Abstract
The rate and character of the arterial pulse has been used for millennia for the diagnosis of a wide range of disorders. Perhaps more useful, however, is the direct cannulation of an artery, which allows quantitative information to be extracted.
The rate and character of the arterial pulse has been used for millennia for the diagnosis of a wide range of disorders. Perhaps more useful, however, is the direct cannulation of an artery, which allows quantitative information to be extracted.
What is the Windkessel effect?
During systole, the left ventricle (LV) ejects around 70 mL of blood into the aorta (the stroke volume, SV). The elastic aortic walls expand to accommodate the SV, moderating the consequent increase in intra-aortic pressure from a diastolic blood pressure (DBP) of 80 mmHg to a systolic blood pressure (SBP) of 120 mmHg. The ejected blood possesses kinetic energy, whilst there is storage of potential energy in the stretched aortic wall. In diastole, recoil of the aortic wall converts the stored potential energy back into kinetic energy. This maintains the onward flow of blood during diastole, thereby maintaining DBP; this is known as the ‘Windkessel effect’. This effect, along with the cardiac valves, converts the sinusoidal pressure wave generated in the heart into a positive and constant pressure at the tissues, much like converting AC to DC electricity. With advancing age, there is degeneration of elastin in the wall of the aorta. The aortic wall becomes less compliant, and its ability to accommodate SV without a large increase in pressure reduces. This accounts for the development of systolic hypertension in the elderly.
What is the arterial pressure wave?
Ejection of blood into the aorta generates both an arterial pressure wave and a blood flow wave. The arterial pressure wave is caused by the distension of the elastic walls of the aorta during systole. The wave propagates down the arterial tree at a much faster rate (around 4 m/s) than the mean aortic blood velocity (20 cm/s). It is the arterial pressure wave that is felt as the radial pulse, not the blood flow wave.
Describe the arterial pressure waveform for the aorta
Starting from end-diastole (Figure 35.1), the pressure generated by the LV ejects the SV into the aorta. The intra-aortic pressure rises to a peak value (the SBP) and then falls to a trough (the DBP). The smooth descent of the curve is interrupted at the dicrotic notch, when the aortic valve closes.
Figure 35.1 The arterial waveform.
How does the arterial pressure waveform differ at peripheral arteries?
The morphology of the arterial pressure waveform differs depending on where it is measured (Figure 35.2). As the site of measurement moves more distally:
The arterial upstroke is steeper and SBP is increased.
DBP is decreased.
Crucially, mean arterial pressure (MAP) is relatively constant wherever it is measured; this is another reason why MAP is the most important measure of blood pressure.
The morphology of the dicrotic notch changes:
– The dicrotic notch is positioned further down the pressure curve.
– Rather than being a sharp interruption in the pressure descent, the dicrotic notch becomes more of a dicrotic wave.
