Arterial and Aortic Pulse Pressure
As the left ventricle ejects blood into the aorta, the aortic pressure increases. The greater the stroke volume, the greater the change in aortic pressure during ejection. The maximal change in aortic pressure during systole (from the time the aortic valve opens until the peak aortic pressure is attained (see Cardiac Cycle) represents the aortic pulse pressure, which is defined as the systolic pressure minus the diastolic pressure. For example, if the systolic pressure is 130 mmHg and the diastolic pressure is 85 mmHg, then the pulse pressure is 45 mmHg.
Pulse Pressure = Systolic Pressure − Diastolic Pressure
The rise in aortic pressure from its diastolic to systolic value is determined by the compliance of the aorta as well as the ventricular stroke volume. In the arterial system, the aorta has the highest compliance, due in part to a relatively greater proportion of elastin fibers versus smooth muscle and collagen. This serves the important function of dampening the pulsatile output of the left ventricle, thereby reducing the pulse pressure (systolic minus diastolic arterial pressure). If the aorta were a rigid tube, the pulse pressure would be very high. Because the aorta is compliant, as blood is ejected into the aorta, the walls of the aorta expand to accommodate the increase in blood volume. As the aorta expands, the increase in pressure is determined by the compliance of the aorta at that particular range of volumes. The more compliant the aorta, the smaller the pressure change during ventricular ejection (i.e., smaller pulse pressure) (see figure). Therefore, aortic compliance is a major determinant, along with stroke volume, of the pulse pressure.
- A highly compliant aorta (i.e., less stiff, normal aorta) has a smaller pulse pressure for a given stroke volume into the aorta than a stiff, low compliant aorta.
- A larger stroke volume (not shown in the figure) produces a larger pulse pressure at any given compliance.
- Aortic compliance decreases with age due to structural changes, thereby producing age-dependent increases in pulse pressure.
- For a given stroke volume, compliance determines pulse pressure and not mean aortic pressure.
- However, because vessels display dynamic compliance, increasing the rate of ventricular ejection (as occurs with increased ventricular inotropy) will increase the pulse pressure compared to the same volume ejected at a lower rate.