Cardiac Output and Cardiac Index

Cardiac Output and Cardiac Index:

Cardiac Output- It is the quantity of blood pumped into the aorta each minute by the heart. This is also the quantity of blood that flows through the circulation and is responsible for transporting substances to and from the tissues. Therefore, cardiac output is perhaps the most important factor that we have to consider in relation to the circulation. The cardiac output is dependent on the volume of blood ejected at each contraction (stroke volume) and the rate of pulsation per minute. The volume of blood ejected per minute is the minute volume.

In a healthy young man this cardiac output amounts to about 5-6 litres/min or 80 ml/kg/min (minute volume). For women, this value is 10 to 20 percent less. In 1870, a German physiologist, A.fick, described a simple method of estimating cardiac output from measurements of oxygen consumption (or carbon dioxide production) and the difference between the oxygen (or carbon dioxide) contents of blood entering and leaving the heart. Thus

Cardiac Output (litres/min) = O2 absorbed by lungs (ml/min) / arteriovenous O2 difference (ml/litre blood)

If blood entering the right atrium contains 160 ml O2/litre while that leaving the left ventricle holds 200 ml/litre, it follows that 40 ml O2 has been picked up by each litre of blood circulated through the heart and lungs. If oxygen is being removed from the inspired air at the rate of 200 ml/min, then it is obvious that 5 litres of blood must have been pumped through the heart and lungs to do the job. Several kinds of flow meters are also available but most of the measurements recorded in the literature are based on Pick’s Principle of some modification of it.

The cardiac output usually remains almost proportional to the overall metabolism of the body. That is, the greater the degree of activity of the muscles and other organs, the greater also will be cardiac output. During exercise, the output may be increased 5 to 6 times more, due to augmented stroke volume and increased cardiac frequency. Anxiety may be accompanied by a 10 to 20 % rise above the normal level. Cardiac output is about 10% lowered during sleep. In severe anemia the arterio-venous difference in O2 content is very low to the extent of 2 ml per 100 ml; due to which cardiac output is increased to the extent of 12 litres per minute. In such cases, the cardiac output is accompanied by a high pressure in the right atrium.

Cardiac Index- Because the cardiac output changes markedly with body size, it has been important to find out some means by which the cardiac outputs of different-sized persons can be compared with each other. Generally, cardiac output increases approximately in proportion to the surface area of the body. Therefore, it is frequently stated in terms of the cardiac index, which is the cardiac output per square meter of body surface area. The normal human being weighing 70 kg has a body surface area of approximately 1.7 square meters, which means that the normal average cardiac index for adults is approximately 3.0 litres per minute per square metre.

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