Thermodilution is one method of estimating cardiac output. This parameter directly tells about the efficiency of this organ. What is the thermodilution method, what are the conditions for its implementation and the limitations of this test?
Thermodilutionis one method of measuring cardiac output, which is the volume of blood pumped by the heart. This test is usually performed during right-sided catheterization and measures the difference in blood temperature between the thermistors placed at different sites in the catheter used in this test.
Contents :
- Heart throw: what is it?
- Right-sided catheterization: what is it?
- Thermodilution: what is it?
Heart throw: what is it?
Cardiac output, or minute capacity, is the volume of blood that is pumped by the heart in one minute. This value is therefore derived from the heart rate and stroke volume, which is the amount of blood that is ejected from the right and left ventricles with each contraction. While the heart rate is very easy to measure, the precise measurement of the stroke volume is possible primarily by invasive methods.
Right-sided catheterization: what is it?
Right-sided catheterization is an invasive test that allows to obtain many parameters of the heart work, including cardiac output. This procedure involves the insertion of a catheter through the jugular or femoral vein into the right atrium, right ventricle, and pulmonary artery. This test tells us a lot about the condition of the circulatory system and the performance of the heart. Thanks to right-sided catheterization, it is possible to measure, inter alia, the pressures in the above-mentioned cavities, vascular resistance, and, if the catheter tip is in the pulmonary artery, cardiac output. How exactly does it take place?
Thermodilution: what is it?
During right-sided catheterization, the measurement of cardiac output is most often performed using thermodilution. What is this method about? The catheter used for right-sided catheterization (the so-called Swan-Ganz catheter) has several tunnels in its cross-section, one of them exits at the end of the catheter, the other several dozen centimeters earlier, so-called thermistors, i.e. temperature sensors, are located at both outlets.
During the examination, the cord that ends earlier, usually located in the right atrium, is given 10ml of cold saline solution, the temperature of which is measured as it flows out of the catheter. The cold fluid immediately mixes with the blood, causing its temperature to change - a slight, temporary drop. The cooled volume of blood flows towards the end of the catheter where a second temperature sensor, most commonly located in the pulmonary artery, measures it.
The greater the cardiac output, the greater the amount of blood flowing through the right atrium and right ventricle, so the lower the drop in blood temperature recorded at the end of the catheter. Based on the difference in temperature and the rate of its change, computers draw a temperature rise and fall curve that indirectly tells about the speed of blood flow. From this graph, stroke volume is calculated and then cardiac output.
The thermodilution method is also called the temperature dilution method. Of course, this method of measurement evaluates the right ventricular projection, if there is no so-called leakage defect, it is equal to the left ventricular projection, because both of these chambers pump the same volume of blood with each contraction.
Another method of measuring cardiac output is Fick's method, used for example in the so-called Leakage defects, i.e. heart defects involving the presence of a defect in the atrial or interventricular septum, which results in mixing of blood between the right and left parts of the heart. In this case, the measurement of cardiac output is based on a formula that takes into account the consumption of oxygen and its tension in the arterial and venous blood.
About the authorBow. Maciej GrymuzaA graduate of the Faculty of Medicine at the Medical University of K. Marcinkowski in Poznań. He graduated from university with an over good result. Currently, he is a doctor in the field of cardiology and a doctoral student. He is particularly interested in invasive cardiology and implantable devices (stimulators).