Day-to-day variability in the normal electrocardiogram.

@article{Michaels1967DaytodayVI,
  title={Day-to-day variability in the normal electrocardiogram.},
  author={Leslie Michaels and Remi J. Cadoret},
  journal={British heart journal},
  year={1967},
  volume={29 6},
  pages={913-9}
}