Cardiac Output Assessment
Calculate cardiac output using the Fick principle: CO = VO₂/(CaO₂ - CvO₂)
For clinical use by healthcare professionals
Fick Equation Calculator
Enter patient parameters to calculate cardiac output
Results
Enter patient parameters and click Calculate to see results
Saved Calculations
No saved calculations yet
Use the "Save Calculation" button to save your results
Clinical Note
The Fick method is considered the gold standard for cardiac output measurement. Always interpret results in the context of the patient's clinical condition.
The Fick Principle
Understanding the foundation of cardiac output measurement
The Fick principle, first described by Adolf Fick in 1870, is a method for calculating cardiac output based on the conservation of mass. It states that the amount of oxygen consumed by the body must equal the amount of oxygen leaving the lungs minus the amount of oxygen returning to the lungs.
The Fick Equation
CO = VO₂ / (CaO₂ - CvO₂)
Where:
- CO = Cardiac Output (L/min)
- VO₂ = Oxygen consumption (mL/min)
- CaO₂ = Arterial oxygen content (mL O₂/dL blood)
- CvO₂ = Mixed venous oxygen content (mL O₂/dL blood)
Step-by-Step Guide
How to use the Fick equation in clinical practice
Step 1: Measure Oxygen Consumption (VO₂)
Oxygen consumption can be measured directly using metabolic carts or estimated based on age, gender, and body surface area. For an average adult at rest, VO₂ is approximately 3-3.5 mL/kg/min or about 250 mL/min total.
Step 2: Measure Arterial Oxygen Content (CaO₂)
Arterial oxygen content is calculated using the formula:
Clinical Applications
When and how to use the Fick method in clinical settings
Indications
- Assessment of cardiac function in heart failure
- Evaluation of shock states
- Valvular heart disease assessment
- Congenital heart disease evaluation
- When thermodilution methods are inaccurate (e.g., tricuspid regurgitation)
Evidence
Why Use
The Fick method is considered the gold standard for cardiac output measurement, especially in patients with:
- Tricuspid regurgitation where thermodilution may be inaccurate
- Intracardiac shunts
- Need for precise hemodynamic assessment in critical care
- Research settings requiring high accuracy
When to Use
Use the Fick method when:
- Other methods of cardiac output measurement are contraindicated or unreliable
- Evaluating patients with heart failure
- Assessing valvular heart disease
- Monitoring critically ill patients requiring precise hemodynamic management
Literature
Original Description:
Fick A. Ueber die Messung des Blutquantums in den Herzventrikeln. Sitzungsberichte der Physikalisch-Medizinischen Gesellschaft zu Würzburg. 1870;2:16–17.
Modern Validation:
Dhingra VK, et al. Validation of a modified Fick method for cardiac output determination in patients on mechanical ventilation. Intensive Care Med. 2002;28(9):1259-64.
Clinical Application:
Hsu PS, et al. Accuracy of the Fick method in cardiac output measurements: A clinical study of comparison with the thermodilution method. Acta Anaesthesiol Sin. 2003;41(3):123-7.
Creator Insights
Historical Context
Adolf Fick described his principle in 1870, long before modern cardiac catheterization techniques were available. His insight that the rate of oxygen consumption must equal the product of cardiac output and the arteriovenous oxygen difference was revolutionary for its time and remains a fundamental principle in cardiovascular physiology.
Frequently Asked Questions
Common questions about the Fick equation and cardiac output measurement
What is the Fick principle and how does it work?
The Fick principle is a method for calculating cardiac output based on the conservation of mass. It states that the amount of oxygen consumed by the body (VO₂) equals the difference between the amount of oxygen delivered to the tissues (arterial oxygen content, CaO₂) and the amount of oxygen returning from the tissues (venous oxygen content, CvO₂), multiplied by the cardiac output. The formula is: CO = VO₂ / (CaO₂ - CvO₂).
How accurate is the Fick method compared to other cardiac output measurement techniques?
The Fick method is considered the gold standard for cardiac output measurement and is often used to validate other techniques. It is particularly valuable in situations where thermodilution methods may be inaccurate, such as in patients with tricuspid regurgitation or intracardiac shunts. However, its accuracy depends on precise measurement of oxygen consumption and proper sampling of arterial and mixed venous blood.
Can the Fick equation be used in all patients?
While the Fick principle applies to all patients, practical limitations exist. The method requires invasive sampling of mixed venous blood, typically via a pulmonary artery catheter, which may not be feasible or indicated in all patients. Additionally, the method assumes steady-state conditions, which may not be present in critically ill patients with rapidly changing hemodynamics.
What is a normal cardiac output and cardiac index?
Normal cardiac output in adults at rest is typically 4-8 L/min. The cardiac index, which normalizes cardiac output to body size, normally ranges from 2.5 to 4.0 L/min/m². Values below these ranges may indicate heart failure or hypovolemia, while values above may suggest conditions like sepsis, anemia, or hyperthyroidism.
How is oxygen consumption (VO₂) measured or estimated?
Oxygen consumption can be measured directly using metabolic carts that analyze inspired and expired gases. However, in clinical practice, it is often estimated based on age, gender, and body surface area. For an average adult at rest, VO₂ is approximately 3-3.5 mL/kg/min or about 250 mL/min total. More accurate formulas include the LaFarge equation for children and the Dehmer equation for adults.
What factors can affect the accuracy of Fick cardiac output measurements?
Several factors can affect accuracy: (1) Errors in measuring or estimating oxygen consumption, (2) Improper sampling of arterial or venous blood, (3) Laboratory errors in blood gas analysis, (4) Rapid changes in the patient's hemodynamic state during measurement, (5) Presence of intracardiac shunts, and (6) Significant changes in hemoglobin concentration between arterial and venous samples.
How is arterial and venous oxygen content calculated?
Oxygen content is calculated using the formula: O₂ content = (1.34 × Hb × O₂ saturation) + (0.003 × PO₂). The first term represents oxygen bound to hemoglobin, where 1.34 is the amount of oxygen (in mL) that can be carried by 1 gram of hemoglobin when fully saturated. The second term represents dissolved oxygen, which is typically negligible at normal PO₂ levels. For arterial content (CaO₂), arterial saturation (SaO₂) and PaO₂ are used; for venous content (CvO₂), mixed venous saturation (SvO₂) and PvO₂ are used.
What is the difference between direct and indirect Fick methods?
The direct Fick method involves actual measurement of oxygen consumption using a metabolic cart, along with measured arterial and mixed venous oxygen content. The indirect Fick method uses estimated oxygen consumption based on predictive equations. While the direct method is more accurate, the indirect method is more practical in many clinical settings where metabolic carts are not readily available.
Can the Fick method be used during exercise?
Yes, the Fick method can be used during exercise and is actually one of the most accurate ways to measure cardiac output during physical activity. This requires simultaneous measurement of oxygen consumption and arteriovenous oxygen difference during exercise. The exercise Fick method is particularly valuable in evaluating patients with heart failure or unexplained exercise intolerance.
How does the Fick calculator handle different units of measurement?
Our Fick calculator standardizes all inputs to ensure accurate calculations. Oxygen consumption (VO₂) is entered in mL/min. Arterial and venous oxygen content (CaO₂ and CvO₂) are entered in mL O₂/dL blood. The calculator automatically converts these values as needed for the Fick equation. The final cardiac output is displayed in L/min, and cardiac index in L/min/m². The calculator also provides options to convert between different unit systems if needed.