The Fick Principle is a foundational method for measuring cardiac output (CO), which reflects the volume of blood pumped by the heart per minute. It is based on the relationship between oxygen consumption and the difference in oxygen content between arterial and venous blood. According to the principle, the amount of oxygen taken up by the lungs per minute must equal the oxygen delivered to tissues by cardiac output multiplied by the arterial–venous oxygen difference.

This method provides an accurate, physiology-based approach for quantifying cardiac function. Unlike indirect methods such as echocardiography, the Fick method is grounded in direct gas exchange principles, making it particularly valuable in research, catheterization labs, and critical care monitoring. Cardiac output assessment is essential for diagnosing and managing conditions such as heart failure, shock, pulmonary hypertension, and congenital heart disease.

The Fick method is considered the gold standard, though its use is limited by the need for invasive measurements (mixed venous oxygen sampling from a pulmonary artery catheter) and challenges in precisely measuring oxygen consumption. In modern practice, estimated VO₂ values are often substituted, but this can reduce accuracy. Nonetheless, the formula continues to serve as a key reference standard against which other methods, including thermodilution and Doppler echocardiography, are validated.