Use a timer control OR develop a windows service which checks against the blood pressure level from database or wherever it is saved in particular time interval. Decide the time interval based on the fluctuation of the pressure.
For sending SMS, you need to learn AT commands.
To know more about AT commands and GSM modem read
Introductin to AT commands[
^]
I would suggest you to read the whole tutorial.
For developing an application for sending and receiving SMS using modem read:
Send and Read SMS through GSM Modem using AT Commands[
^]
Hope this helps.
----
Ankur