- Temporary pacing is performed to support a patient with bradycardia until the cause is reversed or a permanent pacemaker is implanted.
Definitions Types of Pacing
- Transcutaneous pacing
- Pacing via multifunction pads attached to Philips or Zoll defibrillation machine set to pacing mode.
- This mode of pacing relies on electrical conduction of energy through electrodes placed on the chest wall on opposite sides of the heart.
- Patients with a lot of fatty tissue may require higher energy to capture the heart. Always ensure that you have both electrical capture (paced QRS on rhythm tracing) and mechanical capture (palpated pulse or cardiac contraction on echo).
- Patients generally require sedation, as this method is quite uncomfortable for patients, and is only used transiently until a transvenous pacing wire is placed.
- Transvenous pacing
- Pacing via a pacing wire that is inserted through a central large vein into the right ventricle. The pacing wire enters the body through an introducer sheath, and is externally connected to a pulse generator (aka “pacing box”)
- Epicardial pacing
- Pacing electrode wire is loosely attached to the epicardial surface of the heart. This is usually done after cardiac surgery. The wire exits the thoracic cavity via subxiphoid area and connected to an external pulse generator.
- Pace: The ability to deliver an electrical impulse
- Sense: The ability of the pacemaker to detect intrinsic electrical activity from the heart
- Pacing Artifact: The sharp electrical signal recorded when a pacemaker discharges to capture the myocardium.
- Capture: Depolarization of myocardium by an artificial stimulus.
- Capture Threshold: The minimum amount of energy needed to depolarize the myocardium; measured in milliAmps (mA). This measurement is variable, and strongly dependent on the position of the pacing electrode. Electrodes in an optimal position require the lowest energy to capture myocardium.
- Note: Capture threshold should be checked daily as the transvenous pacing wire can migrate into a position that cannot capture the myocardium or requires significantly higher energy to capture.
Settings On the Pacemaker Generator
Output Energy (usually milliamps (mA))
Defines the amount of energy the generator will deliver with each paced stimulus. This is the most commonly changed setting on the pacemaker pulse generator.
This value is often set 2-3 times the capture threshold to ensure enough energy is delivered above the threshold to reliably capture myocardium.
NOTE: Pacing energy can either be set as voltage (V) or current (amps). Conventionally, permanent pacemakers use voltage and temporary pacing systems use current (amps) to specify pacing energy
- Lower Rate (beats per minute – bpm)
- The rate below which the pacemaker will begin to pace. The pacemaker will not allow the heart rate to drop below this setting.
- NOTE: The pacemaker can sometimes pace above the lower rate setting if it’s tracking atrial activity (p-waves) or accelerates in response to activity. However, temporary pacemakers rarely have this feature enabled.
- Sensitivity (millivolts mV)):
- In addition to pacing, the pacemaker also senses for intrinsic electrical activity. Sensitivity defines the minimum amplitude of intrinsic signal before the pacemaker considers it an “intrinsic QRS”.
- Think of this as a fence placed on an ECG. The pacemaker will label a “QRS” any signal that is above this “fence” (see figure).
- NOTE: The word “sensitivity” leads to confusion.
- To lower the sensitivity (detect fewer QRS candidates), one needs to increase the sensitivity value (raise the fence).
- To raise the sensitivity (detect more QRS candidates), one needs to reduce the sensitivity number (lower the fence).
Modes of Pacing
Asynchronous (aka Fixed Rate)- VOO Mode
- Pacemaker paces at the programmed lower rate, and does not sense – it is “blind” to patient’s intrinsic cardiac activity.
- WARNING: Competition may result between patient’s intrinsic rhythm and pacing. If pacing occurs within the vulnerable period within the ST segment, polymorphic VT/VF can result.
Synchronous (aka Demand) – VVI Mode
- Deliver pacing only when when intrinsic rate falls below the programmed lower rate. AKA backup-only pacing, demand pacing.
- If intrinsic rate is above the programmed lower rate: Pacing is inhibited
- If intrinsic rate falls below programmed lower rate: Backup pacing provided.
- The first letter represents chamber being paced.
- The second letter represents chamber being sensed.
- The third letter represents response to sensing.
- Most common forms for temporary pacing are:
- VOO – Pace ventricles, no sensing occurs (asynchronous)
- VVI – Pace ventricles, sense ventricles, inhibit in response to sense ventricular event.
Single Chamber Temporary Pacemaker
Typically ventricle but can be atrial.
- Lowe rate (heart rate)
- Output Energy (mA)
- Sensitivity (mV)
- Ventricular pacing
- Ventricular sensing
- If intrinsic QRS is sensed, ventricular pacing is inhibited
- Ventricular pacing
- No sensing
- Ventricular asynchronous pacing at lower programmed pacing rate
Dual Chamber Temporary Pacing
- Pacing/sensing electrodes are placed in the atrium and ventricles.
