Cardiac Energy Consumption Is Required to Support External and Internal Cardiac Work

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Cardiac energy consumption (which is equivalent to cardiac oxygen consumption) provides the energy for both external work and internal work.

Most of the external work of the heart involves the ejection of blood from the ventricles into the aorta and pulmonary artery. The work of ejecting blood from the ventricles is the stroke work. Stroke work, strictly speaking, is equal to the product of the volume of blood ejected (stroke volume, SV) and the pressure against which the blood is ejected (aortic and pulmonary artery pressure during systole). Because the systolic pressure in the pulmonary artery is about one sixth of the pressure in the aorta, more than 80% of external work is done by the left ventricle. Left ventricular stroke work (SW) is usually calculated as:

Mean arterial pressure (Pa) is used instead of mean arterial pressure during systole because it is more readily available and is a reasonable index of mean systolic pressure.

A small additional component of external work (usually <10%) is kinetic work. Kinetic energy is the energy imparted to blood in the form of flow velocity as it is ejected with each heartbeat. We do not elaborate on this component of external work because it is of little importance in most situations.

Cardiac contractions involve many events that do not result in external work. These include internal mechanical events such as developing force by stretching series elasticity (see Chapter 10), overcoming internal viscosity, and rearranging the muscular architecture of the heart as it contracts. These activities, known as internal work, use far more energy (perhaps 5 times as much) than external work.

Cardiac Efficiency. The efficiency of the heart in performing external work can be estimated by dividing the external work of the heart by the energy equivalent of the oxygen consumed by the heart. Only 5 to 20% of the energy liberated by cardiac oxygen consumption is used for external work under most conditions. Therefore, changes in external work do not reveal much about changes in energy consumption in the heart. This is because internal work, the major determinant of oxygen consumption and, thereby, cardiac efficiency, varies independently of external work. As we shall see, large increases in internal work can occur in the absence of changes in external work. When this happens, oxygen consumption increases and efficiency decreases. The difference between pressure work and volume work illustrates this point.

"Pressure Work" Versus "Volume Work". Most of the cardiac energy devoted to internal work is used to maintain the force of contraction (and, thus, ventricular pressure) rather than to eject the blood. The importance of this is seen by comparing two tasks: lifting a 20-pound weight from the floor to a table and lifting the weight to the table height and continuing to hold it. The second task is clearly more difficult, even though the external work done (i.e., the force multiplied by the distance the object was moved)

in each case is the same. The ventricles not only develop the pressure required to move the blood, but must maintain the pressure during systole. This takes far more energy than the external work alone as calculated from arterial pressure and stroke volume. In fact, if the external work of the heart is raised by increasing stroke volume but not mean arterial pressure, the oxygen consumption of the heart increases very little. Alternatively, if arterial pressure is increased, the oxygen consumption per beat goes up much more. In other words, pressure work by the heart is far more expensive in terms of oxygen consumption than volume work. This makes sense because internal work consumes far more energy than external work.

Afterload. The discussion of pressure work versus volume work emphasizes the importance of afterload as a determinant of energy use and oxygen consumption by the heart. Because of Laplace's law, an increase in ventricular radius is equivalent to an increase in arterial pressure. Thus, an increase in ventricular radius, as can occur with heart failure, also causes a proportional increase in internal work and energy use, independent of any change in external work.

Heart Rate. Thus far, we have considered only the energetic events associated with a single cardiac contraction. The energy consumed per unit time is equal to the energy consumed in a single heartbeat multiplied by the heart rate. It follows that the production of energy from oxidative phosphorylation per unit time must be sufficient to match the energy consumed in a single heartbeat multiplied by the heart rate.

There is another important consideration related to heart rate. Much of the internal work of the heart occurs during isovolumetric contraction, when force is being developed but no external work is being done. If cardiac output is in creased by increasing heart rate, the energy expended in the internal work of isovolumetric contraction increases proportionately. By contrast, if cardiac output is increased by increasing stroke volume, there is a much smaller increase in internal work. This means that increasing cardiac output by increasing heart rate is more energetically costly than the same increase by means of stroke volume.

Contractility. Altered myocardial contractility has significant energetic consequences because of differential effects on external and internal work. Inotropic agents (e.g., nor-epinephrine) may increase pressure work by raising arterial pressure and, thereby, increase internal work. However, in-otropic agents can also cause the heart to do the same stroke work at a smaller end-diastolic volume, reducing both afterload and internal work. During exercise, increased contractility causes end-diastolic volume to decrease despite the increase in venous return. This lowers the contribution of ventricular radius to afterload and avoids the inefficiency of an increase in end-diastolic volume.

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  • latoya
    Why does "pressure work" consume more energy?
    8 years ago

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