A-Fib's Biochemical Origins

[RoadRat]

I'm a paramedic student.
I'll explain the patient which inspired this question, then I'll summarize my question.

Patient:
During an ER clinical, a middle-aged hypotensive pt (70/32 BP) previously diagnosed with A-Fib with RVR presented with a strongly fluctuating heart rate. This pt's heart rate was consistently >100 for the first several hours I was in contact with her. During the last hour of pt contact, her heart rate would swing from 72 to 163 in a mere 10 seconds. Then, just as quickly, her heart rate would fall to 62. Back up to 154. Down to 48. You get the picture.
The pt remained A&Ox4, low but stable BP, and asymptomatic.
No dizziness, nausea, tiredness, anxiety, or any other physical disturbance.
Her 12-lead showed a ST depression. Her ST segment doesn't reach the isometric line until slightly before the following QRS complex. Consistently at least 0.24 seconds.
Pt has a dense medical history of end stage renal failure, liver failure, CHF, and several others I'm sure she forgot to mention.
Pt reported faithful 3x weekly dialysis.
Pt was actually brought to us by EMS at the completion of her dialysis therapy that same day.
Pt was being treated with a norepinephrine drip, an epinephrine drip, an amiodarone drip, and 4lpm O2 via NC. Fluid challenge was not attempted due to renal failure.
(Sorry, I don't have the rates or dosages of the norepi, epi, or amiodarone.)

Question:
What about the irritable cardiac tissue OR the electrolyte imbalance would cause her heart rate to swing so wildly and so quickly?

 

[Doczilla]

Generally, there's two explanations for tachyarrythmias:

Irritability

Or

Excitability

For irritable tissue, (ischemia, injury, toxicity) cellular changes with ion gates and sequestration/leakage of things like calcium and potassium can change the resting membrane potential; making cells more likely to depolarize. Ever seen or heard of Charlie horses, seizures after head trauma, PVC's, things like that? Its the same thing happening in different parts of the body. That's why acute a fib should have a suspicion of ischemia behind it before anything else.

The fluctuations occur because the atrium (which is depolarizing at 200+ times per minute in a fib) can't always carry the conduction to the ventricles. The reason for this is the AV node, and since it's made of muscle (as opposed to nerve) it acts like a speed bump for conduction.

This is a great thing. It can take an atrial rate of 200 and turn it into 100. That's why people with WPW and similar conditions are so prone to tachyarrythmias, because that nerve is bypassing the AV node.

So, the reason these fluctuations occur is because occasionally more impulses escape through the AV node for a period of time. It's completely random. Hence the term "irregularly irregular".