Pacing

Cardiac pacing is repetitive, low-current electrical stimulation of the heart to initiate and maintain rhythm. Pacing:

Terminology

Key pacing terminology includes:

  • Sensitivity
    • Thresholds
    • Undersensing
    • Oversensing
  • Capture
    • Thresholds
  • Pacemaker code nomenclature
  • Pacing modes

Sensitivity

Sensitivity describes the ability of the pacemaker to detect native cardiac rhythm. Sensitivity is:

  • Measured in mV
    • The sensing threshold is the voltage at which native activity is detected
    • The sensitivity setting (usually known as sensitivity) is the voltage that the pacemaker is set to
      • This is lower than the sensing threshold
        “Increasing the sensitivity” therefore requires decreasing the sensitivity setting.
  • Good when the sensing threshold is high
  • Poor when the sensing threshold is low

Undersensing:

  • Failure to sense the normal cardiac rhythm, resulting in inappropriate delivery of a pacing impulse
    If this occurs during chamber repolarisation, arrhythmia can result:
    • Inappropriate atrial pacing may cause AF
    • Inappropriate ventricular pacing may cause polymorphic VT or VF
  • May occur due to:
    • Variability in the sensing threshold
    • Inappropriate setting of the sensitivity
  • Due to the risk of malignant arrhythmias, use undersensing pacing leads should be carefully considered and not used at all outside of a critical care setting

Low atrial sensitivity is less concerning than low ventricular sensitivity as there is less (but not no) R-on-T risk.

An R-on-T can still result if an undersensing atrial lead is used in a DDD mode. This occurs when:

  • The atrial lead fails to sense a native P wave
  • Atrial pacing then occurs during the subsequent (native) QRS
    • The (functioning) ventricular lead studiously ignores any sensed electrical activity that occurs during atrial pacing
      This is known as a blanking period.
    • Therefore, the ventricular lead does not register any native ventricular activity that follows from the paced atria
  • After the programmed A-V interval, the ventricular lead paces
  • This pacing occurs during repolarisation

Oversensing:

  • Inappropriate inhibition of pacing due to misinterpretation of other electrical activity as chamber depolarisation
    Sources include:
    • Diathermy
    • Muscle activity
      • Shivering
    • T-wave
    • Other chamber depolarisation

Capture

Capture describes how easily the pacemaker can cause a depolarisation. Capture is:

  • Measured in mA
    • The capture threshold is the current at which a pacing impulse causes chamber depolarisation
    • The capture setting is a current well above the capture threshold
  • Good when the capture threshold is low
  • Poor when the capture threshold is high

Pacing Codes

A generic 3 or 5 digit code is used to describe pacing modes:

This code is used by the North American Society of Pacing and Electrophysiology (NAPSE), and the British Pacing and Electrophysiology Group (BPEG), and is abbreviated to the NBG (NAPSE/BPEG Generic Code).

  • Chambers paced
    A, V, D (dual, i.e. both atria and ventricles), O (neither).
  • Chambers sensed
    A, V, D, O.
  • Response to sensing
    • I: Inhibition
      Pacemaker is inhibited by a sensed signal. e.g. VVI, ventricular pacing is inhibited by sensed ventricular activity.
    • T: Trigger
      Pacemaker triggered by sensed signal.
    • D: Inhibition and triggering
      Requires a dual-chamber system.
  • Rate Response
    Only relevant in permanent systems. Describes if the pacemaker changes rate due to sensed change in metabolic demand.
    • R: Responsive
      Responds to either movement or ↑ minute ventilation.
    • O: No response
  • Multisite pacing
    Are multiple cardiac chambers paced:
    • O: None
    • A: Atria
    • V: One or both ventricles
    • D°: Any combination of ventricles

