Cardiac Index Calculator
Estimate your cardiac index with our free cardiovascular system calculator. See reference ranges, risk factors, and next-step guidance.
Reviewed for accuracy by Rahul Singh, Health & Wellness Specialist
Medical disclaimer: This calculator is provided for educational and informational purposes only and does not constitute medical advice, diagnosis, or treatment. Results are general estimates and may not reflect your individual circumstances. Always consult a qualified healthcare professional before making decisions about your health.
Cardiac Index Calculator
Calculator
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Formula: CI = CO / BSA, where BSA = 0.007184 x H^0.725 x W^0.425
Worked example — Cardiac Index: 2.87 L/min/m2 (Normal)
Formula
CI = CO / BSA, where BSA = 0.007184 x H^0.725 x W^0.425
Where CI = Cardiac Index (L/min/m2), CO = Cardiac Output (L/min), BSA = Body Surface Area (m2) calculated using the DuBois formula, H = Height in centimeters, and W = Weight in kilograms. Cardiac output can be measured directly or calculated as Heart Rate x Stroke Volume.
Worked Examples
Example 1: Normal Adult Cardiac Index
Problem:A 70 kg, 170 cm adult has a cardiac output of 5.2 L/min measured by thermodilution. Calculate the cardiac index.
Solution:BSA = 0.007184 x 170^0.725 x 70^0.425 = 0.007184 x 39.45 x 7.56 = 1.81 m2 Cardiac Index = CO / BSA = 5.2 / 1.81 = 2.87 L/min/m2 This value falls within the normal range of 2.5-4.0 L/min/m2
Result:Cardiac Index: 2.87 L/min/m2 (Normal)
Example 2: Low Cardiac Index in Heart Failure
Problem:A 90 kg, 175 cm patient in the ICU has a heart rate of 110 bpm and stroke volume of 35 mL. Calculate the cardiac index.
Solution:BSA = 0.007184 x 175^0.725 x 90^0.425 = 0.007184 x 40.21 x 8.26 = 2.06 m2 CO = HR x SV = 110 x 35 / 1000 = 3.85 L/min Cardiac Index = 3.85 / 2.06 = 1.87 L/min/m2 This is below 2.2, indicating low cardiac output syndrome
Result:Cardiac Index: 1.87 L/min/m2 (Low - requires intervention)
Frequently Asked Questions
What is cardiac index and why is it clinically important?
Cardiac index (CI) is the cardiac output normalized to body surface area (BSA), expressed in liters per minute per square meter. It provides a more accurate assessment of cardiac performance across patients of different body sizes than cardiac output alone. A normal cardiac index ranges from 2.5 to 4.0 L/min/m2 in healthy adults. Clinicians use cardiac index to evaluate heart function in critical care settings, guide fluid resuscitation, assess the severity of heart failure, and monitor the response to inotropic medications. Low cardiac index values below 2.2 L/min/m2 often indicate inadequate tissue perfusion requiring intervention.
How is body surface area calculated for cardiac index?
Body surface area (BSA) is most commonly calculated using the DuBois and DuBois formula published in 1916: BSA = 0.007184 multiplied by height in centimeters raised to the power of 0.725, multiplied by weight in kilograms raised to the power of 0.425. Other formulas exist, including the Mosteller formula (square root of height times weight divided by 3600) and the Haycock formula often used in pediatrics. The DuBois formula remains the most widely used in clinical practice and research for normalizing hemodynamic parameters. Average adult BSA is approximately 1.7 m2 for women and 1.9 m2 for men.
What is the difference between cardiac output and cardiac index?
Cardiac output (CO) is the total volume of blood pumped by the heart per minute, typically measured in liters per minute, calculated as heart rate multiplied by stroke volume. Normal resting cardiac output ranges from 4 to 8 L/min. Cardiac index normalizes cardiac output by dividing it by body surface area, producing a value in L/min/m2 that accounts for body size differences. For example, a CO of 5 L/min is normal for a 60 kg patient (CI about 3.1) but may represent inadequate perfusion in a 120 kg patient (CI about 2.2). This normalization makes cardiac index more useful than cardiac output for clinical decision-making across diverse patient populations.
What are the normal ranges for cardiac index values?
Normal resting cardiac index in healthy adults ranges from 2.5 to 4.0 L/min/m2, with an average around 3.0 to 3.5 L/min/m2. Values between 2.2 and 2.5 L/min/m2 are considered borderline low and may warrant monitoring. A cardiac index below 2.2 L/min/m2 generally indicates low cardiac output syndrome, while values below 1.8 L/min/m2 suggest cardiogenic shock with critically impaired tissue perfusion. Values above 4.0 L/min/m2 may indicate a hyperdynamic state, which can be seen in sepsis, anemia, thyrotoxicosis, or arteriovenous shunting. During exercise, cardiac index can increase to 7 to 8 L/min/m2 in trained athletes.
