Ejection fraction is written EF or LVEF, in capital Latin letters, as a percentage with a whole number: EF 58 %. Cardiac output is denoted CO, cardiac output, or Q̇, and is written in litres per minute, while stroke volume is SV, in millilitres.
Ejection fraction is a share, not an amount: it tells what part of its volume the ventricle pushes out per beat, but not how much blood the body receives. How much the body receives is told by cardiac output, and a large, weakened ventricle with a fraction of 25 % can pump as many litres as a healthy one with a fraction of 60 %.
The left ventricle beats inside the ultrasound fan: the amber border of the cavity contracts around the letters EF and moves back out, the wall thickens as it does so, the leaflets of the mitral valve swing open, and during filling a red Doppler flow flares up beneath them. On the right a blue volume bar moves between the marks of 120 and 50 millilitres, a marker runs along the ECG at the bottom, and speckle shimmers in the fan.
Quantity data sheet
share per beat · litres per minute sheet 1/1 · rev. 2026-09Ejection fraction and cardiac output EF · % · CO · L/min
A dimensionless share and a derived SI unit · quantities: ejection fraction, %, and volumetric blood flow, L/min
A pump that does not push everything out
The left ventricle never empties to the bottom: of the one hundred and twenty millilitres of blood gathered during relaxation it pushes out about seventy, and fifty are left in the cavity until the next beat. The ratio of what is pushed out to what was gathered is called the ejection fraction, and it is the most quoted number in cardiology: it decides who gets drugs, who gets a defibrillator, and whether chemotherapy can continue.
The body, however, needs not a percentage but litres. Stroke volume multiplied by heart rate gives cardiac output — about five litres per minute at rest and up to thirty in a skier on a climb — and it can stay normal even when the fraction has halved, because a diseased heart stretches and takes in a larger volume in order to push out the same seventy millilitres as before.
The sheet shows a beating ventricle on an echocardiograph screen next to its pressure–volume loop, so that both quantities can be seen in a single beat.
01 · Definition
volumes · fraction · flow · loopEjection fraction is the share of the ventricle's end-diastolic volume that it pushes out in a single contraction: the difference between the volumes at the end of filling and at the end of ejection, divided by the volume at the end of filling. The difference itself is called stroke volume, and stroke volume multiplied by the heart rate is the cardiac output, the volumetric flow of blood through the heart.
The fraction describes the pump, and the output describes the supply. Because people of different sizes need different flows, the output is divided by body surface area to give the cardiac index, which can then be compared between people without correcting for height and weight.
Both quantities live in the pressure–volume loop: in one beat the ventricle fills at low pressure, then contracts with the valves closed, ejects blood into the aorta and relaxes. The width of the loop is the stroke volume, its area is the work of the heart per beat, and the position of the left edge shows how much blood is left in the cavity and how strongly the ventricle contracts.
A small fraction, normal litres
On the left is the apical view on the echocardiograph screen: the grey wall of the ventricle, the black cavity with amber discs by Simpson's method, the mitral valve at the bottom, red flow during filling and blue during ejection; the green ECG under the fan sets the beat. On the right is the pressure–volume loop of the same ventricle: an amber marker traces it with every beat, the width of the loop is the stroke volume, the red fill is the work per beat, and the dashed lines mark the volumes at the end of filling and at the end of ejection.
Output holds while the heart stretches
A weakened ventricle keeps a normal output at the price of stretching: it takes two hundred and fifty millilitres into the cavity in order to push out the same sixty, and its filling pressure rises, and it is this pressure that forces fluid into the lungs and causes breathlessness. That is why the fraction reveals the disease long before the output falls, and the output shows how long it is still compensated; the clinician looks at both quantities and at the pressure behind them.
02 · Conversion
flow · index · stroke volumeLitres per minute, per day and in cubic metres
Enter the cardiac output in any flow unit, and the sheet converts it to the others, recalculates it into the cardiac index for people of different sizes, and shows what stroke volume is needed at different heart rates to get the same flow.
A rule to remember: five litres per minute is about seven tonnes of blood per day, almost a milk tanker.
