Cardiac Output
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Direct answer
Cardiac output is stroke volume times heart rate — about 70 mL times 75 beats per minute, or 5 L/min at rest, indexed to 3 L/min per square metre of body surface. The Fick principle measures it as oxygen consumption divided by the arteriovenous oxygen difference: 250 mL of oxygen per minute divided by 5 mL per decilitre gives 5 L/min. Clinically it is measured by thermodilution through a Swan-Ganz catheter or estimated by echocardiography. Output is governed by four factors — preload, contractility, afterload and heart rate — but Guyton's central point is that the heart is a slave to venous return: at a right atrial pressure near zero, the venous return curve crossing the cardiac function curve fixes output at about 5 L/min, with a mean systemic filling pressure of 7 mm Hg driving flow back to the heart. In strenuous exercise output reaches 20-25 L/min, and up to 35 in endurance athletes.
What you must remember
- Core numbers: cardiac output 5 L/min (4-8), cardiac index 3 L/min/m2; stroke volume 70 mL; ejection fraction 60 per cent; heart rate 60-100.
- Fick arithmetic: oxygen consumption 250 mL/min with an arteriovenous difference of 4-5 mL/dL gives 5-6 L/min — the classic calculation the screening paper reproduces with changed numbers.
- Measurement options: direct Fick, indicator dilution, thermodilution (Swan-Ganz), Doppler echocardiography; the stroke volume volume via pulse pressure is a rough bedside surrogate.
- Frank-Starling law: stroke volume rises with end-diastolic volume up to about 200 mL, when the sarcomeres at 2.2 micrometre reach optimal overlap; failing hearts operate on a flatter, right-shifted curve.
- Venous return side: mean systemic filling pressure averages 7 mm Hg; right atrial pressure 0 mm Hg; the intersection of the venous return and cardiac curves — the equilibrium point — sets the actual cardiac output.
- Reflex effects: the Bainbridge reflex stretches atrial receptors to speed the heart; the atrial reflexes and respiratory variation shape output beat to beat; the Anrep and Bowditch effects raise contractility with load and rate respectively.
- High-output states: anaemia, thyrotoxicosis, arteriovenous fistula, beriberi and pregnancy — all low-resistance, high-flow physiology that still can end in failure.
The Fick calculation in practice
Suppose a patient consumes 250 mL of oxygen per minute, arterial oxygen content is 20 mL/dL and mixed venous oxygen content is 15 mL/dL. The arteriovenous difference is 5 mL/dL, i.e. 50 mL per litre of blood, so cardiac output equals 250 divided by 50, which is 5 L/min. Now push the same patient into sepsis: tissues extract less, the venous content rises to 18, the difference shrinks to 20 mL per litre, and output computes to 12.5 L/min — the hallmark of distributive (warm) shock. Reverse the scenario in cardiogenic shock: extraction climbs to 10 mL/dL, and 250 divided by 100 gives 2.5 L/min. The Fick principle is nothing more than conservation of oxygen, yet the same arithmetic separates the three shock physiologies the exam tests. Pair it with the venous-return view: in the septic patient total peripheral resistance has collapsed, the venous return curve steepens, and the heart — still vigorous — is simply pumped faster by the returning column.
Viva angles examiners love
Examiners probe two confusions. First, "which is the most important determinant of cardiac output" — in Guyton's framing, venous return: the heart pumps what it receives, and constriction of venous reservoirs raises output far more than a small change in contractility. Second, the difference between cardiac output and cardiac index: a 5 L/min output in a small woman is not the same physiology as 5 L/min in a tall man; dividing by body surface area normalises it to about 3 L/min/m2. A favourite extension is why a standing soldier faints on parade: venous pooling in dependent veins cuts venous return, preload falls to the steep part of the Starling curve, output and cerebral flow dip — the whole event is a venous return story, which is why leg movement prevents it.
Frequently asked questions
What is the normal cardiac output and cardiac index at rest?
Cardiac output is about 5 L/min (70 mL x 75 beats/min) and the cardiac index is about 3 L/min per square metre of body surface area.
How is cardiac output measured by the Fick principle?
Divide oxygen consumption (about 250 mL/min) by the arteriovenous oxygen difference in mL per litre of blood; a difference of 5 mL/dL yields 5 L/min.
Which factor most directly limits cardiac output in a normal person?
Venous return: the heart pumps whatever the systemic veins deliver, so mean systemic filling pressure of about 7 mm Hg and right atrial pressure set the operating point.
What happens to cardiac output in severe anaemia?
It rises — low viscosity and tissue hypoxia reduce peripheral resistance and drive flow to 7-8 L/min or more, a classic high-output state.
What is the significance of the Frank-Starling upper limit?
Beyond an end-diastolic volume near 200 mL, sarcomeres overstretch, contractility plateaus and then falls, which is why over-transfusion can decompensate a failing ventricle.