- This allows:
- Pacing in atrium
- Pacing the ventricles
- Achieve AV synchrony: (A stands for Atrial)
- A-sense –> trigger V-pace (called tracking)
- A-pace –> trigger V-pace
- Achieve narrow QRS (natural QRS depolarization) if sinus node dysfunction.
- A-pace –> V-sense )
- Lower Heart Rate
- Output (mA) for both atria and ventricle
- Sensitivity for both atria and ventricle
- Upper rate limit
- PVARP -Post-Ventricular Atrial Refractory Period.
- A-V Interval (Paced)
- The ability of the pacemaker to see what electrical activity is being generated by the patient’s own heart to prevent any competition between between pacing and intrinsic heartbeat.
- Sensitivity is measured in millivolts (mV).
- Programming the sensitivity value too low will allow the pacemaker to see all signals and label more QRS candidates. However, the pacemaker may also see small electrical signals (i.e. noise/artifact), and label them as QRS complexes.
- Failure to detect QRS signals.
- This results can be due to:
- Poor pacing wire position – the wire is too far from the healthy myocardium to see good-amplitude signals. Sometimes the wire can get stuck in the annulus, where signals are too small to be seen.
- Pacing wire is in an area of scar, which has small electrical signals, or no electrical signals at all.
- Sensitivity value is programmed too high (fence is too high), and QRS complexes are not detected.
- Asynchronous pacing (cannot see intrinsic QRS)
- Competition between intrinsic and paced QRS.
- Reduce sensitivity value.
- Reposition the pacing wire.
- The pacemaker sees extra signals and mislabels them as QRS. These extra signals can be noise, T-waves, and P-waves.
- This can be due to:
- Setting sensitivity value too low.
- Excessive electrical artifact (LVAD, electromagnetic interference, movement).
- Lack of pacing (lack of output)
- Raise the sensitivity value (to make it less sensitive).
- Reduce artifact (i.e. remove sources of electromagnetic noise, reduce movement).
Failure to Capture
- Pacing artifact are seen, but no paced QRS appears. This is a lack of capture problem.
- P-wave sensed appropriately because the device attempts to pace after each sensed P-wave (pacing artifact follows each P-wave).
- Lack of capture can be due to inadequate programmed pacing energy or migration of the pacing wire.
- First pacing spike falls too close to the native QRS – the second native QRS is not seen by the pacemaker (undersensed).
- This is an inappropriate pacing problem (lack of pacing inhibition).
- Reduce sensitivity value to increase detection of QRS complexes.
- The pacemaker is not pacing when it should – this is a lack out output problem.
- The pacemaker believes it is seeing QRS complexes, when in fact, it is detecting noise. This leads to inadvertent inhibition of pacing.
- NOTE: this problem is intermittent because pacing occurs eventually.
- Increase sensitivity number (move fence above the noise).
Undersensing with physiologic non-capture
- Pacing artifact falls on the the native QRS – the part of QRS where the ventricle is refractory.
- The pacer not detecting intrinsic rhythm. This is an inappropriate pacing (lack of inhibition) problem.
- Inappropriate pacing is due to undersensing because the device does not see intrinsic QRS complexes, which would normally inhibit pacing.
- Decrease the sensitivity value, and ensure it is programmed to a mode with inhibition like VVI.
Pacemaker functioning normally
- The pacer spikes fall into the QRS, this is called pseudofusion.
- Important to know what a intrinsic, paced, fusion, and pseudo-fusion beat should look like.
- The pacemaker paced in the vulnerable part of the ST segment of the intrinsic QRS. This is sometimes called “R-on-T phenomenon”, and can sometimes cause ventricular fibrillation.
- This is caused by inability to sense intrinsic QRS to avoid pacing immediately after the QRS. This can result from asynchronous pacing mode, or if the sensitivity value is set too high to detect intrinsic QRS complexes.
- Program pacemaker to a mode with inhibition (VVI, DDD, DDI).
- Reduce sensitivity value to “see” more QRS complexes.
- Authors: Dr. Daniel Durocher (MD, FRCPC, Cardiologist, ICU Fellow), Dr. Dimitar Saveski (MD, FRCPC, Cardiology Fellow)
- Graphics generously given by Dr. Craig Ainsworth (Cardiologist and Critical Care)
- Staff Reviewer: Dr. Pavel Antiperovitch (MD, FRCPC[Cardiology], Cardiac Electrophysiologist)
- Copy Editor: <Pending>
- Last Updated: February 17 2022