Wiring an analog input to an existing circuit

First off, let me declare my variables.

  1. I’m terribly new.
  2. I want to learn.
  3. Not even sure this is even valid.
  4. Sorry if this is in the wrong section.

Background: I want to buy a cheap pulse oximeter (http://www.amazon.com/Finger-Pulse-Oximeter-SM-110-Carry-Wrist/dp/B003TJH3LI). Notice there is no functionality to record trends in o2 sat/ HR.

Goal: I would like to, if possible, attach an analog input into the existing circuit to be able to graph/write/whatever. record the changes over time.

Questions:

  1. Is this feasible?
    (I know i haven’t purchased the meter yet, and don’t have the circuit)
  2. Is there anything in general that I would try to identify to connect to the right location?
    Thanks!

Goal: I would like to, if possible, attach an analog input into the existing circuit to be able to graph/write/whatever. record the changes over time.

Questions:

  1. Is this feasible?

No, because there’s probably no point in the device where you can get that analog value.

  1. Is there anything in general that I would try to identify to connect to the right location?

Without having the schematics of that device I would rather try to catch the display signals and interpret them than to search for an analog output of the wanted values.