Perhaps you could use the zero crossing as a reference, if your signal is with respect to 0 volts. The p signal will rise above 0 and then fall. You could compare the data and look for some point slightly above 0 to start the capturing of the p wave. I would look for two or three increasing points as a criteria, then start recording the data until it falls back to some point near zero. The qrs would be different because it rises above 0, then falls below 0, then returns to 0. You would have to again compare each data point and look for three increasing values, then start capturing data. Look for a zero crossing, continue capturing, the look for a return to 0 to end the recording. This is quite a challange, but it can be done by data comparisons between successive points. Sorry I don't have time right now to create a sample vi, but you can experiment with loops and shift registers to compare successive data, or event structure to signal when to start the capture and when to stop. Maybe a state machine would work. Or a two parallel loop process where one loop just compares for increasing or decreasing data, and sets flags to start and stop capturing data, and the other loop captures the data only when the flag is set.
