Question 1

a) Outline the advantages and disadvantages of the following modes of epicardial pacing post cardiac surgery.

  1. AAI 
  2. VVI 
  3. DDD

    (80% marks)

A patient arrives ventilated to the ICU following a CABG and aortic valve replacement. They are on low dose noradrenaline and a native rate of 60 beats/minute in sinus rhythm. They have VVI epicardial pacing wires in situ with a pacemaker backup rate of 40 beats/minute.

You are called to the patient when the monitor trace below is demonstrated (see Figure 1.1)

SAQ image which is not the original CICM image

b) State the pacing issue, and list the specific actions you would perform with the pacing box. (20% marks)

(Image removed from report.)

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College answer

Not available.


Advantages and disadvantages of various pacing modes is a topic that has come up in the past, specifically in the form of Question 20 from the first paper of 2007. To see it return in the 2020s is a positive sign - repeating questions about important topics is not a failure of SAQ design, but a way of emphasising important material (because assessment drives learning), and prevents a drift towards esoterica which can occur if the examiners intentionally avoid repeating SAQs.

The answer to that old SAQ is reproduced below with zero modification. 

A Comparison of AAI, VVI and DDD Pacing Modes
Pacing Mode Description Advantages Disadvantages
AAI atrial demand pacing
  • Only one lead required
  • Preserves AV synchrony in the presence of an intact AV node
  • Only paces on demand; allows intrinsic rhythm while the rate is high enough
  • Useless if the AV node is dysfunctional
  • Useless in the presence of AF
  • Increased risk of perforating the thin atrial wall
VVI ventricular demand pacing
  • Only one lead required
  • Only paces on demand; allows intrinsic rhythm while the rate is high enough
  • Asynchronous AV contraction
  • Difficult to assess ST changes (LBBB-like QRS morphology)
DDD dual chamber demand pacing
  • AV synchrony is preserved
  • "Pacemaker syndrome" is thus avoided
  • Advantageous in patients with normal atrial activity and a dysfunctional AV node
  • Versatile mode, with hemodynamic advantage 
  • Two leads required
  • Bundle of His is bypassed
  • Possibility of "endless loop" tachycardia
  • Increased risk of perforating the thin atrial wall

The other (20% mark) part of this question, which relates to the troubleshooting of the pacemaker circuit, is impossible to reconstruct from the wording of the question, because "(Image removed from report.)" The post-AVR patient described in the stem could theoretically develop any range of temporary pacing wire complications. For the purpose of keeping things simple, undersensing was used here, because it fits the scenario. So:

Pacing issue:  undersensing. This is a situation where the pacemaker, instructed to inhibit itself whenever a viable P or QRS comes along, instead paces irresponsibly, ignoring normal cardiac activity. This pacemaker is in essence asynchronous.


  • Reduce the output to minimum immediately, to minimise the risk of R on T phenomenon. This should be safe, as the patient has a native rate of 60.
  • Keep the rate 40, i.e. lower than the patients native rate
  • Increase the sensitivity (i.e. keep reducing the sensitivity value) until no pacing spikes are seen. The value at which this happens is the sensitivity threshold.
  • Reduce the sensitivity setting to half of this value, to prevent this from happening again.


Gillis, Anne M., et al. "HRS/ACCF expert consensus statement on pacemaker device and mode selection." Journal of the American College of Cardiology 60.7 (2012): 682-703.