Like some of the others who have recently commented on this forum on the topic, I occasionally get irregularity during exercise -- both PAC and PVC. However, unlike most people I'm normally consciously aware of the internal processes going on in myself (and have the ability to consciously control and manipulate it to some degree). So, I can elaborate, in quite some detail, about what exactly is happening.
For me, PVC's typically occur during a specific transition period when the heart rate is slowing down from a high rate back to normal rate and, in myself, when it happens, it tends to be when it's crossing the 140 BPM threshold. It tends to be linked to respiration and appears to be an extreme form of sinus arrhythmia where the prolongation of a cycle reaches the point that it overlaps with the following cycle and simply gets dropped.
Second, there also appears to be not one, but two separate signals that drive the heart beat, with one being the "slow rate" driver, the other the "high rate" driver. During the transition period, they may overlap and you get a brief period of conflict, while one passes off control to the other.
PAC's are less frequent, but I had several instances of this during today while doing the Harvard Step Test and while measuring the heart rate and observing its activity and electrical signaling. They, too, appeared to be linked to a cross-over threshold, and seem to be the result of conflicting signals coming from two separate drivers. During the slow-down, there appeared to be a tendency for a *forced* slow-down to occur (as has occasionally occurred during other Harvard Step Test followups), with the higher rate superposing itself on top of this and (to a limited degree) going in tandem with it. When coming off a Harvard Step Test, the first 60 seconds will generally be in the 150 BPM area. During the following 30 seconds it may drive downward to under 130 or even under 120. If it drives dramatically downward (the "forced slowdown"), even to under 120, that's when the conflict may occur. The rate will dramatically switch from as high as 140-150 to around 120.
There are definitely two signaling sources operating the heart. One is internal to the heart, the other appears to be coming from the brain, itself. The forcing is the one coming from the brain, while the one in the heart is still driving at around 140 BPM. The closest analogy is the effect of slamming on the brakes while the car is running on ice.
Another analogy is a controller for a stepper motor driver suddenly deciding to drop the "step" signal rate while the motor is still running at high speed. In extreme cases, this can cause the motor to lock up.
Because of the ability to consciously control the heart beat, I'm not entirely sure if any of this is a case of the "observer getting in the way", when it happens. Today's case was unusual, because I saw the signs of what appeared to be a brief third signal superposed on the other two, over a single-beat period, at two times about 5 seconds apart from one another.
I'll be keeping watch over this. It's fascinating to see all this complexity.
Sounds good. If you have any concerns about this, you Should consider talking to an MD.
Complex situations are difficult to understand and sometimes doctors decide to measure surface potentials to understand any abnormal heart beats. If the situation is really complex, they go inside your heart with a probe for a closer look at the local electrograms. Sometimes that is the only way to know the difference between a benign underlying cause, and something more serious. These days portable electronics are available too to help with these evaluations.
I too have had what I'm told are PACs coming down off of high respiration rates, as well as extremely sinus heart rates such as what you'd see with bursts of sprinting; 200bpm and a little above. As you say, there's a common crossing point where the PACs begin, and also instead of a stutter in the pulse, there's more like a pause, then the pulse continues. Then as repiration and pulse slows, the PAC's subside.
I want to point out that with a near lifetime history of SVT, I'm generally more atune to my heart than others may be. I had initially assumed what I was feeling during these events were PVCs. It wasn't until I was hooked up to an EKG recorder was administered the standardized treadmill test to failure (nearing 14 minutes) that my cardiologist noted that I was throwing PACs and NOT PVCs as I had bvelieved they were. He wasn't concerned at the number seen during the cooldown period. For me, PVCs occur in the "normal" heart rate range, 60-100bpm; rates you'd see during everyday activities. So all of that was to ask you: How do you know what you're experiencing is PAC or PVC?
I have noticed that PVCs hit hard than PACs do. PVCs have a more forcefull "bump" than PACs How about you?
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