An echocardiogram, which is an ultrasound petformed to assess the structure of the heart, is shortened to "echo."
What you're referring to is called an electrocardiogram, shortened to "EKG" or "ECG," and it measures several aspects of the heart rhythm.
Side note: You might say "right index finger" in formal/professional settings, but 99.9% of native speakers would understand it the way it is, and some might even say it that way. This is not really worth mentioning for context but might help with language mastery.