I got a pulse oximeter from ebay, including a probe, for under $300.

Ohmeda Biox 3740 Pulse Oximeter

ohmeda biox 3740 front

ohmeda biox 3740 back 25 pin serial interface

RS232 interface

2400 baud, 8 bits, 1 stop, no parity
and in the ASCII setup, i have to check the "force incoming data to 7bit ascii"

some sample outputs (live output is once every 2 seconds):

:SaO2= 96 PR= 78   
:SaO2= 98 PR= 80 ** LOW QUALITY SIGNAL
:SaO2= 97 PR= 81 ** PROBE OFF PATIENT
:SaO2=--- PR=--- ** PROBE OFF PATIENT

OHMEDA BIOX 3740 PULSE OXIMETER
TREND DATA OUTPUT
12 SECONDS PER DATA POINT

:SaO2=--- PR=---   
:SaO2= 98 PR= 64   


a site with information on connecting medical devices to computers

Mac OS X and Linux things