Pacing Modes

Basic Pacing Modes
Mode Action Indications Contraindications
AAI
  • Atrium sensed
  • Atrium paced at the set rate if the native atrial rate is less than the set rate
  • Pacing inhibited in response to sensing
  • Sinus bradycardia
    ↓ Risk of new AF in the post-operative period.
  • High grade AV block
    Or high risk of such.
  • AF
  • Poor atrial sensitivity
    Relative contraindication, may produce AF in atrial modes or VF in DDD A-V sequential modes.
VVI
  • Ventricle sensed
  • Ventricle paced at the set rate if the native ventricular rate is less than the set rate
    RV free wall is depolarised first, leading to a pseudo-LBBB pattern and dyssynchronous LV ejection that ↓ LV EF.
  • Pacing inhibited in response to sensing
  • AV block
  • Ventricular standstill
  • Poor ejection
    Ventricular pacing results in dyskinetic contraction of the ventricle in addition to the loss of atrial kick, and can adversely affect haemodynamics.
  • Poor ventricular sensitivity
    Absolute contraindication, may produce VF.
DDD
  • Both chambers sensed
  • Both chambers paced
  • Either chamber inhibited in response to sensing
  • Atrium paced at the set rate if the native atrial rate is less than the set rate, as AAI
  • Ventricle paced at the sensed atrial rate if it exceeds the set rate, and no AV nodal conduction occurs during the set A-V interval
  • Ventricle paced at the set rate if no AV nodal conduction occurs (during the set A-V interval) following an atrial pacing impulse
  • AV block
  • Sinus bradycardia and risk of AV block
  • Asystole
  • Poor atrial or ventricular sensitivity
  • Pacemaker mediated tachycardia
DDI
  • Both chambers sensed
  • Both chambers paced
  • Either chamber inhibited in response to sensing
  • Atrium paced at the set rate if the native atrial rate is less than the set rate; as AAI
  • Ventricle paced at the set rate if the ventricular rate is less than the set rate; as VVI
    This prevents potentially detrimental rapid ventricular pacing occurring if an atrial tachycardia occurs. It is also more tolerant of a prolonged PR interval than DDD.
  • AV block when ventricular pacing is poorly tolerated
  • AF or flutter with inadequate ventricular rate
  • Pacemaker mediated tachycardia
  • Poor atrial or ventricular sensitivity
VOO
  • No chamber sensed
  • Ventricle paced at a set rate
    High risk R-on-T.
  • Emergency
  • Weaning from CPB
  • Pacing dependence in OR with diathermy use
  • No availability of skilled practitioner with defibrillator
DOO
  • No chamber sensed
  • Both chambers paced at a set rate at a set A-V interval
    High risk R-on-T.
  • As VOO
  • As VOO

Temporary Systems

Temporary systems include epicardial and transvenous systems. These systems provide for:

  • Setting capture and sensitivity
    • Levels should be determined following a pacing check
    • Default settings prior to performing a pacing check include:
      • Output: 10-15mA
      • Sensitivity:
      • Rate: ~10 higher than native rate
        Particularly prior to pacing check, to ↓ risk of R-on-T.
  • Performing rapid atrial pacing

Pacing Check

Full pacing checks cannot be performed if:

  • The underlying rate is inadequate:
    • Symptomatic hypotension
    • MAP <50mmHg
  • Temperature <36°C
  • Hypokalaemia
    K+ <4.5mmol/L.

Temporary epicardial systems should be checked frequently (at least once per nursing shift) to ensure capture and sensitivity thresholds and safe and functional. The full check requires:

  • Preparation
    • ECG and BP monitoring
    • Explain to the awake patient that they may feel pre-syncopal
  • Sensitivity testing
    Test of the pacemakers detection of native cardiac output.
    • Set the pacing rate below the native rhythm
      • 20bpm below a regular rhythm
      • 30bpm below an irregular rhythm
    • Set the output of both leads to 0.1mA
      This activates both chambers but ensures that R on T will not occur as the output is too low to achieve capture. No pacing spikes will be visible on the ECG.
    • Check ventricular sensitivity threshold
      • Start at the lowest value (0.8mV for the ventricular lead)
      • ↑ Sensitivity (mV) until the sensing light is no longer flashing, and the pacing light flashes instead. Then ↓ sensitivity until all native beats are sensed. This is the sensing threshold. The sensitivity setting of the pacemaker is set based upon this threshold.
    • Check atrial sensitivity
      As ventricular sensitivity.
    • Check ventricular capture threshold
      • Set the pacing rate 10bpm above the native rhythm
      • ↑ Output from 0.1mA until 1:1 capture is achieved
        This is the capture threshold. The capture setting should be set to the greater of double the threshold or 5mA, whichever is greater.
    • Check atrial capture
      As ventricular capture.
Setting Sensitivity
Lead Threshold Setting
Atrial <1mv Avoid as backup mode. Inappropriate in ward.
1-4mV 0.5mV
>5mV 1mV
Not checked 0.5mV. Inappropriate in ward.
Ventricular <2mV Leave pacing off.
2-6mV 1mV
7-10mV 2mV
>12mV 3mV

Rapid Atrial Pacing

Rapid atrial pacing is a technique for reversion of some atrial tachyarrhythmias by pacing the atria at 20-30bpm faster than the native atrial rate (usually at 260-340bpm).

Rapid atrial pacing is also known as atrial overdrive pacing.

