Clinical Information
The function of the heart is the transport of the blood to deliver oxygen, nutrients and chemicals to the cells of the body, to ensure their survival and proper function and to remove the cellular wastes. Cardiac Output indicates how well the heart is performing this function. Cardiac Output is regulated principally by the demand for oxygen by the cells of the body. If the cells are working hard, with a high metabolic oxygen demand then the Cardiac Output is raised to increase the supply of oxygen to the cells, while at rest when the cellular demand is low, the Cardiac Output is said to be baseline. Cardiac Output is regulated not only by the heart as a pump, but also by the function of the vessels of the body as they actively relax and contract thereby increasing and decreasing the resistance to flow.
When Cardiac Output increases in a healthy but untrained individual, most of the increase can be attributed to an increase in Heart Rate (HR). Change of posture, increased sympathetic nervous system activity, and decreased parasympathetic nervous system activity can also increase Cardiac Output. HR can vary by a factor of approximately 3, between 60 and 180 beats per minute, whilst Stroke Volume (SV) can vary between 70 and 120 ml, a factor of only 1.7.
A parameter related to SV is Ejection Fraction (EF). EF is the fraction of blood ejected by the Left Ventricle (LV) during the contraction or ejection phase of the cardiac cycle or Systole. Prior to the start of Systole, the LV is filled with blood to the capacity known as End Diastolic Volume (EDV) during the filling phase or diastole. During Systole, the LV contracts and ejects blood until it reaches its minimum capacity known as End Systolic Volume (ESV); in that moment it does not empty completely. Clearly the EF is dependent on the ventricular EDV which may vary with ventricular disease associated with ventricular dilatation. Even with LV dilatation and impaired contraction the Cardiac Output may remain constant due to an increase in EDV.
Stroke Volume (SV) = EDV – ESV
Ejection Fraction (EF) = (SV / EDV) × 100%
Cardiac Output (CO) = SV × HR
Cardiac Index (CI) = Cardiac Output / Body Surface Area (BSA) = SV × HR/BSA
HR is Heart Rate, expressed as BPM (Beats Per Minute)
BSA is Body Surface Area in square meters
Diseases of the cardiovascular system are often associated with changes in Cardiac Output, particularly the pandemic diseases of hypertension and heart failure. Cardiovascular disease can be associated with increased Cardiac Output as occurs during infection and sepsis, or decreased Cardiac Output, as in cardiomyopathy and heart failure. The ability to accurately measure Cardiac Output is important in clinical medicine as it provides for improved diagnosis of abnormalities, and can be used to guide appropriate management. Cardiac Output measurement, if it was accurate and mini-invasive, would be adopted as part of every clinical examination from general observations to the intensive care ward, and would be as common as simple blood pressure measurements are now. Such practice may revolutionize the treatment of many cardiovascular diseases including hypertension and heart failure. This is the reason why Cardiac Output measurement is now an important research and clinical focus in cardiovascular medicine.
