From the Guidelines
The Fick cardiac output calculation is a method used to measure cardiac output, which is the volume of blood pumped by the heart per minute, and is calculated by the formula: Q = V˙o2 / [C(a-v)O2], where Q indicates cardiac output, V˙o2 indicates oxygen consumption, and [C(a-v)O2] indicates the arteriovenous O2 content difference 1.
Calculation Details
The Fick principle calculates cardiac output by measuring oxygen consumption and the difference in oxygen content between arterial and venous blood.
- The formula is based on the principle that the rate of oxygen consumption is equal to the product of cardiac output and the arteriovenous oxygen content difference.
- In clinical practice, this typically involves drawing blood samples from an arterial line and a pulmonary artery catheter while measuring oxygen consumption through indirect calorimetry.
- Normal cardiac output ranges from 4-8 liters per minute in adults at rest.
Clinical Considerations
The Fick method is considered the gold standard for cardiac output measurement because it directly measures the heart's function based on metabolic demands.
- However, it's invasive and resource-intensive, so it's typically reserved for research settings or complex clinical scenarios where accurate hemodynamic assessment is crucial.
- Alternative methods like thermodilution via pulmonary artery catheters or non-invasive techniques like echocardiography are more commonly used in everyday clinical practice 1.
Limitations and Alternatives
While the Fick method is highly accurate, its invasiveness and resource requirements limit its use in many clinical settings.
- Other methods, such as thermodilution, may be used to estimate cardiac output, but these methods have their own limitations and potential sources of error 1.
From the Research
Fick Cardiac Output Calculation
The Fick cardiac output calculation is a method used to estimate cardiac output, which is the volume of blood the heart pumps per minute. This calculation is based on the Fick principle, which states that the rate of oxygen consumption is equal to the product of cardiac output and the arteriovenous oxygen difference.
Formula and Variables
The Fick cardiac output calculation can be represented by the following formula:
- Cardiac Output (CO) = Oxygen consumption (VO2) / Arteriovenous oxygen difference (Ca - Cv) Where:
- VO2 is the oxygen consumption, which can be measured directly or estimated using standard equations 2, 3, 4, 5
- Ca is the arterial oxygen content
- Cv is the venous oxygen content
- The arteriovenous oxygen difference (Ca - Cv) can be calculated using arterial and venous blood gas analysis 6, 3
Measurement Methods
There are different methods to measure the variables needed for the Fick cardiac output calculation, including:
- Direct measurement of oxygen consumption using respiratory gas monitoring systems 5
- Estimation of oxygen consumption using standard equations 2, 4
- Measurement of arteriovenous oxygen difference using pulse and fiberoptic oximetry 5
- Measurement of hemoglobin concentration using in vitro oximetry 5
Comparison with Other Methods
The Fick cardiac output calculation has been compared with other methods, such as thermodilution, which is considered a gold standard for measuring cardiac output. Studies have shown that the Fick method can provide accurate estimates of cardiac output, but may have some limitations, such as a higher degree of error in certain patients 2, 4. However, other studies have found that the Fick method can be a reliable and accurate way to estimate cardiac output, especially in emergency conditions 3.