What is the feedback loop that regulates the artificial heart's pump rate, assuming it exists at all?

I would guess so because beta-blockers are also prescribed for anxiety as well as various heart conditions, and breaking the feedback loop is the intended effect. I'd imagine therefore that you'd be a bit less anxious as a result.

It's the body telling the heart "Pump more, we need extra oxygen".

Not having a heart doesn't change his mood, his mood changes the heartbeat.

That's why there were so surprised he could play sports - without that acceleration he might not have enough bloodflow for his activities.

I wonder if it has a knob to manually adjust the flowrate.

Or maybe since it's mechanical it's fine for it to run at max at all times? That might not be good for the rest of the vascular system though.

If that's bad for the vascular system then an idea would be to measure O2 saturation in the toes, and adjust the flow if it gets low.

Even if it is, I assume everything that comes with knowing you don't have a heart is too much of a factor for this to be measured/evaluated.