Pacemakers: The Basics

From Ask Dr Wiki

Pacemaker Tutorial

Michael W. Tempelhof, MD Duke University Medical Center

INDEX:

I. Pacemaker Terminology II. Codes III. Pacemaker Systems and Types IV. Indications for Placement V. Pacemaker Configurations VI. Complications/Problem Shooting

I. Pacemaker Terminology

Rate: the heart rate at which the pacemaker will pace. Standby rate: the lowest rate at which the pacemaker will pace. Capture: depolarization and resultant contraction of the myocardium in response to a pacemaker generated electrical stimulus. Sensing: Dependent on the amplitude, slew rate and signal frequency, it describes the pacemakers ability to recognize a native electrical signal. Sensitivity: the minimum intracardiac signaling required by the pacemaker to initiate a pacemaker response. Threshold: minimum quantity, of either amplitude (milliamperes, volts) pulse duration (milliseconds), charge (microcoulombs) or energy (microjoules) produced by the pacemaker that persistently produces an action potential and myocardial contraction. Mode: indicates pacemaker capabilities: fixed rate or demand. Fixed rate: pacemaker is one that fires at a specific preset rate, regardless of the patient's own heart rate. Demand: that is, only when the patient's heart rate falls below a preset value. Pulse interval: the total time of the AV and VA intervals. Programmed standby rate: number of pulses per minute. Hysteresesis: an intentional prolonged pulse interval in order to allow the generation of a spontaneous-intrinsic electrical depolarization event. Atrioventricular (AV) interval (ie AV delay): described for dual-chamber pacemakers. The equivalent to a native PR interval. Represents the time (msec) between an atrial event and a paced ventricular event. Time that the pacemaker discerns whether or not to pace dependant upon sensing a native R wave. Allows the ventricle time to fill following an atrial contraction. Ventriculoatrial (VA) interval: described for dual-chamber pacemakers. Represents the time (msec) between a ventricular event and a paced atrial event.

II. Pacemaker Codes. A potential five letter system. 1. Chambers Paced. 2. Chambers Sensed. 3. Mode of Response to Sensed Stimulus. 4. Programmable Rate Responsive. 5. Tachyarrhythmia function. III. Pacemaker SYSTEMS and types

UNIPOLAR: a single negative electrode is in contact with the heart. The circuit is completed by placing a second-indifferent electrode elsewhere in the body.

  a. Great intrinsic signal strength
  b. Requires pocket stimulation.  And incompatible with ICDs and susceptible to electromagnetic interference. 

BIPOLAR: two electrodes in contact with the heart with circuit competed across a small field.

  a. Less susceptible to interference and far-field sensing.
  b. Combatable with ICDs.
  c. Pacemaker artifact on EKGs.  

MODE: fixed rate or demand.

 a. Fixed rate: fires at a specific-preset rate, regardless the intrinsic-native electrical activity.
    1. only require a triggering mechanism.
    2. an antiquated pacemaker.
b. Demand: will trigger when intrinsic heart rate falls below a programmable limit.
       1. Requires both a sensing and triggering mechanism.
       2.  Can be converted to fixed-rate via magnet.

• Asynchronous: describes a fixed-type pacemaker triggering at preset intervals. • Synchronous:

  a. Demand pacemakers
  b. Atrial-synchronous pacemakers

3. Rate-Responsive:

 a. Direct metabolic sensors
 b. Indirect metabolic sensors
 c. Non-Metabolic Physiological sensors
 d. Direct Activity sensors

2. Synchronous Pacemakers Demand:

  Pulse interval programmed and allows generation of native electrical-myocardial coupling

Atrial Synchronous Pacemaker:

  SA node firing is sensed by pacemaker and using an AV interval the pacemaker generates ventricular contraction via intrinsic SA nodal firing.

3. Rate-Responsive Direct metabolic sensors:

  1. Mixed Venous O2 sats
      Low O2 sat will trigger pacer to increase HR.
  2. Central Venous PH
      Low blood pH from exercise or disease will trigger an increase in HR.

b. In-Direct metabolic sensors:

  1. Ventilation rate (estimates oxygen intake).
      measures changes in chest impendance as a correlate to metabolic activity and changes HR accordingly.
  2. Mixed Venous Temperature
      Via a ceramic thermometer, blood temperature  is utilized as a marker of metabolic demand. 
c. Non-Metabolic Physiological sensors 

1. QT interval

      Pacing leads directly measure QT interval which shortens under stress or exercise. 
  2. Ventricular Depolarization Gradient
      Directly measures the QRS ventricular potential: a direct correlate of HR. 
  3. Stroke Volume
      using impedance extrapolations SV is  measured and correlated to metabolic demands. 
  4. Mean Arterial Blood Pressure
      Rate and magnitude of pressure change is calculated and correlated to metabolic demand. 

d. Direct Activity sensors

  1. Motion detection 
    Utilizes an accelerometer or vibration sensor to asses activity/exercise and change HR accordingly.
    Specificity of sensors continues to be problematic.

