Cardiac Output

Cardiac output is the amount of blood pumped by the heart per unit time () and is determined by four factors: two factors that are intrinsic to the heart—heart rate and myocardial contractility—and two factors that are extrinsic to the heart but functionally couple the heart and the vasculature—preload and afterload.

Heart rate is defined as the number of beats per minute and is influenced mainly by the autonomic nervous system. Increases in heart rate increase cardiac output as long as ventricular filling is adequate during diastole. Contractility can be defined as the intrinsic level of contractile performance that is independent of loading conditions. Contractility is difficult to define in an intact heart because it cannot be separated from loading conditions.^{[12] [13]} For example, the Frank-Starling relationship is defined as the change in intrinsic contractile performance based on changes in preload. Measurement of cardiac output in a living organism can be determined by using the Fick principle, a schematic depiction of which is illustrated in Figure 18-7 .^[1]

The Fick principle is based on the concept of conservation of mass such that the O₂ delivered from pulmonary

Figure 18-7 Illustration demonstrating the principle of determination of cardiac output according to Fick's equation. If the O₂ concentration of the pulmonary artery (CpaO₂ ), the O₂ concentration of the pulmonary vein (CpvO₂ ), and O₂ consumption are known, cardiac output can be calculated. (From Berne RM, Levy MN: The cardiac pump. In Cardiovascular Physiology, 8th ed. St Louis, CV Mosby, 2001, pp 55–82.)

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The amount of O₂ delivered to the pulmonary capillaries by way of the pulmonary arteries (q₁ ) equals the total pulmonary arterial blood flow () times the O₂ concentration in pulmonary arterial blood (CpaO₂ ):

q₁ = × CpaO₂

The amount of O₂ carried away from the pulmonary venous blood (q₃ ) is equal to the total pulmonary venous blood flow () times the O₂ concentration in pulmonary venous blood (CpvO₂ ):

q₃ = × CpvO₂

The pulmonary arterial O₂ concentration is the mixed systemic venous O₂ , and the pulmonary venous O₂ concentration is the peripheral arterial O₂ . O₂ consumption is the amount of O₂ delivered to the pulmonary capillaries from the alveoli (q₂ ). Because q₁ + q₂ = q₃ ,

(CpaO₂ ) + q₂ = (CpvO₂ )

q₂ = (CpvO₂ ) − (CpaO₂ )

q₂ = (CpvO₂ ) − CpaO₂

= q₂ /(CpvO₂ − CpaO₂ )

Thus, if the O₂ concentration in the pulmonary artery (CpaO₂ ), the O₂ concentration in the pulmonary vein (CpvO₂ ), and O₂ consumption (q₂ ) are known, cardiac output can be determined.

The indicator dilution technique is another method of determining cardiac output, and it is also based on the law of conservation of mass. The two most commonly used indicator dilution techniques are the dye dilution and the thermodilution methods. Figure 18-8 illustrates the principles of the dye dilution method.^[1]