Describe the thermodilution method for measuring cardiac output.

Describe the thermodilution method for measuring cardiac output.

• A. Inject a known bolus of warm saline into the pulmonary artery and measure downstream temperature
• B. Inject a known bolus of cold saline into the left ventricle and measure aorta temperature
• C. Inject a known bolus of cold saline into the right atrium and measure the temperature downstream in the pulmonary artery
• D. Inject a known bolus of saline into the coronary artery and measure coronary venous temperature

Answer: (C)

Thermodilution measures cardiac output by using a temperature indicator dilution technique. A known volume of cold saline is injected into the venous system, and a temperature sensor downstream tracks the resulting temperature drop as the bolus passes through the circulation. The sensor is placed in the pulmonary artery, so the bolus travels first through the right heart and then into the lungs. By analyzing the temperature versus time curve with the Stewart–Hamilton equation, the cardiac output is calculated: a larger, faster-disappearing temperature dip corresponds to a higher cardiac output, while a slower, smaller dip points to a lower output. Injecting the cold saline into the right atrium and measuring the temperature downstream in the pulmonary artery provides the correct first-pass dilution signal that the equation uses. Injecting into the left ventricle or into a coronary artery would not yield the proper dilution curve for this method, and measuring in the wrong vessel would misrepresent the cardiac output.

Thermodilution measures cardiac output by using a temperature indicator dilution technique. A known volume of cold saline is injected into the venous system, and a temperature sensor downstream tracks the resulting temperature drop as the bolus passes through the circulation. The sensor is placed in the pulmonary artery, so the bolus travels first through the right heart and then into the lungs. By analyzing the temperature versus time curve with the Stewart–Hamilton equation, the cardiac output is calculated: a larger, faster-disappearing temperature dip corresponds to a higher cardiac output, while a slower, smaller dip points to a lower output. Injecting the cold saline into the right atrium and measuring the temperature downstream in the pulmonary artery provides the correct first-pass dilution signal that the equation uses. Injecting into the left ventricle or into a coronary artery would not yield the proper dilution curve for this method, and measuring in the wrong vessel would misrepresent the cardiac output.