While we gather Heart Rate data from the player, Nevermind actually looks at a metric called "Heart Rate Variability" (HRV) when "reading" the player's fear and stress levels.
What's really cool about HRV is that it doesn't need any permanent or long-term calibration. For the purpose of Nevermind, all it needs is a brief period of time to get a base-line reading of the player's natural heart beat cadence.
While Heart Rate is essentially the speed at which a person's heart beats, HRV is the consistency of the length of time in-between beats. It may sound counterintuitive, but it's a good thing to have an "inconsistent" heart rate. In other words, you want it to be something like "fast" "fast" "fast" "slow" "slow" "slow." That means you're feeling good - you're calm but alert! When you get stressed, though, your "fight or flight" system kicks in and your heart beat becomes more consistent. It looks something like "fast," "fast," "fast," fast," "fast," etc. And this transition from inconsistent to consistent can happen extremely quickly in the human body. When the game senses that that change has occurred, it knows you've started to become anxious and will respond by becoming harder.
So, it doesn't really matter what your Heart Rate is when you play Nevermind at any given time. Once the game gets that initial base-line reading of your "typical cadence" for that moment, all it needs to do is look for any deviation in that cadence and it will be able to accurately respond from there.
Furthermore, Nevermind starts at the Neurostalgia Institute, a low-stress, clinic-like hub area intended to ease you into the narrative and setting of the world. Secretly, we are also using this period to calibrate the sensor to have a baseline off which to build when things get more intense.
Watch the video in the Biofeedback section above for more details on how this works!