How is cardiac output measured in clinical settings?
Cardiac output can be measured using several techniques. The thermodilution method via a pulmonary artery (Swan-Ganz) catheter remains the clinical gold standard, where cold saline is injected and temperature changes downstream are analyzed. The Fick method calculates cardiac output from oxygen consumption divided by the arteriovenous oxygen content difference. Non-invasive methods include echocardiography using Doppler measurements of blood flow through the aortic valve, impedance cardiography measuring thoracic electrical impedance changes, and pulse contour analysis from arterial waveform monitoring. Each method has different accuracy profiles, with thermodilution and Fick considered most reliable for critically ill patients.
What conditions cause a low cardiac index?
A low cardiac index can result from numerous cardiac and non-cardiac conditions. Heart failure, both systolic and diastolic, is the most common cause, where weakened ventricular contraction or impaired filling reduces stroke volume. Acute myocardial infarction directly damages myocardial tissue, decreasing contractility. Valvular heart disease such as severe aortic stenosis or mitral regurgitation impairs efficient blood ejection. Cardiac tamponade and constrictive pericarditis restrict cardiac filling. Massive pulmonary embolism increases right ventricular afterload dramatically. Severe arrhythmias including ventricular tachycardia or complete heart block compromise coordinated cardiac contraction. Hypovolemia from hemorrhage or dehydration reduces preload and consequently cardiac output.
What is stroke volume index and how does it relate to cardiac index?
Stroke volume index (SVI) is the stroke volume divided by body surface area, expressed in mL/beat/m2, with a normal range of 33 to 47 mL/beat/m2. It represents the volume of blood ejected per heartbeat normalized to body size. The relationship between SVI and cardiac index is direct: cardiac index equals stroke volume index multiplied by heart rate divided by 1000. A low SVI with a compensatory high heart rate may maintain a normal cardiac index temporarily, but this compensation increases myocardial oxygen demand and is unsustainable. Tracking SVI helps clinicians distinguish between conditions requiring volume resuscitation (low SVI from hypovolemia) versus inotropic support (low SVI from pump failure).
How does cardiac index change with age and physiological states?
Cardiac index decreases naturally with aging, declining approximately 1% per year after age 30, primarily due to reduced stroke volume and decreased maximal heart rate. Newborns have a relatively high cardiac index of 3.0 to 4.5 L/min/m2 which gradually decreases through childhood. Pregnancy increases cardiac index by 30 to 50 percent, peaking in the second trimester due to increased blood volume and decreased systemic vascular resistance. Exercise can increase cardiac index three to four fold in healthy individuals. Fever increases cardiac index by approximately 10 percent for each degree Celsius rise. Anesthesia and mechanical ventilation typically reduce cardiac index by 10 to 25 percent through various mechanisms including decreased venous return.
What treatments are used when cardiac index is dangerously low?
Treatment of low cardiac index depends on the underlying cause and follows a systematic approach. Volume resuscitation with crystalloids or colloids is first-line when hypovolemia is suspected, guided by dynamic preload indicators. Inotropic agents such as dobutamine, milrinone, or levosimendan increase myocardial contractility to improve stroke volume. Vasopressors like norepinephrine may be needed if low cardiac index is accompanied by hypotension. Mechanical circulatory support including intra-aortic balloon pumps, ventricular assist devices, or extracorporeal membrane oxygenation provides temporary support in refractory cardiogenic shock. Addressing reversible causes such as percutaneous coronary intervention for acute myocardial infarction or pericardiocentesis for tamponade is essential.
Why is the DuBois formula preferred for BSA calculation in hemodynamics?
The DuBois and DuBois formula has remained the standard for BSA calculation in hemodynamic assessments for over a century due to its validated accuracy across diverse adult populations and its extensive use in establishing normal reference ranges for cardiac index and other hemodynamic parameters. Since all published normal ranges for cardiac index (2.5 to 4.0 L/min/m2) were derived using DuBois BSA values, using alternative formulas could produce slightly different cardiac index values that do not correspond to established clinical thresholds. The formula has been validated against direct measurements in numerous studies and performs well for adults with BMI between 15 and 40. For obese patients or pediatric populations, alternative formulas like Haycock or Mosteller may be more appropriate.
References
Reviewed for accuracy by Rahul Singh, Health & Wellness Specialist · Editorial policy
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