Cardiac output equals heart rate multiplied by stroke volume
Mnemonic
“Output is rate times stroke, and stroke is preload, afterload and contractility”:
- Cardiac output = heart rate x stroke volume
- Normal is about 5 litres per minute, or a cardiac index of 2.5 to 4 l/min/m2
Stroke volume has exactly three determinants:
- Preload - end diastolic volume, governed by venous return. Raising it increases stroke volume by the Frank-Starling mechanism
- Afterload - the resistance the ventricle ejects against. Raising it reduces stroke volume
- Contractility - the intrinsic force at any given fibre length, raised by sympathetic stimulation, calcium and inotropes
“Fill it, squeeze it, or unload it” is therefore the whole of haemodynamic management, and every vasoactive drug acts on one of the three.
Mean arterial pressure = cardiac output x systemic vascular resistance, the circulatory version of Ohm’s law, which is why hypotension is always a failure of output, of resistance, or of both.
Expansion
Cardiac output = heart rate x stroke volume
Typical resting adult values:
- Heart rate about 70 beats per minute
- Stroke volume about 70 ml
- Cardiac output about 5 litres per minute
- Cardiac index (output divided by body surface area) about 3.0 litres per minute per square metre
- Ejection fraction (stroke volume divided by end-diastolic volume) about 60 per cent
In maximal exercise cardiac output can rise fourfold to fivefold in an untrained adult, and up to sevenfold in an athlete. Most of the early rise comes from heart rate, because stroke volume plateaus at around 40 to 50 per cent of maximum oxygen uptake.
At very high rates, above roughly 180 beats per minute, diastole becomes so short that filling falls and cardiac output begins to decline.