High pass filter problem

So I am designing a low pass filter to filter out the high frequency noise from the ECG wave signal.  But I have no idea of the frequency of the ECG wave.   I think I might make a rough guess to start trials and errors.

I looked at the the MIT Open Course ECG slide again and found that there are 6 wave signals in a period of about 17mm or (17mm * 0.04 second/mm =  17 * 40 mS = 700mS).

In other words, the average frequency of the 7 waves would be roughly 7 / 700mS = (7 * 1000) / 700
= 10Hz!

Now I seem to be in big trouble, because if the ECG signal is only 10Hz, then the high pass filter I made earlier must also have filter away the ECG signal, together with the 50Hz line frequency.

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