognize the

arrhythmia, and then treat it appropriately. Ventricular arrhythmias are initially recognized by the

device when the ventricular rate exceeds a programmed rate cutoff. If this ventricular rate is

maintained for a given number of beats, the device will recognize the fast rhythm as VT or VFand begin to treat the arrhythmia. Nonsustained VT typically does not last long enough to meet

criteria for the device.

Because the initial detection of the arrhythmia is based entirely on rate, it may be difficult for thedevice to discern whether a fast rhythm is ventricular or supraventricular in origin. Therefore,

once VT or VF has been identified by the rate cutoff, ICDs use a series of programmable

algorithms to discriminate true ventricular arrhythmias from other tachyarrhythmias such assinus tachycardia, supraventricular tachycardia, or atrial fibrillation with rapid ventricular rates.

These algorithms are designed to limit ICD therapy to only true VT or VF. These algorithms are

very effective but not perfect. When optimally programmed, inappropriate shocks due to

nonventricular rhythms can be minimized. Nonetheless, inappropriate ICD shocks remain one

of the most common adverse events associated with ICD use.

The goal of ICD therapy is to terminate potentially life-threatening ventricular arrhythmias.

Modern devices perform this function with a combination of high-voltage defibrillation shocks

and less painful bursts of rapid pacing. Typical programming includes several initial attempts to pace the ventricle at a rate slightly faster than the ventricular tachycardia (termed

"antitachycardia pacing"). In some patients, this will terminate the arrhythmia, and a shock can

be avoided. However, if antitachycardia pacing is not successful, then the device will deliver aseries of high-voltage shocks until the arrhythmia terminates

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IN THE ED: Approximately 15%-30% of patients will experience an appropriate ICD shock

within three years of implant. When a patient receives an ICD shock, the goal of the physician

is to determine whether the therapy was appropriate. An important consideration is the number

of shocks a patient receives. Most often, a single shock represents appropriate therapy for VT or

VF. If the patient is otherwise stable, hospitalization is usually not warranted. A common

practice is to have the patient call his or her physician's office to arrange follow-up within 24

hours. In this situation, if it is determined that the ICD shock was appropriate, the focus should

be on addressing any exacerbating factors, most often congestive heart failure.

When a patient receives more than two shocks within 24 hours he or she should be evaluated

immediately, as this may indicate inappropriate shocks, failed device therapy, or recurrent

ventricular arrhythmias. Whereas shocks preceded by syncope or presyncope are usually

appropriate, shocks without such a prodrome may be inappropriate. Inappropriate shocks may

result from sinus tachycardia, SVT, or atrial fibrillation with rapid ventricular response, external

electromagnetic interference (electrical "noise"), or true device or lead malfunction. The precise

management of inappropriate ICD shocks will depend on the underlying cause. The term

electrical storm describes multiple appropriate shocks within 24 hours, and indicates recurrent

VT or VF. This is a potentially life-threatening condition, and these patients should be

hospitalized with a focus on correcting any underlying causes such as ischemia, electrolyte

abnormalities, or worsening heart failure. Often, these patients will require short- or long-term

antiarrhythmic medications to suppress recurrent ventricular arrhythmias.

Even single ICD shocks can result in considerable distress and anxiety for the patient and canreduce their quality of life. Attention should be paid to the patient's psychological status after anyICD shock, and when needed, anxiolytic medications should be offered. Less commonly,

professional psychological treatment may be required

Suggested Readings

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8/13/2019 Pacemakers Ct 12292012


The Antiarrhytmic Versus Implantable Defibrillators (AVID) Investigators. A comparison ofantiarrhythmic-drug therapy with implantable defibrillators in patients resuscitated from near-

fatal ventricular arrhythmias. New Engl J Med. 1997;337:1576

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