Revertable and Non-revertable Rhythms
Revertable Rhythms Non-revertable Rhythms
  • Atrial flutter
  • AV junctional tachycardia
  • Paroxysmal re-entrant SVT
  • AF
  • VF
  • Sinus tachycardia

Steps:

  • Ensure accurate invasive arterial monitoring
  • Apply defibrillation pads
    In case of precipitation of a malignant arrhythmia.
  • Stages:
    1. Set rapid atrial pacing rate ~20bpm below the flutter rate
    2. Commence rapid atrial pacing, ↑ rate as you do so until at 20-30bpm above the flutter rate
      Trial for periods up to 20s.
  • If effective, SR should be restored following the rapid pacing period

Troubleshooting

Pacing problems include:

  • Failure to pace
  • Failure to capture
  • Irregular pacing
  • Malignant arrhythmias
  • Pacemaker-mediated tachycardia
  • Crosstalk

Failure to Pace

This can be an emergency in the pacing dependent patient. If unstable:

  1. Ensure the box is connected
  2. Press the emergency button
    Sets DOO at maximal output (usually 20mA atria, 25mA ventricular).
  3. Connect the pacing box directly to the pacing lead (bypassing the connecting leads)
  4. Replace the pacemaker
  5. Institute transcutaneous pacing
  6. Place a transvenous wire

Pacemaker should deliver a pacing impulse, however:

  • No pacing spike is seen
  • Chamber capture does not occur
    The chamber rate is less than the set rate.

Causes and management include:

  • Wire disconnection or migration
    • Box is on, and mode is appropriate
    • Box is connected to cables
    • Wires are connected to the cables
      Consider replacing cables or cleaning connections.
    • Wire location at the skin
  • Oversensing
    • Sensitivity check
    • Control other sources of electrical interference
  • Crosstalk
    • Discuss with electrophysiology
    • Disable less-effective system

Failure to Capture

This can be an emergency in the pacing dependent patient. If unstable, follow the advice for failure to pace.

Pacemaker delivers a pacing impulse but capture does not occur.

Causes and management include:

  • Wire disconnection and migration
    • Box is on, and mode is appropriate
    • Box is connected to cables
    • Wires are connected to the cables
      Consider replacing cables or cleaning connections.
    • Wire location at the skin
  • ↑ Electrical resistance
    • Check capture threshold
    • Consider echocardiography
      Potential for pericardial collection to ↑ capture thresholds.
  • AF
    Prevents atrial capture.
    • Rhythm control

Irregular Pacing

Pacing spikes occurring irregularly, or only irregularly capture the chamber.

Causes and management include:

  • Native rate similar to set rate
    • Assess underlying rate by ↓ set rate
    • Assess for respiratory change in native R-R interval
    • Set rate either well above or below the native rate
  • Tracking atrial arrhythmia
    • Control arrhythmia
      • Cardioversion
    • Change to non A-V sequential mode
      • VVI
      • DDI
  • Undersensing
    • Check sensitivity
    • Disable affected lead if undersensing
  • Oversensing
    • Sensitivity check
    • Control other sources of electrical interference

Malignant Arrhythmia

Pacing system delivers an impulse during a vulnerable part of ventricular repolarisation, resulting in polymorphic VT or VF. Causes and management include:

  • Undersensing of atrial or ventricular leads
    • Commence CALS
    • Pause pacemaker
    • Defibrillation
    • Following ROSC:
      • Repeat sensitivity check
      • Review telemetry

Pacemaker-mediated Tachycardia

Broad complex tachycardia occurring just below the upper ventricular rate of the pacing system. Pacemaker-mediated tachycardia:

  • Occurs when there is both:
    • A-V sequential pacing mode (e.g. DDD)
      i.e. A mode with both:
      • Atrial sensing
      • Ventricular pacing
    • Slow retrograde AV nodal conduction or accessory pathway
  • Results from:
    • Ectopic (or paced) ventricular depolarisation
      Ectopy required so that the AV node is not in a refractory state.
    • Retrograde conduction via the AV node into the atria
    • Atrial activity (via retrograde conduction) is sensed, resulting in delivery of a ventricular impulse after the programmed A-V interval

Many pacing systems include a PVARP, which prevents sensing of an atrial impulse shortly after delivery of a ventricular one in order to ↓ risk of pacemaker-mediated tachycardia.

Therefore, for this to occur the retrograde AV nodal conduction needs to be slower than the PVARP so it is registered when the impulse reaches the atria.

Pacemaker-mediated tachycardia may occur with permanent systems but is usually self-terminated, as the system recognises the arrhythmia and defaults to a DDI mode.

Management:

  • Pause pacing
    This will terminate a pacemaker-mediated rhythm.
  • ↑ PVARP
    Will be limited by the set A-V interval and the set rate.
  • Change to DDI or VVI

Crosstalk

Crosstalk occurs when there are two pacing systems in use. Crosstalk:

  • Usually occurs when there is a temporary epicardial system and a permanent system in place
  • May result in failure to pace if:
    • The faster system fails to capture the chamber it is pacing
    • The slower system senses the attempt to the faster system, and inhibits its own pacing

References

  1. Bersten, A. D., & Handy, J. M. (2018). Oh’s Intensive Care Manual. Elsevier Gezondheidszorg.