Cardiac Function Curves Physiology Notes

Cardiac Function Curves Introduction

Cardiac function curves are Frank-Starling curves that demonstrate the capacity of ventricles to pump blood and maintain blood circulation throughout the body. Most of the cardiac function curves are obtained from animal experiments by using heart-lung preparation. However, these curves are considered to represent the functions of ventricles in the human heart.

Cardiac function curves are of two types:

Cardiac output curves

1. Venous return curves.
2. Cardiac Output Curves

Cardiac output curves are the curves that show the relationship between cardiac output and right atrial pressure.

Right atrial pressure, in turn, depends upon venous return.

Read And Learn More: Medical Physiology Notes

Normal Cardiac Output Curves

• Normally, the left ventricular output is 5 liters/minute, when the pressure in the right atrium is 2 mm Hg.
• When the atrial pressure rises between 4 and 8 mm Hg, the left ventricular output also increases.
• It increases to about two and halftimes of normal (basal) output, i.e. the output increases to about 13-14 liters/minute.
• And, this is the maximum limit for the increase in cardiac output.
• Further increase in right atrial pressure does not increase the ventricular output and the curve shows a plateau.
• Right ventricular output is 5 liter/minute when the right atrial pressure is zero.
• This reaches the maximum, i.e. 13-14 liters/minute when the atrial pressure increases between 2 and 4 mm Hg.
• Thus, the cardiac output curves demonstrate that the cardiac output is directly proportional to atrial pressure up to a certain extent.
• The Plateau of the curve shows that the heart can control the output by itself if the atrial pressure rises beyond + 8 mm Hg.
• It is due to the fact that in normal conditions, the venous return is decreased when the atrial pressure raises above + 8 mm Hg.

Factors Affecting Cardiac Output Curves

Shifting of cardiac output curve to the left indicates an increase in cardiac output and shifting to the right indicates a decrease in cardiac output.

The conditions which shift the cardiac outcurve to the left or right are discussed below:

Shift to Left

When there is an abnormal increase in the functioning of the heart (hyperreflective heart), the cardiac output curve is shifted to the left indicating an increase in cardiac output.

It occurs in the following conditions:

1. Combined stimulation of sympathetic and parasympathetic nerves supplying the heart: It causes hyperexcitation of the heart resulting in increased rate and force of contraction.

• The cardiac output increases up to 25 liters/minute (i.e. the plateau is shifted to the left).
• The increase in output is about twice the maximum output in normal conditions (13-14 lifers/ minute)

2. Hypertrophy of the heart: It increases cardiac output up to 10-19 liters/minute. It is because of the increase in the force of contraction.

3. The excitation (by cardiac nerves) of the heart along with hypertrophy of the ventricles:

• In this condition, the cardiac output is elevated above 35 liters/minute.
• It occurs in Marathon runners. The increase in cardiac output is an important factor in the prolonged running time of Marathon runners.

Shift to Right

When the functioning of the heart decreases (hypo-effective heart), the cardiac output curve is shifted to the right.

It occurs in the following conditions:

1. Stimulation of parasympathetic nerve fibers of the heart
2. Inhibition of sympathetic nerves to the heart
3. Myocardial infarction
4. Diseases of the valves in the heart
5. Congenital heart diseases.

Venous Return Curves

• The venous return curves are the curves that demonstrate the relationship between venous return (blood flow in the vascular system) and right atrial pressure.
• The venous return curves are also called Vsiemc vascular function curves.
• Normally, 5 liters of blood returns to the heart every minute. When the right atrial pressure increases, the venous return decreases due to back pressure.
• When venous return decreases, the cardiac output also decreases.

Analysis Of Cardiac Function Curves

The relation of cardiac output and venous return with right atrial pressure is determined when cardiac output curves and venous return curves are merged together.

Coupling Of Cardiac And Vascular Functions

• Cardiac output represents cardiac function and venous return represents vascular function.
• Coupling or merging of cardiac output (cardiac function) curves and venous return (vascular function) curves show that when venous return is normal (5 liters/minute), the cardiac output, as well as the right atrial pressure, are normal.

• The relation between cardiac output and venous return under normal conditions is represented by (A).
• When the venous return increases (B), the cardiac output also increases along with an increase in right atrial pressure.
• Thus, any factor that alters venous return alters the cardiac output also.