Numbers ought to be obvious and straightforward in all walks of life, one would think. But there are many sets of numbers in medicine that confuse people.

The other day a patient told me tearfully that her brother’s heart was only working at 25%. I told her I was pretty sure that he had only lost half of his pump function and not 75%.

I explained that a beating heart never contracts so completely that there is no blood left inside it. Instead, only about 55% of the blood inside it is pushed out with every beat. That 55% is what we call the normal ejection fraction (EF).

Systolic heart failure with only 25% of the blood volume pumped out is definitely a significant problem, and associated with a risk of deadly rhythm disturbances like ventricular tachycardia.