DDD? VVI? Pacemakers in the ED

This post is inspired purely by my ignorance, and a referral the other day.

Referred a pleasant chap with bradycardia to the cardiology SpR. However my usual witty banter and razor sharp clinical acumen ground to halt when asked about his pacemaker….

Me: “It was inserted for bradycardia 3 years ago”

Cardiology: “What mode?”

Me: “Huh?”

Cardiology: “Is it DDD?”

Me: “Erm…”

Cue a swift retreat to the notes to find out. So in proper Problem Based Learning style, I’m here to get my head round pacemakers. We’ll start with a look at pacemaker modes (that DDD thing), and then a quick overview of pacemaker problems you might encounter in the ED.

Permanent pacemakers are implanted devices that provide electrical stimuli to the heart to induce contraction. They comprise of:

  • A pulse generator – which is the small device you can feel under the patients skin. This includes the battery, and all the clever electronics. It is implanted under the skin (usually on the left side of the chest).
  • Pacing leads. These wires are implanted transvenously and the tips embedded in the endocardium of the desired chamber. The wires deliver the electrical impulses to the heart, and can also pick up intrinsic activity and relay those signals back to the pulse generator.

Pacemaker modes

The pacemaker mode is described using a sequence of letters…..

  1. The first letter tells us which chambers of the heart are paced. This could A (the atria), V (ventricles) or D (dual, or both).
  2. The second letter tells us which chambers the pacemaker is ‘sensing’. Again, this could be A (atria), V (ventricles), D (dual) or O (none, no sensing occurs).
  3. The third letter tells us what the pacemaker does it if detects activity in a ‘sensed’ chamber. The response could be T (triggers the pacemaker), I (inhibits the pacemaker), D (dual – this means that atrial activity would trigger the pacemaker, but if atrial and ventricular activity are detected, the pacemaker is inhibited).
(There are also a fourth and fifth letter, but for our purposes the first three are all we need to know about)
After going through it, I’m still confused, so lets look at some examples:

VVI Mode



VVI pacemaker

The simplest mode. If we look at the codes above we can work out that this pacemaker will

Pace the ventricles only…..

Sense activity in the ventricles only….

Be inhibited (i.e not fire) if it senses activity.




DDD pacemaker

DDD is the most common setting out there. This means that:

The pacemaker paces both atria and ventricles (D)…..

and the pacemaker senses in both the atria and ventricles (D).

It has a dual response, meaning it responds differently depending which chamber it detects activity in (D):








Of course, occasionally pacemakers malfunction. When this happens the unfortunate patient often turns up at the ED. As you can imagine, they are often pretty unwell – usually complaining of dizziness or syncope. Your admission ECG will be essential in working out what is going on.

For simplicity I split the causes of pacemaker failure into 3 groups

1. Pacemaker not pacing heart.

You will see no pacing on the ECG and the patients underlying rhythm will be revealed, for example:

2. Pacing but not causing contraction; you will see pacing spikes not followed by cardiac activity:

These problems can occur for many reasons, battery failure, lead fracture or dislodgement, but this is not really important in the ED. What is important is that the patient is now relying on their underlying rhythm.

3. Pacing when its not supposed to pace. The pacemaker is happily pacing away regardless of what the patients heart is doing. This can be a mild irritation, or a life threatening emergency. If the pacemaker spike happens to fall on the vulnerable part of the t-wave it can trigger VF.

Other problems caused by inappropriate pacing include include pacemaker syndrome and pacemaker mediated tachycardia, which I will discuss further below.


1. A. B. C.

A cliché I know, but when in doubt, its the way to go. Manage these patients according to their presenting rhythm, go down your ALS algorithms for arrhythmia if required. The presence of a pacemaker does not change this intial management. Cardioversion and defibrillation can be performed as normal, remembering to keep the pads at least 10cm from the pacemaker. Patients presenting in bradycardia may requir external pacing again performed as normal.

An ECG is a must (d’uh), and a CXR. You may be able to visualise a fracture of a pacing lead on the x-ray:

On point to remember – pacing leads reach the heart via the veins and the SVC. This means that poking around with a CVC guide-wire in the same vein may not be a great idea. You may trigger the pacemaker, or dislodge a thrombus which not infrequently develop round the leads. If in doubt, femoral is the way to go…

2. Get specialist advice. A cop out on my part yes, but as far as I know most ED’s will not have the equipment or expertise to assess and adjust a pacemaker. You may have access to a magnet, which can be used to reset a pacemaker, usually to a basic asynchronous mode. However the variations in devices and settings mean it is not possible to generalise too much. Bottom line, leave it alone unless you know what you’re doing.


Specific pacemaker problems

  • Pacemaker mediated tachycardia. This happens like this – a ventricular contraction is transmitted back up via the AV node. This is sensed by the atrial lead and another ventricular contraction is stimulated. The pacemaker ends up sensing its own activity and gets itself in an endless loop. Now you shouldn’t see any daft heart rates here, as the pacemaker as a limit, but it can be enough to cause tachyarrythmia related symptoms. This is one occasion where resetting the pacemaker with the magnet will relieve the problem, and may be needed ASAP in an emergency.
  • Pacemaker syndrome. Usually occurs soon after implantation, with the patient presenting with symptoms of congestive cardiac failure.  This occurs with ventricular pacing. Ventricular contractions are no longer synchronised with those of the atria, and atrial filling of the ventricles is lost. Therefore, cardiac output drops, and heart failure develops. Manage the heart failure  and get someone with the skills needed to fine tune the pacemaker.
  • Infection around the generator or the leads can occur. As with any infection of a indwelling device, this needs taking seriously. Screen for severe sepsis, load with IV abx and get a surgical assessment to see if there is anything that needs draining. These patients can deteriorate rapidly if the sepsis not managed aggressively.
  • Erosion of pacemaker site – unusual, but erosion of the pulse generator through the skin is possible. IV abx and send back to cardiology to get it sorted out.
  • Twiddler’s syndrome. Worth mentioning just for the name. Some patients will persistently twiddle and mess about with the pacemaker, to the point where they can twist or damage the leads. If this occurs then the pacemaker may malfunction.
That quite enough on pacemakers for now. Take home points for me are:
1. Pacemaker codes: 3 letters. The first tells you which chambers are paced, the second tells you which chambers are sensed, and the third tells you what happens when it senses activity.
2. If you suspect a pacemaker is not working, manage the patients arrythmia as normal, but get specialist guidance in case the pacemaker needs adjusting.
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