WHAT IS CARDIAC OUTPUT (THERMODILUTION) TEST?
Cardiac output (CO) is the product of heart rate (HR) and stroke volume (SV). It is the volume of blood ejected from the heart over a period of 1 minute. The determinants of cardiac output are preload, afterload, and heart rate in beats per minute and stroke volume in millilitres per beat (CO = HR × SV). Stroke volume is the volume of blood ejected with each ventricular contraction and is the difference between the volume of the left ventricle at end diastole and the volume remaining in the ventricle at end systole. In an average-sized adult at rest, cardiac output is approximately 4-8 L/min. In diseased states, cardiac output is usually found to be less than normal and may be so low that adequate blood supply to the body’s tissues cannot be delivered. A low cardiac output may be the result of the poor filling of the ventricle (reduced preload) or poor forward emptying of the ventricle (increased afterload). Some causes of low resting cardiac output are diminished myocardial function resulting from myocardial infarction, aortic stenosis, arterial hypertension, and cardiomyopathy. The thermodilution method of cardiac output determination measures the change in core temperature in the pulmonary artery before and after injection of a specific quantity of injectate of known temperature. The change in temperature reflects the cardiac output inversely and is used to plot a cardiac output/thermodilution curve. A low cardiac output produces a greater change in temperature for a longer period of time than does a high cardiac output.