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Printed on 7/20/2026
For informational purposes only. This is not medical advice.
Cardiac output (CO) is the volume of blood the heart pumps per minute — the product of stroke volume and heart rate — and cardiac index (CI) normalizes it to body surface area for interpretation across patients of different sizes. Use it alongside Ejection Fraction to separate a pump function problem (low EF) from a rate/volume problem (low CO with preserved EF). Calculate BSA first if you don't already have it. In the ICU, cross-check hemodynamic status with Mean Arterial Pressure and Shock Index, and correlate valve-driven CO limitations with Aortic Valve Area or Mitral Valve Area when valvular disease is present.
Formula: Cardiac Output (L/min) = Stroke Volume (mL) × Heart Rate (bpm) ÷ 1000. Cardiac Index = Cardiac Output ÷ BSA (m²).
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Cardiac output is fundamentally simple: it's the volume of blood ejected by the left ventricle with each beat (stroke volume, SV) multiplied by how many times the heart beats per minute (heart rate, HR). Stroke volume itself is most commonly derived from echocardiography using the left ventricular outflow tract (LVOT) velocity-time integral (VTI) method — SV = LVOT cross-sectional area × LVOT VTI — or measured directly via thermodilution with a pulmonary artery catheter, or by cardiac MRI volumetric analysis (SV = end-diastolic volume − end-systolic volume). Body surface area (BSA), calculated from height and weight using the Mosteller or Du Bois formula, is needed to normalize the raw cardiac output value into cardiac index, since a cardiac output of 5 L/min means something very different in a 50 kg patient versus a 120 kg patient. Use the [BSA Calculator](/tools/bsa-calculator) if you don't already have this value from a recent encounter.
Cardiac output (L/min) = Stroke Volume (mL) × Heart Rate (bpm) ÷ 1000. For example, a stroke volume of 70 mL and a heart rate of 75 bpm gives a cardiac output of 5.25 L/min — right in the middle of the normal 4-8 L/min resting range for an average-sized adult. Cardiac index (L/min/m²) = Cardiac Output ÷ BSA. This normalization matters enormously in practice: a cardiac output of 4.0 L/min might be entirely adequate for a small 45 kg patient (BSA ~1.4 m², CI ~2.9) but represents significant hypoperfusion in a large 110 kg patient (BSA ~2.3 m², CI ~1.7, in the cardiogenic shock range). Always interpret cardiac output through the lens of cardiac index when comparing across patients or tracking trends in someone whose volume status or weight is changing.
Normal resting cardiac index is 2.5-4.0 L/min/m². A cardiac index below 2.2 L/min/m² suggests inadequate perfusion, and below 1.8 L/min/m² — especially combined with elevated filling pressures (pulmonary capillary wedge pressure >18 mmHg) — meets hemodynamic criteria for cardiogenic shock, per the classic Forrester and more recent SCAI shock staging classifications. A cardiac index above 4.0 L/min/m² suggests a hyperdynamic state: sepsis (early, warm shock), severe anemia, thyrotoxicosis, arteriovenous fistula, liver failure, or pregnancy. Always interpret the number alongside the clinical picture — a 'normal' cardiac output in a septic patient who needs a CI of 5-6 to meet peripheral oxygen demand may actually represent relative cardiac insufficiency, which is why trending values and correlating with lactate, mixed venous oxygen saturation, and clinical exam matters more than a single isolated number.
Intensivists and critical care nurses
Track cardiac index trends in patients with pulmonary artery catheters or non-invasive cardiac output monitors to titrate inotropes, vasopressors, and fluid resuscitation in cardiogenic or septic shock.
Cardiologists managing advanced heart failure
Calculate cardiac index from echo-derived stroke volume to objectively assess forward flow in patients being evaluated for inotrope therapy, mechanical circulatory support, or transplant listing.
Cardiac anesthesiologists and cardiac surgery ICU teams
Monitor cardiac output intraoperatively and post-bypass to guide inotrope weaning and detect low-output syndrome after cardiac surgery, one of the most common early postoperative complications.
Sonographers and cardiology fellows reading echo studies
Cross-check the cardiac output value auto-calculated by echo machine software using the LVOT VTI method, and understand how stroke volume derivation feeds into the broader hemodynamic assessment.
Residents and critical care fellows
Use cardiac output and index calculations to build intuition for the relationship between preload, afterload, contractility, and heart rate — the four determinants of cardiac performance taught in every critical care curriculum.
A cardiac output of 4.5 L/min is normal in a small patient and low in a large one. Cardiac index removes body size as a confound and is the number that actually correlates with shock staging thresholds.
Different measurement methods have different error profiles — thermodilution can be affected by tricuspid regurgitation and irregular rhythms, while echo LVOT VTI method is sensitive to LVOT diameter measurement error (which is squared in the area calculation). Use the same method consistently when trending a patient over time rather than comparing across methods.
At very high heart rates, diastolic filling time shortens enough that stroke volume drops, partially offsetting the rate increase — this is why sustained SVT or rapid AF can paradoxically reduce cardiac output despite the faster rate, especially in patients with diastolic dysfunction or valve disease.
Septic patients often need a higher-than-normal cardiac index (5-6+ L/min/m²) to meet the elevated peripheral oxygen demand of systemic inflammation — a CI of 3.0 that would be reassuring in a healthy postoperative patient may represent relative cardiac insufficiency in severe sepsis.
Because LVOT cross-sectional area uses the radius squared, a small measurement error (even 1-2 mm) in LVOT diameter produces a disproportionately large error in calculated stroke volume and cardiac output — this is the most common reason echo-derived CO differs meaningfully from invasive thermodilution measurements.
Cardiac output fluctuates with volume status, position, medications, and measurement technique. A single low reading in isolation is far less clinically useful than a consistent downward trend correlated with clinical deterioration (rising lactate, falling urine output, cool extremities).
Normal resting cardiac index is 2.5-4.0 L/min/m². Below 2.2 suggests inadequate perfusion; below 1.8 with elevated filling pressures meets criteria for cardiogenic shock. Above 4.0 suggests a hyperdynamic state (sepsis, anemia, thyrotoxicosis, AV fistula).
Use cardiac output/index calculations in ICU hemodynamic monitoring, heart failure assessment, perioperative cardiac monitoring, and whenever stroke volume has been measured by echo, thermodilution, or cardiac MRI and needs to be converted into a clinically interpretable flow value.
Accuracy depends entirely on the accuracy of the input stroke volume measurement, which varies by method (echo LVOT VTI is sensitive to diameter measurement error; thermodilution is affected by tricuspid regurgitation and arrhythmia). A single measurement is less useful than a trend, and normal values must be interpreted in the context of the clinical demand state (e.g., sepsis requires higher-than-normal CI).
Disclaimer: This tool is for educational and informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider with questions about your health.
April 21, 2026 · trust-baseline
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OpenBody MetricsCalculate body surface area (BSA) with Du Bois, Mosteller, and Haycock formulas for chemotherapy dosing, cardiac index, and clinical calculations.
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