With the establishment of atrial pacing leads, work commenced on re-establishing atrioventricular synchrony in order to achieve physiologic pacing.

The ECG appearance of dual chamber pacing (red highlight) is atrial pacing (Ap) followed soon after by ventricular pacing (Vp) with the intervening interval referred to as the atrioventricular delay (AV).Let us revisit the three letter code in relation to dual chamber pacing.

Theoretically, there are a large number of possible dual chamber groupings with many of the exotic combinations incorporated into programmable pacing systems, but rarely or never used. The earliest atrial leads were non-steroid eluting and unipolar and had very high stimulation thresholds and poor sensing. Consequently, early dual chamber systems with very limited programmability, mainly provided asynchronous atrial pacing combinations.DOO AsynchronousD Atrial and ventricular pacingO No atrial or ventricular sensingO No chamber response

DOO: There is unipolar atrial and ventricular pacing (red highlight). A ventricular ectopic (yellow highlight) is not sensed and pacing continues unabated with probable atrial pacing, but not ventricular (red vertical arrow) as it lies within the ventricular refractory period. The next atrial stimulus artefact occurs at the commencement of a sinus P wave (blue highlight) and is an atrial fusion beat. Then follows a series of sinus P waves prior to and faster than the asynchronous atrial stimulus artefacts (green highlight). Because the underlying rhythm is complete heart block, ventricular pacing is not interrupted following the ectopic.DVI D Atrial and ventricular pacingV Ventricular sensing only I Ventricular inhibited.Atrium triggers ventricle after AV delay.Effectively AOO + VVI pacing. Very rarely if ever used today.

DVI: Unipolar atrial and ventricular pacing (red highlight) with atrial pacing (red vertical arrow) and ventricular pacing (blue vertical arrow). There is a sinus P wave (green vertical arrow) which is not sensed, and the asynchronous atrial stimulus artefact occurs soon after at the commencement of the conducted QRS. The AV delay is now shortened to 110 ms (yellow highlight) which is called safety pacing. In some DVI pacing systems, safety pacing was an non-programmable feature designed to prevent ventricular pacing within the T wave of the native QRS which lay within in the AV delay. The AV shortening was designed to prevent the stimulus artefact from precipitating a ventricular tachyarrhythmia.There is a sensed ventricular ectopic (blue highlight) and despite asynchronous atrial pacing, the next atrial stimulus artefact has been reset. This is because all early dual chamber pacemakers had ventricular (V-V) based timing in contrast to today where it is atrial (A-A) or hybrid timing. With A-A timing (red horizontal arrows), the next atrial stimulus artefact should occur where the red vertical stippled arrow lies. With V-V timing it occurs later (blue horizontal arrows).DDI D Atrial and ventricular pacingD Atrial and ventricular sensing I Ventricular inhibited.Atrial sensing does not trigger ventricle.Effectively AAI + VVI pacing, but a sensed P wave does not trigger AV synchrony. Designed at the same time as DDD pacing, it remains useful to prevent upper rate ventricular pacing with atrial tachyarrhythmias when mode switching is ineffective. For full physiological effect, maximum atrial pacing is desirable.

DDI: There is dual chamber pacing (red highlight). P waves (yellow highlight) are sensed and will inhibit atrial pacing, but there is no AV synchrony with atrial sensing.VDD V Ventricular pacing onlyD Atrial and ventricular sensingD Ventricular inhibited.AV synchrony (A triggers V)Chronologically, VAT or atrial sensing- asynchronous ventricular pacing was available in the very early 1970’s, but clinically very unsuccessful. During the early 1990’s, atrial sensing-ventricular inhibited pacing (ASVIP) was also unsuccessfully attempted in young patients with complete heart block.

VDD: Atrial sensing (red vertical arrows) triggers ventricular pacing (red highlight). With sinus bradycardia below the programmed low rate (carotid sinus massage), there is loss of AV synchrony (red stippled vertical arrow) and non-physiologic ventricular pacing occurs (yellow highlight).A P wave sensed early after ventricular pacing will trigger the ventricle but cannot violate the upper rate limit and thus the AV delay is prolonged (blue highlight) to the maximum ventricular pacing rate.

VDD: Response to ventricular ectopics. The compensatory pause may (yellow highlight) or may not (blue highlight) allow the next sinus P wave to be sensed depending on the programmed low rate.In the era of limited programming, VDD pacing could result in incessant pacemaker mediated tachycardia, if retrograde conduction resulted in a P wave outside the refractory period and triggered ventricular pacing as a tachycardia near the upper rate limit. Drug therapy, magnet application or even pulse generator removal was necessary if programming was not available.

VDD: There is atrial sensing-ventricular sensing (red highlight). A run of pacemaker mediated tachycardia (yellow highlight) terminates in a pause without a P wave and thus there is a ventricular paced beat (blue highlight).An alternative explanation is a short run of atrial tachycardia.DDD D Atrial and ventricular pacingD Atrial and ventricular sensingD Atrial and ventricular inhibitedAV synchrony (A triggers V)Now established as the default dual chamber pacing mode with the other modes used only in specific circumstances.

DDD: There is dual chamber pacing (red highlight) and atrial sensing-ventricular pacing (yellow highlight). If atrial pacing and a sinus P wave occur at the same time atrial fusion results (A fusion, blue highlight).

DDD: Atrial sensing-ventricular pacing (red highlight) followed by atrial sensing-ventricular sensing with negligible change in the AV delay.

DDD: Response to ventricular ectopics. The compensatory pause may (red highlight) or may not (yellow highlight) allow the next sinus P wave to be sensed depending on the programmed low rate (850 ms-70 bpm). Once again there is atrial fusion (blue highlight).Here is a summary of all the pacing modes as discussed.

There are a number of other programmable modes not discussed. VDI will be discussed with mode switching algorithms, DDT with biventricular pacing and OOO may be used temporarily during magnetic resonance imaging.Harry Mond