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Temporary Pacing

BY Ram ST7 ICM Royal Papworth Hospital

Some history and why?

Temporary pacing wires have been used since the 1960s to help manage cardiac rhythms in post cardiac surgical patients. Arrhythmias remain common problems following cardiac surgery with up to 40% of patients experiencing atrial tachyarrhythmias


  • High risk surgical patients
  • Risk factors include
  • advancing age, valvular surgery, poor left ventricular function, structural heart disease, diabetes mellitus, preoperative beta-blocker or digoxin-use, and a history of arrhythmias.

Position of Pacing Wires

  • Ventricular wires are sutured onto the diaphragmatic surface of the right ventricle, while the atrial wires are attached to the right atrium with sutures or clips
  • The atrial wires are then brought rightwards of the sternum, ventricular wires are brought out through the skin leftwards of the sternum

Pacing check

  • Slowly reducing the pacing rate until no pacing spikes are seen reveals both the underlying heart rate and rhythm.


  • Sensitivity--->measured in millivolts
  • refers to the ability of the pacemaker to sense the current created by the underlying cardiac electrical activity
The general range is 0.4-10mV for the atria, and 0.8-20mV for the ventricles
  • A pacemaker that is unable to sense the underlying rhythm is said to be undersensing and has a high sensitivity value.
  • An undersensing pacemaker--> is unable to discern where in the cardiac rhythm a pacing spike should be delivered, which risks delivery of random pacing signals.
  • An undersensing pacemaker increases the chances of pacing on a T wave in a so-called R on T phenomenon, which can lead to ventricular fibrillation


  • An oversensing pacemaker means that the pacemaker ‘over-reads’ the cardiac rhythm.
  • In this setting, the opposite is true. The pacemaker will not discharge a pacing spike when in fact it should.
  • Pacing up will not occur for patients with an underlying heart rate below the set pacing rate.


  • The minimum current that the pacemaker can sense is termed the sensing threshold and the sensitivity is often set to half this value.


  • It is the current delivered by the pacemaker in milliamps.
The minimum current required to induce an actional potential in the myocardium is termed the capture threshold and can be observed practically as the minimum current required to produce regular pacing spikes with appropriate morphological rhythm changes
  • The output is often set to a value double that of the capture threshold.
  • Low output risks pacing signals having no, or indeed variable, effects on cardiac conduction.
  • Inappropriately high output risks extracardiac pacing such as of the phrenic nerves and acceleration of fibrosis at the lead/myocardium interface.

To check the ventricular sensing threshold

  • Place in VVI, AAI or DDD, increase sensitivity value until the sense indicator stops flashing
  • asynchronous pacing should take place, then turn sensitivity down until sense indicator flashes which each endogenous depolarization
  • this equals the pacing threshold
  • set pacing generator at half the pacing threshold
  • Why is the sensitivity threshould kept half of the observed value?
  • to ensure that the cardiac electrical activity will be sensed even if the electrode tip overgrows with fibrous tissue.

Ventricular Capture Threshold

1. The ventricular sensing threshold must first be assessed as satisfactory.2.The pacing rate should be increased to 10 bpm above the underlying heart rhythm, with care taken in tachycardic patients.3.Increase vent output until regular pacing spikes are seen on the rhythm strip along with morphological changes.4.This value is the capture threshold, and the ventricular output should be set to a value double that of the capture threshold.

  • The output is doubled to compensate for any increase in resistance/ fibrous growth on the leads.
  • If capture threshold approximates the maximal output setting --> an early referral to electrophysiology
The atrial capture threshold should be assessed in a similar fashion.

The pacing code


  • The ventricle is sensed, and if it produces a QRS, the pacemaker sits quietly and waits.
The VVI pacemaker will only fire if there has been no ventricular electrical activity within the interval specified by its set rate.


  • DDD is a sequential pacing mode
  • In addition to ventricular pacing, DDD provides an atrial kick, which contributes an extra 25% to the cardiac output.
  • For patients who develop a high-grade atrioventricular block, they are not at risk of suffering from a low cardiac output state.


  • In AAI the atrium is sensed as well as paced.
  • There is no reaction if the atrium fires normally- the pacemaker listens for normal atrial activity, and does not interfere with it.
  • If the atrial activity is occurring at a rate slower than the set pacemaker rate, the pacemaker will step in and fire

Problems associated with pacing

  • Failure to pace occurs when the pacemaker is set to deliver electrical impulses but no corresponding pacing spikes are seen.
  • There is a problem either with the pacemaker, the pacing wires, or at the lead/myocardium interface.
  • R on T
  • A premature ventricular complex occurring at the critical time during the T wave of the preceding beat precipitates ventricular tachycardia and fibrillation.
  • Occurs if loss of sensing of the intrinsic rhythm becomes evident.


Complications from pacing/wires

  • Tamponade, ventricular perforation, ventricular arrhythmias, and disruption of coronary anastomoses.
  • Pacing wires are also at risk of localized infection and myocardial damage.
  • More common issues pertain to their function

Any questions?