π« Cardiac Index Calculator
Cardiac output normalized to body size. Normal: 2.5 to 4.0.
Need BSA? Our BSA calculator computes it from height and weight.
Educational only. Hemodynamic interpretation belongs to the clinical team.
Cardiac output alone is hard to interpret: 4 liters a minute is plenty for a small woman and inadequate for a large man. Cardiac index fixes that by dividing output by body surface area, putting everyone on the same scale. Normal is 2.5 to 4.0 L/min/m2. This calculator does the division and places the result on that scale.
How to Use This Calculator
Enter cardiac output in liters per minute and body surface area in square meters.
Enter cardiac output from thermodilution, Fick, or echo measurement.
Enter BSA, or compute it with our BSA calculator.
Read the index: under 2.2 is the cardiogenic shock range, 2.5 to 4.0 normal, above 4 high output.
The Formula and the Normal Range
Indexing to BSA works because metabolic demand scales with surface area: the body's tissues need perfusion proportional to their mass and metabolic rate, and BSA approximates that better than weight. A 50 kg woman with a CO of 3.5 has a CI of about 2.5 (normal for her size); a 100 kg man with the same 3.5 has a CI near 1.7 (shock range). Same output, opposite meanings, which is the entire argument for the index.
Low index with signs of poor perfusion (cold extremities, low urine output, altered mentation, rising lactate) defines cardiogenic shock physiology and drives ICU decisions about inotropes and mechanical support. High index appears in distributive states like sepsis, severe anemia, hyperthyroidism, and large AV fistulas: the pump works overtime because demand or shunting demands it. The number never stands alone; it is read with blood pressure, filling pressures, lactate, and the patient in the bed.
Worked Example
Cardiac output 5.0 L/min, BSA 2.0 m2: CI = 2.5 L/min/m2, the bottom of normal. The same 5.0 L/min in a patient with BSA 1.6: CI = 3.1, comfortably mid-normal. And 5.0 L/min with BSA 2.4: CI = 2.08, below the shock threshold, which would prompt a hard look at perfusion markers. Three patients, one cardiac output, three different clinical stories. That is why ICU flowsheets chart the index, not the raw output.
How Cardiac Output Gets Measured
Pulmonary artery catheters measure CO by thermodilution: cold saline injected, temperature change downstream reveals flow. The Fick method divides oxygen consumption by the arterial-venous oxygen difference, the physiological gold standard. Doppler echo estimates stroke volume from LVOT diameter and velocity-time integral times heart rate, noninvasive and repeatable at the bedside. Pulse-contour devices derive it from the arterial waveform. Each method has error bars of 10 to 20 percent, so trends matter more than single readings, and the index inherits that uncertainty. Our Fick cardiac output calculator and Doppler echo calculator show the two most-used methods.
Clinical Context That Changes the Reading
A CI of 2.3 in a warm, mentating patient making urine is observed; the same 2.3 in a cold, confused, oliguric patient is treated. Context also includes what the heart is working against: a normal index with a MAP of 55 is still shock (distributive), while a low-normal index with hypertension may just need afterload reduction. Athletes resting at 2.2 with a heart rate of 45 are not in shock; they are efficient. Numbers serve the clinical picture, never the reverse, which is worth stating plainly on a page full of numbers.
For informational purposes only, not medical advice. Consult a healthcare professional.
Frequently Asked Questions
What is a normal cardiac index?
2.5 to 4.0 L/min/m2. Below 2.2 with signs of poor perfusion suggests cardiogenic shock physiology; above 4.0 suggests a high-output state such as sepsis, severe anemia, or hyperthyroidism.
How do you calculate cardiac index?
Divide cardiac output (L/min) by body surface area (m2). Example: CO 5.0 L/min / BSA 2.0 m2 = CI 2.5 L/min/m2. This calculator does it instantly.
What is the difference between cardiac output and cardiac index?
Cardiac output is total flow in L/min; cardiac index normalizes it to body size by dividing by BSA. The index lets clinicians compare perfusion adequacy across different-sized patients on one scale.
What causes low cardiac index?
Pump failure (MI, cardiomyopathy), severe bradycardia, obstructive causes (tamponade, massive PE), and hypovolemia. Clinically it matters when paired with poor perfusion signs: cold extremities, low urine output, altered mentation, rising lactate.
What causes high cardiac index?
Sepsis, severe anemia, hyperthyroidism, large arteriovenous fistulas, beriberi, and liver failure. The heart pumps extra because tissue demand is high or blood is shunting past tissues.
How is cardiac output measured?
Thermodilution via pulmonary artery catheter, the Fick method (oxygen consumption over AV oxygen difference), Doppler echocardiography (LVOT area x VTI x heart rate), or pulse-contour analysis. Each carries 10-20% error, so trends beat single values.
What cardiac index indicates cardiogenic shock?
Below about 2.2 L/min/m2 with clinical signs of hypoperfusion (hypotension, cold extremities, oliguria, altered mentation, high lactate). The number plus the patient defines shock, never the number alone.
Why index to body surface area?
Because tissue perfusion needs scale with body size and metabolic demand, which BSA approximates better than weight. Indexing puts a 50 kg woman and a 100 kg man on the same interpretive scale.
Can athletes have low cardiac index at rest?
Yes, and it is normal. A trained athlete with a resting heart rate in the 40s may show a CI near 2.2 with warm extremities and perfect mentation: high stroke volume, low rate, adequate perfusion. Context rules.
Does cardiac index change with exercise?
Dramatically. CI can triple or quadruple with maximal exercise as heart rate and stroke volume rise. Resting reference ranges apply only at rest; exercise circulation is interpreted against workload.