Man, it's not every day I get to bust out "Simulacron and Simulation" knowledge.
The question you must ask yourself is are you aiming to emulate the flatline effect or are you trying to represent it? Are you trying to make a realistic clone of a heart rate monitor or an artistic interpretation of it?
If you try to emulate it, the best you can do is get a sound that most people are going to think is a sample, but if you lose accuracy people are going to be distracted from the point you were trying to make in the first place. However if done well, I think this is more impressive.
Wheras if you try to represent a flatline using a guitar, people will think "Woah! That guitar sounds like heartrate monitor", but in reality it still sounds very much like a guitar. Part of this will likely include determing what makes a heart rate monitor sound like a heart rate monitor, exaggerating that, and applying it to your guitar.