Question 3

Outline your approach to the evaluation and treatment of a cardiac surgical patient who returns to your Intensive Care Unit with temporary atrial epicardial pacing wires and problems with atrial pacing.

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College Answer

Evaluation of problems with atrial pacing requires careful evaluation of the rhythm strip (and/or ECG), and systematic examination of pacing leads (from patient to pacemaker, including ensuring atrial wires are connected to atrial port of pacemaker). Specific problems to be excluded include:
·           excessive sensitivity to electrical activity (resulting in inappropriate/excessive inhibition of atrial pacing). Excessive sensitivity is usually due to settings on the pacemaker, and may be confused with return of spontaneous atrial activity (including AF). Treatment is to increase the absolute value of sensitivity (making it harder to inhibit).
·           relative insensitivity to electrical activity (resulting in atrial pacing when not appropriate). Insensitivity is due to the specific setting of sensitivity (including deliberate setting of AOO mode). Treatment is to decrease absolute value of sensitivity (making it easier to inhibit).
·           inability to capture (resulting in no atrial activation). Inability to capture is usually due to some specific mechanical problem including: wires no longer connected to atrium [potentially activating ventricle, diaphragm or nothing at all], wires not tightly connected to cable, cable not connected to correct port, or setting of output too low relative to requirements. Treatment is to tighten and confirm all external connections, then increase output if possible. Bipolar leads may be tried in reverse positions, or attempt to convert to unipolar pacing. Positioning of patient may also facilitate capture (short term solution). Other specific treatments to be considered include treatment of underlying arrhythmias/bradycardias with appropriate medications.


It is difficult to say precisely what ones management would be if the college doesn't specify what "problems" there are with the atrial pacing.

A guide to troubleshooting the pacemaker circuit is offered elsewhere.

In brief:

1) Start with the box.

  • Is it even on?
  • Is the battery dying?
  • Are the wires detached from the pulse generator?
  • Are the leads connected?
  • Was the temporary pacing wire pulled out in course of a recent pressure area care?
  • Are the epicardial electrodes displaced? Is the transvenous electrode tip wiggling uselessly in the venticle?
  • Is there any weird twitching in the chest wall muscles of the patient? Is the ventilator demonstrating some bizarre sawtooth pattern, suggesting that the diaphragm is being paced?

Ok, so the hardware is intact. if there is output failure, its not because of the leads or the battery. Move on to the software.

2) Check the sensor threshold.

  • Put the pacemaker in a VVI, AAI or DDD mode.
  • Change the rate to one which is much lower than the patients native rate.
  • Observe the sense indicator.
  • Keep increasing the sensitivity.
  • Find the sensitivity maximum - where the pacemaker is picking up NONE of the endogenous electrical activity.
  • Now keep decreasing the sensitivity.
  • Find the sensor threshold - where the sensor picks up EVERY endogenous electical event (i.e. no pacing spikes are visible)

Crank the sensitivity setting up to double the sensor threshold.

This should take care of oversensing as a cause of pacing failure.

3) Now, check the output threshold.

  • Set the pacemaker well above the native rate.
  • Start reducing the output.
  • Find the capture threshold - where a QRS complex no longer follows each pacing spike.

Crank the output to double the capture threshold.

4) Still not working?

  • Roll the patient to one side, and then another. Sometimes this influences the position of the transvenous pacing wire tip just enough to get you some capture.
  • Reverse the leads. Sometimes this works, but logically - it shouldnt.
  • Convert to unipolar pacing. Attach the negative lead to the positive electrode, and the negative lead to the subcutaneous tissue of the chest.
  • Give up. Time to pace externally while waiting for another wire to be floated, or the epicardial leads to be resited.