Calculating FICK Cardiac Output and Input

Summary / Explanation

Introduction

The Fick principle, named after Adolf Eugen Fick (1829-1901), is a fundamental concept in cardiovascular physiology that provides a method for calculating cardiac output. Cardiac output refers to the volume of blood pumped by the heart per unit of time, which is typically measured in liters per minute (L/min).[1] The Fick principle is crucial in determining cardiac output by relating tissue oxygen consumption and oxygen concentrations in arterial and venous blood.

The Fick Principle

The Fick principle is based on the idea that the oxygen consumption rate by peripheral tissues is directly proportional to the product of the amount of oxygen delivered to the tissues and the arterial-venous oxygen concentration difference. This principle can be expressed by the following equation:

Cardiac output = Tissue oxygen consumption/(Arterial oxygen content - Venous oxygen content)

Where:

• Cardiac output (CO) is the blood volume pumped by the heart per minute (measured in L/min).

• Tissue oxygen consumption (VO₂) is the rate at which oxygen is consumed by the body tissues per minute (measured in mL/min).

• Arterial oxygen content (CaO₂) is the oxygen content in arterial blood (measured in mL/L).

• Venous oxygen content (CvO₂) is the oxygen content in venous blood (measured in mL/L).[2][3]

Components of the Fick Equation

Tissue oxygen consumption (VO₂): This represents the amount of oxygen utilized by body tissues. Oxygen consumption is typically measured in milliliters per minute and is a key parameter in the Fick equation. VO₂ can be influenced by metabolic rate, physical activity, and overall health. Ideally, it is measured through a closed respiratory system that records inspired and expired oxygen.[4][5]

Arterial oxygen content (CaO₂): This component refers to the oxygen content in arterial blood and is determined by the partial pressure of oxygen (PaO₂) in the arterial blood and the oxygen saturation of hemoglobin. Ideally, the CaO₂ should be measured from the left atrium, where the blood is oxygenated by the lungs before oxygen extraction by the tissues.

Venous oxygen content (CvO₂): CvO₂ represents the oxygen content in venous blood, which returns to the heart after oxygen has been delivered to the tissues. The difference between CaO₂ and CvO₂ is a crucial factor in the Fick equation, reflecting the amount of oxygen extracted by the tissues. Ideally, the CvO₂ should be measured from the pulmonary vein.

Direct Fick Method

The direct Fick measurement is the gold standard but is challenging to perform, as it requires a closed respiratory system and a pulmonary artery catheter.

Indirect Fick Method

The indirect Fick method is often used, as it is more practical and less risky.

VO₂: Validated measurements have estimated adult tissue oxygen consumption to be 125 mL/min/m² at rest. For adults aged 70 or older, oxygen consumption is estimated at 110 mL/min/m². Therefore, tissue oxygen consumption can be estimated using the following equation:

• VO₂ (mL/min) = 125 (mL/min/m²) × body surface area (BSA; measured in m²).

• For patients aged 70 or older, the equation is as follows: VO₂ (mL/min) = 110 (mL/min/m²) × BSA (m²).

CaO₂: Arterial oxygen content can be determined by measuring the peripheral arterial blood gas oxygen saturation and multiplying it by the hemoglobin oxygen-carrying capacity.

CvO₂: Venous oxygen content can be determined by measuring the oxygen saturation from a central venous catheter and multiplying it by the hemoglobin oxygen-carrying capacity.

Maximal hemoglobin carrying capacity is described below: 

Hemoglobin oxygen capacity is the maximum volume of oxygen that combines with 1 g of hemoglobin (ie, Hüfner constant), which varies between 1.34 and 1.39 under physiological conditions.[6] The calculation is based on the molecular weight of hemoglobin and the ability to bind 4 oxygen molecules. At a body temperature of 37 °C and atmospheric pressure of 760 mm Hg, the Hüfner constant is calculated to be 1.58 mL/g. However, the actual hemoglobin carrying capacity is less than calculated, so the coefficient often used is 1.31 mL/g.[7]

Thus, the maximum oxygen-carrying capacity of blood (mL of O₂ per dL of blood) is the oxygen bound by hemoglobin plus the amount of oxygen dissolved in the blood. As the oxygen dissolved in blood is a minor contributor, oxygen-carrying capacity can be estimated as the amount of oxygen bound to hemoglobin multiplied by oxygen saturation.

Both arterial and venous blood content are calculated using the following equation: 

• Blood oxygen content = Hgb (g/dL) × 1.31 (mL/g) × Measured O₂ saturation.

Using these substitutions, we arrive at the following:

• Cardiac output (CO) = Tissue oxygen consumption (VO₂)/(Arterial oxygen content − Venous oxygen content).

• CO (L/min) = (10 × 125 × BSA)/([SaO₂ − SvO₂] × Hgb x 1.31).

Clinical Applications

The Fick principle and the associated calculation of cardiac output have several clinical applications:

Assessment of cardiac function: Utilizing the Fick principle to measure cardiac output, clinicians can evaluate the heart's efficiency in pumping blood to meet the body's oxygen demands.[8][9][10]

Monitoring oxygen delivery: Utilizing the Fick principle to estimate oxygen delivery to tissues is crucial for maintaining cellular function. Monitoring oxygen delivery is crucial in critical care settings and during surgical procedures.[11][12]

Research and exercise physiology: The Fick principle finds extensive use in research and exercise physiology, particularly in studying cardiovascular function. In addition, it is instrumental in assessing the effects of exercise or drugs on cardiac output and oxygen consumption.[13]

Limitations and Considerations

While the Fick principle is a valuable tool, it has some limitations and considerations:

• The Fick principle involves invasive blood sampling from arterial and venous sites, which can pose risks, especially in critically ill patients.

• The assumption of a steady state of oxygen consumption limits accuracy in dynamic situations, such as rapid changes in a patient's condition or during exercise.

• The Fick principle assumes no oxygen consumption by the lung. Therefore, the difference between arterial and venous tissue oxygen extraction may be overestimated in cases of acute lung injury, such as pneumonia. 

• The Fick principle may not accurately reflect cardiac output in ventilation-perfusion imbalances, as it assumes normal gas exchange in the lungs.

• The mixed venous pulmonary artery oxygen content is used to measure venous blood oxygen content, which does not account for oxygen consumption in the lung tissue. Therefore, this equation may underestimate tissue oxygen consumption in situations with significant lung oxygen consumption.

• The assumption of constant hemoglobin levels may not hold true in conditions involving acute bleeding, transfusions, or fluctuations in hemoglobin levels.

• The Fick principle may be less applicable in pediatric and neonatal populations due to challenges in obtaining accurate blood samples, shunting, and developmental variations.

• Accurate implementation demands specialized equipment and expertise, constraining its feasibility in resource-limited settings.[3]

Conclusion

Fick cardiac output quantitatively assesses the heart's capacity to pump blood and supply oxygen to tissues. Despite its invasive nature and underlying assumptions, the Fick principle retains significance in clinical practice, research, and exercise physiology. Additionally, the indirect Fick method, which is well-validated, serves as a reliable alternative.

*(see above asterisk) The partial pressure of oxygen (PaO2) multiplied by the oxygen solubility coefficient (0.0031) determines the amount of dissolved oxygen, which is minimal compared to the oxygen bound to hemoglobin. Thus, the total arterial oxygen content comprises the amount of bound oxygen along with the relatively small amount of dissolved oxygen.[7]