If the fraction has fallen from 60 to 50 %, that is a drop of ten percentage points, but of one sixth of the starting value, and in medical reports the two are constantly confused. Cardio-oncologists, for example, consider a fall in the fraction of more than ten points to a level below 50 % significant, and the word “point” here decides whether chemotherapy is stopped.
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04 · Measuring instruments
Fick · thermodilution · echo · MRIFour ways to count blood you cannot see
The bookkeeping of oxygen
In 1870 Adolf Fick reasoned that all the oxygen absorbed by the lungs in a minute is carried away by the blood, so the blood flow equals the uptake divided by the difference in oxygen content between arterial and mixed venous blood. He never once measured cardiac output himself — that needs blood from the pulmonary artery — but his principle still serves as the reference against which other methods are compared.
Ten millilitres of cold
A catheter with a small balloon on its tip floats with the blood flow through the right side of the heart into the pulmonary artery. A cooled solution is injected into the atrium, a thermistor downstream draws a cooling curve, and the faster the blood carries the cold away, the smaller the area under the curve and the larger the output; this is an inverse problem, like that of a swimmer by whom the strength of a current is judged.
Tracing the outline twice per beat
The echocardiographer traces the inner outline of the ventricle at the end of diastole and at the end of systole in two mutually perpendicular views, and the machine slices the cavity into twenty discs and adds up their volumes. This is how the recommended Simpson method works; three-dimensional echo needs no assumptions about shape, but it too depends on where a person draws the line along a blurred border.
A stack of circles instead of geometry
A magnetic resonance scanner images the ventricle in a dozen parallel short-axis slices, each as a cine loop of one beat, and the volume is obtained by an honest summation of areas with no assumptions about the shape of the cavity. That is why MRI is regarded as the reference for ejection fraction, and a discrepancy of a few percentage points from echo is usual and expected.
05 · Writing rules
method · points · litresA percentage next to the method, litres next to the minute
The fraction is written as a whole number of percent with the method stated, changes in percentage points, and the output in litres per minute with one decimal place.
The first line on the right writes a share and at once adds a percent sign, turning 58 % into a quantity a hundred times smaller, and the second does not say whether the fraction fell by twelve points or by twelve percent of the starting value. Litres without a minute are a volume, not a flow, an index without a square metre loses its meaning, the fraction cannot be called cardiac output, and hundredths of a percent ascribe to the tracing a precision it does not have.
06 · Neighbouring units
litre · millimetre of mercury · beats per minuteThe output is built from a litre of volume and a beat per minute, and the loop in which the fraction lives is drawn in millimetres of mercury against millilitres.
About five litres of blood in the whole body, and the heart pumps the same amount in a minute.
The height of the loop; its area in mmHg·mL is the work of the heart per beat.
The second factor of the output and the main lever of its growth under exertion.
07 · Historical section
archive · from Fick to SimpsonHow we learned to weigh a beat
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Half the fraction, the same litres
Two bars are the full volumes of a healthy ventricle of 120 mL and of a stretched one of 250 mL; the red part is what each pushes out per beat, 70 and 60 mL. The amber frame passes from one to the other, and the number under a bar is the fraction.
The second heart holds up its output by beating faster while heavily stretched, and it is this stretching that in time ruins it.
A percentage that depends on the hand
Ejection fraction has no reference standard that could be put into a device, and no ventricle phantom that looks equally like all hearts. It is computed from two volumes, each built up from discs along an outline drawn by a person on a blurred border, so the accuracy of the method is determined less by the ultrasound than by the hand and eye of the one tracing.
Reproducibility studies show that two experienced specialists on the same two-dimensional recording differ by several percentage points, and on a repeat study on another day the spread reaches ten points. So the 40 % limit in the guidelines is a convention for choosing treatment, and a person with a fraction of 38 % in one study and 43 % in another is diagnosed on the whole picture, not on a single number.
Cardiac output is built more honestly: it is a flow, and it can be checked against the Fick principle, which rests on the law of conservation of matter. But it too changes from minute to minute with heart rate, breathing and body position, and so an intensive care protocol records not one value but a series of measurements.
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