Lessons · Medicine foundations · cardiac output
Cardiac output: how much blood the heart moves in a minute
Cardiac output is the volume pushed out by one beat times the number of beats in a minute: stroke volume × heart rate.
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What it is for
It is the first question of almost every cardiovascular viva, and the examiner is not testing the multiplication. They are testing whether you can say which of the two numbers changed, and whether the unit you produced was per minute.
How to think about it
Two numbers, one multiplication, and then a unit decision. Millilitres times beats per minute gives millilitres per minute; divide by 1000 for litres per minute, which is how cardiac output is usually said out loud. Write both if you are unsure which the question wants.
Worked example
CO = stroke volume × heart rateThe formula. Volume per beat, times beats per minute.
Stroke volume 70 mL, heart rate 72 beats per minuteThe two figures, from the question.
70 × 72 = 5040 mL/minThe multiplication. Millilitres a minute.
5040 / 1000 = 5.04 L/minAnd in litres, which is the form people say aloud.
Your turn
Stroke volume is 60 mL and the heart rate is 80 beats per minute. Write the line that gives the cardiac output in millilitres per minute.
CO = 60 × 80 = mL/min
Solve one, graded on the server
The trap
Handing back millilitres a minute when the question asked for litres a minute, or the other way round. It is a factor of a thousand and the arithmetic is otherwise perfect, which is exactly the kind of mistake that survives being checked.