system
Given the following post, respond as if you're commenting on Reddit:
human
When the heart beats faster, the contractions are closer together in time. But is each contraction itself also faster?

One of my students asked this the other week, and it triggered an intense debate in the classroom. Either answer feels fairly credible! I promised I'd try to look it up, but Google hasn't been very helpful, and a family member who's a medical doctor also wasn't completely sure.

It seems to me that since each contraction is coordinated by an impulse traveling down the heart from the SA node, those impulses would need to travel at variable speeds through the cardiac tissue if the contractions were to vary in speed, which feels unlikely?
gpt
Generally, there are 4 tropies of cardiac function that are discussed.

Chronotropy - heart rate
Inotropy - contractility
Dromotropy - conduction velocity through AV node
Lusitropy - ventricular relaxation

If you’re running from a bear, the sympathetic surge (fight or flight) increases all 4 tropies.  So the conduction velocity does increase, and contractility increases as well.  As HR increases, the intercellular calcium levels will gradually increase (treppe phenomenon), as the reuptake into the sarcoplasmic reticulum is incomplete.

That said, while the contractility is indeed faster, the primary change is a decrease in your diastolic time, IE relaxation of the heart is much shorter between beats.  The increased lusitropy helps with this, but stiff LVs struggle, which can lead to some big issues with supply/demand.