Technical question

If I have a slow rate, say 40 and I have a PM set with a 60 minimum, what happens when the PM fires because I'm running slow? My heart is still running at 40 so it will fire again soon after. Why don 't I wind up with an irregular cadence and a rate closer to 100? (40 plus 60?)
Maybe a dumb question.

4 Comments

Pacemaker vs own rhythm

by golden_snitch - 2014-08-18 02:08:54

If the pacemaker base rate is higher than your own heart rate, the pacemaker will simply "overdrive" your rhythm. Once the heart cells have been stimulated, they go into a refractory period, in which they are unable to respond to another impulse. So, even if your sinus node would fire, they'd not react to that impulse.

Inga

Follow up

by Jackw - 2014-08-18 11:08:30

Inga:

Many thanks for the answer. A follow up question. A heart can easily run at 140+ when exercising, which means that there is a fairly short latency, so how is this situation different? I can see that refractory period would apply if the natural impulse and the PM pulse were close together, but if the natural pulse is, say, in the the middle of the 60 natural sinus rate, what is to keep the muscle from contracting? Does this mean that my natural system doesn't work at all? Then the PM is doing all the work and I would guess the battery life would be less than the 10 years I was told to expect.

I still find this all a bit puzzling.

Base rate

by golden_snitch - 2014-08-19 02:08:34

Refractory periods shorten the higher the heart rate. But ask your doctor, he might be able to explain this better. I have kind of been there, done that with the situation you describe - natural rate at 40, pacemaker at 60 - and I can assure you that you'll only have a pacemaker rhythm in that situation, the ECG won't show the sinus node kicking in.

If you have a natural heart rate of 40, and the pacemaker is set at 60, then the pacemaker will do all the work, at least at rest. But since you said your rate drops to the 40's only sometimes, you might not be paced much at all.

Battery life depends on a lot more than just pacing percentage. More important is for instance the amplitude (output voltage). So, you can still very well get more than 10 years out of the battery. I even went from 100% atrial pacing to 100% atrial AND ventricular pacing, and that did not change anything in the battery life. Voltage, impedance, features activated, ... - that's all more important than the pacing percentage.

Inga

You asked about rates

by Jackw - 2014-08-20 10:08:03

Inga

Thanks immensely for sharing your knowledge and experience with a newbie.

My rate is tough to track because the aflutter kicks in. Exercise, especially aerobic stuff like running, seems to get it going more vigorously so my HR monitor reads 2x. (I have what is called 2x aflutter) when I came in from running tonight I put it on and it read 120 or so for a while with some fluctuations down to as low as 98. Now after supper and taking my beta blocker I am reclining quietly and it is running at 68-70 so the flutter has settled down. What I need to pin down with the doc is whether I can expect to run at least 55-60 after an ablation, without a PM and no beta blockers. He was sort of inclined to do the PM before or along with the ablation. Maybe just a very conservative approach but since I would be in over night after the ablation hooked up to a monitor the risk seems manageable.

One last point is I had a cardioversion early this year to get rid of the flutter. It worked but only for eight weeks or so. My rate at rest was running as low as the 40s at night and early morning according to the Holter monitor before the flutter came back. It was then that the docs first started talking about needing a PM within the next 2-3 years. Now the flutter is back so we are talking ablation which seems like the right way to go. Just not sure about the PM. One thing that makes me lean toward a PM is that my exercise rate doesn't ever get above 130 even if I am exercising hard. What happens is I run out of oxygen and that keeps me from pushing too hard.

Thanks again. This site is very helpful.

Jack

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