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Mode switching: Fast and slow pacing

By Assoc Prof Harry Mond
May 26, 2020

This is a rate histogram from a 24-hour Holter monitor study on a pacemaker patient.

How would you interpret this?

Normally, we see a diurnal pattern of rate changes during the day with lower rates at night, but rarely a straight line, unless there is complete heart block (slow) or a regular continual rapid tachyarrhythmia (fast). This, however, is both fast and slow.

This is the first time I have seen the pattern of the above histogram but recognised it immediately and could write a report without even looking at the tracings.

The straight lines indicate cardiac pacing at the upper and lower heart rates.

There is a low rate (70 bpm red highlight).

This is non-rate adaptive ventricular pacing (VVI), rate 70 bpm with underlying atrial fibrillation being very likely.

Note with the second tracing, the rate is very fast at the beginning and slows abruptly to the low rate.


Now let us look at the upper rate (110 bpm blue highlight).

Once again, the rate response is almost flat and shows mainly pacing at the upper rate, although with conduction the occasional intrinsic QRS complexes were also rapid and irregular and obviously atrial fibrillation. The slight irregularity throughout was due to ventricular ectopy (0.4%). These occurred in clusters corresponding to the fuzzy lines.

What is the diagnosis?

Dual chamber pacemaker with intermittent sensing of fibrillatory P waves and mode switching programmed OFF.  If turned ON, then intermittent failure of atrial sensing.

The issue are poor fibrillatory P wave voltages, resulting in intermittent sensing.

Automatic mode switching (AMS®) was an Australian invention (Telectronics, Sydney, now Abbott) in about 1990 with the release of the dual chamber minute ventilation rate adaptive pacing system, which was able to be programmed to much higher upper rates than activity. The acronym AMS was owned by Telectronics and now Abbott. It was to prevent upper rate pacing with atrial tachyarrhythmias and in particular atrial fibrillation.

This is what we used to see before we had mode switching: Upper rate pacing with atrial tachyarrhythmias.

I called this “Saturday night syndrome” as it almost always occurred on Saturday night, when I was at the movies and I was paged out (no mobiles and I had to beg the theatre to let me use their phone). The diagnosis was confirmed by programming VVI 30 bpm.                       The emergency room protocol was to give beta blockers to “slow” the heart rate, which was nonsense, but occasionally reverted the arrhythmia.

How do we rectify in this case the problem with mode switching, whether it be ON or OFF?

Atrial sensitivity can be increased (lower value) but usually the fibrillatory P waves are so fine that it doesn’t help much.                                                                                                            Instead program DDIR if the atrial fibrillation is paroxysmal or VVIR if permanent. Because of the flat rate response at rest we need rate adaptive pacing.


Now that we know how to interpret the histogram try this one!

Once again, there is an implanted pacemaker, because of the flat line overnight.

If you look carefully, there are two low rates!


Here is how I interpret the histogram.

 There is a programmed low rate of 70 bpm, but when asleep the rate falls to 60 bpm, which technically violates the lower rate limit. This is due to a programmable function called rest rate and this example is from Abbott.

The activity sensor determines when the patient is resting (day or night) and drops the lower rate. With Medtronic, it is time dependent and you must enter the time you go to bed and get up. In both situations, when the sensor is activated, the rest rate is turned OFF.


About Assoc Prof Harry Mond

In 49+ years as a practicing cardiologist, Dr Harry Mond has published 260+ published manuscripts & books. A co-founder of CardioScan, he remains Medical Director and oversees 500K+ heart studies each year.

Download his full profile here.

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