IV. Indications for Pacer Placement

 Class I: Conditions in which permanent pacing provides a definite benefit. Provided that the condition is not transient in etiology.  Class II: Conditions in which permanent pacing may be indicated as there is conflicting evidence.  Class IIA: Where evidence favors efficacy, with dome discrepancy in consensus.  Class IIB: the usefulness/efficacy is not well established by evidence/opinion.  Class III: Conditions in which permanent pacing is not useful/effective and in some cases may be harmful.

Pacemaker Indication Classifications Class I – Conditions for which there is evidence and/or general agreement that permanent pacemakers should be implanted. Class II – Conditions for which permanent pacemakers are frequently used but there is divergence of opinion with respect to the necessity of their insertion.  Class IIa: Weight of evidence/opinion is in favor of usefulness/efficacy  Class IIb: Usefulness/efficacy is less well established by evidence/opinion Class III – Conditions for which there is general agreement that pacemakers are unnecessary.

IV. Indications for Pacer Placement Class I

 Symptomatic 3rd degree heart block  Symptomatic 2nd degree, type 2  Symptomatic 2nd degree, type 1  Symptomatic bradycardia - <40  Post MI 2nd degree AV block  Any post MI 3rd degree block  Mobitz 2 with trifascicular or bifascicular block  Bradycardia > 40 beats/min with associated dizziness or confusion  Unavoidable drug induced bradycardia alternative  Bradycardia-induced ventricular arrhythmia  Non ischemic, exercise-induced AV block

Class IIA

 Probably beneficial and weight of evidence and opinion lead toward pacemaker placement  Asymptomatic 3rd degree heart block with stable ventricular escape > 40  Any Mobitz 2 not above  Asymptomatic Mobitz 1 at intra- or infra-His levels when found incidentally at the time of electrophysiologic study performed for other reasons.  1st degree AV block with hemodynamic compromise because of effective AV dissociation.  Bifascicular or trifascicular block accompanied by attributable syncope.  Symptomatic sinus bradycardia (<40) without clear association between bradycardia symptoms  Syncope and abnormal response to CSM

Class IIB

 Chronic heart rates of < 30 bpm and minimal  Syncope with significant bradycardia that can be reproduced by a tilt study  Hypertrophic cardiomyopathy

Class III

 Syncope of undetermined etiology  Asymptomatic sinus bradycardia  Asymptomatic sinoatrial block or sinus arrest  Asymptomatic prolonged RR intervals with atrial fibrillation or other causes of transient ventricular pause  Asymptomatic second-degree Mobitz I

Atrial Pacemaker Spikes • Pacemaker spike is seen before each P wave. • Note the normal natively conducted QRS complex.

Ventricular Pacemaker Spikes  Pacemaker spike preceding each paced QRS complex.  Pacemaker beats are wide and resemble bundle branch block beats. Note: the first native-narrow P-QRS complex.

V. Pacemaker Configurations Variations of • Chambers sensed • Chambers paced • Inhibit or Triggering mode • Responsive or not • Anti-tachyarrhythmia

Examples 1. VOO 2. VVI 3. AAI 4. VOO 5. DDD

Pacemaker Configurations: VOO  Asynchronous ventricular pacing (no sensing, no inhibition) paces the heart at a fixed rate with no regard for underlying cardiac activities.  Most pacers can be programmed to this mode temporary to avoid interference from surgical electrocautery.

Pacemaker Configurations: VVI  Pacemaker senses presence or absence of a ventricular QRS complex.  If QRS complex is sensed, the pacer counter is reset without triggering a pulse.  If no QRS complex is sensed then a ventricular stimulus is delivered.

Pacemaker Configurations: AAI  Pacemaker senses presence or absence of SA nodal activity.  If SA nodal activity is sensed then pacer resets the refractory period.  If NO SA nodal activity is sensed then the pacer triggers a pulse stimulating atrial depolarization to be conducted through normal distal conductive tissue.  Primary use for Sick sinus syndrome in the absence of other atrial or AV nodal disease.

