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Cardiac Function CurveSingle cardiac function curve
Graph of a single cardiac function curve A graph showing the relationship between right atrial pressure (x-axis) and cardiac output (y-axis). It shows a steep relationship at relatively low filling pressures and a plateau, where further stretch is not possible and so increases in pressure have little effect on output. The pressures where there is a steep relationship lie within the normal range of right atrial pressure found in the healthy human during life. This range is about -1 to +2 mmHg. The higher pressures normally only occur in disease, in conditions such as heart failure, where the heart is unable to pump forward all the blood returning to it and so the pressure builds up in the right atrium and the great veins. Swollen neck veins are often an indicator of this type of heart failure. At low right atrial pressures this graph serves as a graphic demonstration of the Frank Starling mechanism, that is as more blood is returned to the heart, more blood is pumped from it without extrinsic signals. In vivo however, extrinsic factors such as an increase in activity of the sympathetic nerves, and a decrease in vagal tone cause the heart to beat more frequently and more forcefully. This alters the cardiac function curve, shifting it upwards. This allows the heart to cope with the required cardiac output at a relatively low right atrial pressure. We get what is known as a family of cardiac function curves, as the heart rate increases before the plateau is reached, and without the RAP having to rise dramatically to stretch the heart more and get the Starling effect. Graph of family of cardiac function curves
Graph of a family of cardiac function curve at various heart rates. As the demand for cardiac output increases, the heart rate increases and the curve shifts upwards along the red line. This prevents the right atrial pressure needing to increase.
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