**Core Concept**
Cardiac output is the volume of blood the heart pumps per minute and is a crucial parameter in assessing cardiovascular function. Various methods can be used to determine cardiac output, each with its own strengths and limitations.

**Why the Correct Answer is Right**
The V/Q (ventilation-perfusion) ratio is a measure of the relationship between alveolar ventilation and perfusion in the lungs. While it is essential in understanding respiratory physiology and can be used to diagnose conditions like pulmonary embolism, it does not directly measure cardiac output. Cardiac output can be determined using methods like Fick's principle, echocardiography, and thermodilution, which directly measure or estimate the heart's pumping capacity.

**Why Each Wrong Option is Incorrect**
**Option A:** Fick's principle is a valid method for determining cardiac output. It is based on the principle that the amount of oxygen consumed by the body is equal to the amount of oxygen delivered to the body by the heart. Fick's principle can be used to calculate cardiac output by measuring the difference in oxygen content between arterial and venous blood.

**Option C:** Echocardiography is a non-invasive imaging technique that can be used to estimate cardiac output by measuring the velocity of blood flow through the heart's chambers and valves. Doppler echocardiography can provide information on cardiac output, making it a useful tool in clinical practice.

**Option D:** Thermodilution is a method that uses a catheter to inject a cold saline solution into the pulmonary artery, and then measures the temperature change in the blood as it passes through the heart. This method can be used to estimate cardiac output by calculating the volume of blood that has passed through the heart.

**Clinical Pearl / High-Yield Fact**
When assessing cardiac output, it is essential to consider the clinical context and the limitations of each method. For example, Fick's principle may not be accurate in patients with severe respiratory disease, while echocardiography may not be feasible in patients with poor acoustic windows.

**✓ Correct Answer: B. V/Q ratio. V/Q ratio is not a method for determining cardiac output.**