How to save an ECG signal data into an SD card !?

Given that the range of voltages that the Arduino can measure (slowly) on an analog pin is 0 to 5.0V (for 5.0V Arduinos), your voltages range from 1/100000th to 60/100000th of the range from 0 to 1023. The Arduino would need to return values from 0.0123 to 0.6138 to represent the signal that you want to input to it. Given that signal, the Arduino will consistently return 0.