Pacemaker Configurations: VDD  Pacer will sense both chambers but only inhibit or trigger in the ventricle.  If no atrial or ventricular activity is sensed then ventricular Stimulus occurs.  Configuration is most useful in patients with complete AV block.

Pacemaker Configurations DDD  Pacer that senses both chambers and capable of inhibiting and triggering in both chambers.  Unique ability to maintain physiologic timing between atrial and ventricular systole.  As illustrated above, has different functions depending on pacer programming.

Pacemaker Configurations DDD • Example of unique ability of DDD pacer to sense atrial activity and trigger ventricular systole. • To sense absence of atrial activity, trigger atria and sense absence of ventricular activity and trigger ventricular systole. • Sense absence of atrial activity; trigger atria systole and sense native ventricular complex and inhibit pacer stimulus.

VI. Complications/Problem Shooting

4 Primary Categories I. Failure to Pace II. Failure to Capture III. Undersensing IV. Inappropriate Pacing rates

I. Failure to Pace

 Prolonged time of no pacing spikes despite lack of native QRS complexes.  Pacemaker artifacts do not appear on the ECG Etiology: • Cross talk • Oversensing • Broken or dislodged leads • Battery depletion

Failure to Pace: Cross Talk Less common in Bipolar systems  Electrical event in one chamber is sensed by adjacent chamber lead causing an inappropriate inhibition of pacemaker.  Often associated with atrial lead misplacement or dislodgement, high atrial current output, high ventricular sensitivity

Failure to Pace: Oversensing • activity other than intrinsic ventricular activity is sensed by the ventricular sensing channel and results in failure to output as a result of inappropriate pacemaker inhibition.

II. Failure to Capture  Ventricular pacer spikes with NO subsequent pacer QRS complex.  No evidence of depolarization after pacing artifact Etiology:  Increase in pacing threshold  Lead dislodgement/fracture  RV infarct

Increase in Pacemaker threshold DRUGS: B blockers including sotalol, Class IA anti arrythmics:

    lidocaine, procainamide, quinidine,

Non-dihydropiyride Ca Channel Blockers

     Verapamil

Other causes: • Electrolyte Imbalances Hyperkalemia, acidosis/alkalosis. b. Hypoxemia.

III. Failure to Sense  Inappropriate pacemaker spikes result as a failure of the pacemaker unit to sense the intrinsic QRS complexes.  The failed spikes do not capture the ventricle as they occur during the absolute refractory period.  An intrinsic depolarization that is present, yet not seen or sensed by the pacemaker

Etiology Failure to Sense  Low amplitude intracardiac signal  Lead misposition/displacement  Lead Fibrosis  Inappropriately programmed sensitivity  Lead maturation  Change in the native signal

IV. Inappropriate Pacing rates • Pacemaker Syndrome • Pacemaker-mediated tachycardia

1. Pacemaker Syndrome  Loss of AV synchrony that may result in hemodynamic compromise.  Loss of AV synchrony results in Atrial distention  Most common in VVI pacing  Occurs shortly after pacemaker insertion

Pacemaker Syndrome  Patients complain of CHF like symptoms related to Atrial distention  Orthostatic dizziness  Fatigue and lethargy  Exercise intolerance  Chest fullness or pain  Uncomfortable pulsations in the neck or abdomen  Right upper quadrant pain  Syncope or near syncope  PND orthopnea  elevated neck veins, rales, and pedal edema. Physical exam can often reveal  cannon A-waves

 Diagnosis by evidence of Atrial Ventricular dyschrony on EKG.  Generally treated by switching to dual chamber pacing or adjusting the timing of the AV delay in dual chamber pacers

Pacemaker-Mediated Tachycardia (PMT): run-away pacemaker  Re-entrant tachycardia seen most commonly with dual chamber pacemakers.  Ventriculoatrial conduction results in a retrograde atrial depolarization that is sensed and causes an inapprioprate paced ventricular depolarization. Treatment with Magnet placement to temporary shut off pacer.

Retrograde Conduction May Be Caused By:  Loss of A-V synchrony due to:  Loss of sensing/capture  Myopotential sensing  Premature ventricular contraction (PVC)  Magnet application

Example of Pacemaker-Mediated Tachycardia (PMT):  A DDD pacer with normal function until a retrograde PVC is conducted resulting in atrial activation.  Retrograde atrial activation is ventricularly sensed and initiates ventricular pacing.  Note that atrial pacing spikes are no longer evident once PMT occurs.