Fick Cardiac Output Calculator

Variables and formula

Cardiac output is a hemodynamic measure of cardiac function which defines the volume of blood pumped by the left ventricle during one minute.

The variables used in the calculation are described in the following table:

The equation used in the above calculator is based on the Fick principle which assumes that the rate of blood flow is correlated with oxygen consumption.

Oxygen consumption cannot, however, be determined routinely, therefore a simplified estimation method, accounting for oxygen content in arteries and veins, has been devised.

The arterial and venous oxygen content (or saturation) is estimated though hemoglobin levels (obtained from blood test)

Therefore, the Fick based estimation becomes:

Cardiac Output = O₂ consumption / Arteriovenous O₂ difference = (125 ml O₂/min/m² x BSA) / [13 x Hb x (S_aO₂ - S_vO₂)]

Where BSA is calculated through Haycock’s formula:

Body Surface Area (BSA) = 0.024265 x (Height in cm)^0.3964 x (Weight in kg)^0.5378

Cardiac output explained

CO is the heart rate multiplied by stroke volume, which results in a volume of blood in millilitres, pumped by the heart in one minute. Stroke volume is the amount of blood released in circulation by the hear with each contraction (beat).

The cardiac output of an individual with an average hear rate at rest of 70 beats per minute and a stroke volume of 70 mL, would be 4,900 mL/min.

Associating this with the estimate that each person has approximately 5 L of blood in the circulatory system, we can reach the conclusion that during a minute, at a resting pace, the heart can circulate almost the entire volume of blood in the body.

There is another calculation method for CO, method that is used in the Fick equation calculator:

CO = VO₂ / (Ca - Cv)

Where:

■ VO₂ is the oxygen consumption in ml/min of pure gaseous oxygen;

■ Ca is the oxygen concentration of oxygenated blood taken from the pulmonary vein;

■ Cv is the oxygen concentration of deoxygenated blood from an intravenous cannula.

The cardiac output depends on the regulation of both the heart rate and stroke volume.

Low CO is characteristic of heart failure and its severity can increase up to cardiogenic shock which is characterized by the following:

■ Systolic blood pressure of <90 mmHg (or a drop of >30 mmHg);

■ Urine output of <0.5 ml/kg/hour;

■ Heart rate >60 beats per minute;

■ With or without evidence of congestion.

Increases in CO can be observed during infections or inflammation, as the body’s fight reaction.

In some cases, combined cardiac output may be estimated, which is the sum of outputs from both right and left ventricle. This is used to evaluate fetal circulation because of its specific physiology.

References

1. Wilkinson J. Haemodynamic calculations in the catheter laboratory. Heart. 2001; 85(1): 113–120.

2. LaFarge CG, Miettinen OS. The estimation of oxygen consumption. Cardiovasc Res. 1970; 4(1):23-30.

3. Fagard R, Conway J. Measurement of cardiac output: Fick principle using catheterization. Eur Heart J. 1990; 11 Suppl I:1-5.

4. Mahutte CK, Jaffe MB, Chen PA, Sasse SA, Wong DH, Sassoon CS. Oxygen Fick and modified carbon dioxide Fick cardiac outputs. Crit Care Med. 1994; 22(1):86-95.