Cardiac Index Calculator
Estimate your cardiac index with our free cardiovascular system calculator. See reference ranges, risk factors, and next-step guidance.
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\nCardiac Index = CO / BSA = 5.2 / 1.81 = 2.87 L/min/m2\nThis 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\nCO = HR x SV = 110 x 35 / 1000 = 3.85 L/min\nCardiac Index = 3.85 / 2.06 = 1.87 L/min/m2\nThis 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.