(1)
Department of Cardiovascular, Neural and Metabolic Sciences, Istituto Auxologico Italiano, Milan, Italy
“Central arterial blood pressure” is the term commonly used to describe blood pressure in the ascending aorta, on leaving the left ventricle.
At the beginning of the new millennium, some studies had already pointed out the importance of central systolic blood pressure and of central pulse pressure (systolic – diastolic blood pressure) as cardiovascular prognostic factors, much more significant than peripheral blood pressure values measured in the brachial artery by means of traditional sphygmomanometers.
However, it was only after the Conduit Artery Functional Evaluation (CAFE) study published its outcomes that central systolic blood pressure started to be in the spotlight.
The Anglo-Scandinavian Cardiac Outcomes Trial (ASCOT) had stressed a greater reduction in cardiovascular events in patients treated with calcium channel blockers (amlodipine) compared with patients treated with β-blocker (atenolol), without any difference being noted in the reduction of brachial systolic blood pressure values between the groups treated. The CAFE study, a branch of the ASCOT study, supplied reasonable arguments to explain this more marked reduction in cardiovascular events in subjects treated with vasodilators (Fig. 4.1) [1]. More than 2000 subjects, taking part in the ASCOT study, were measured for central blood pressure. The CAFE study showed that the decrease in central systolic blood pressure and in central pulse pressure was greater in subjects taking some vasodilators with respect to those treated with non-vasodilator drugs (diuretic or β-blocker), despite similar brachial systolic blood pressures (Fig. 4.1). On the basis of the same brachial blood pressure, in the group treated with amlodipine, central systolic blood pressure was significantly lower compared with the group treated with atenolol. Therefore, the authors concluded that the greater reduction in cardiovascular events in the group treated with vasodilators could be caused by a greater effect of these drugs in lowering central systolic blood pressure, with respect to β-blockers. In conclusion, it is reasonable to infer, from the outcomes of the CAFE study, that peripheral blood pressure is not always the best method to assess the effects of drugs on blood pressure and that central systolic blood pressure and central pulse pressure are able to evaluate the real load imposed on the left ventricle much better than peripheral systolic blood pressure and peripheral pulse pressure.
Fig. 4.1
Outcomes of the CAFE study [1]. The two upper lines refer to systolic blood pressure values recorded in the brachial artery; the two lower lines refer to central systolic blood pressure values
Assessment of central arterial pressure values is important because of the difference between peripheral and central blood pressure values. It is well known that mean arterial pressure is characterized by relative “steadiness” in the arterial tree. In other words, it tends to remain constant along the arterial system, from the ascending aorta to peripheral arteries. Diastolic blood pressure behaves in the same way: the difference between central and peripheral diastolic blood pressure values is insignificant (usually below 1 mmHg in the brachial artery in relation to the ascending aorta). On the contrary, peripheral systolic blood pressure values (in the radial, brachial, and femoral arteries) are higher than the ones measured in the ascending aorta. In fact, the difference between blood pressure in the aorta and in the brachial artery is, on average, about 15 mmHg, but even higher differences can be recorded, up to 30–40 mmHg in young adults. This is called the “amplification phenomenon of arterial pressure”.
However, don’t you think that this phenomenon is rather weird? Do you think it is normal that a hydraulic circuit, like the hemodynamic circuit, has blood pressure values lower just after the pump (on leaving the left ventricle) with respect to the periphery? Don’t you consider all this to be strange? Tell me in what mechanical system conceived by human beings, the magnitude of the force at the periphery of the system is greater than the one near the engine. Let us consider an ordinary grass watering system made up of a pump and a hosepipe. It is obvious that the pressure exerted by the water gradually decreases as we leave the pump. Well, do you think it is normal that, in the cardiovascular system, on the contrary, the pressure in the peripheral arteries is higher than near the cardiac pump? How can this phenomenon be explained?
The study of the role of wave reflections can help to explain this amplification phenomenon of arterial pressure.
4.1 Reflected Waves
In the first part of this book, we have seen that blood pressure is changed and modulated by the viscoelastic properties of the aorta and large arteries.
Then, we investigated the clinical significance of arterial stiffness and the evolution of the pressure waveform resulting from the interaction between the heart and large arteries.
However, the relationship between the left ventricle and the aorta cannot explain, in itself, all the phenomena defining blood pressure values and the blood pressure waveform. Therefore, let us now introduce the second parameter characterizing pulse pressure: wave reflections.
The existence of reflected waves is typical of any hydrodynamic system but with different expressions. The concept of wave reflection can be easier to understand if we consider what happens when we drop a stone into the center of a basin full of water. From the point where the stone impacts, a concentric wave is created and travels towards the edges of the container. Here, the wave does not stop, but when it hits the external edges, it creates a wave, which moves back towards the center of the basin (Fig. 4.2a). This is a “reflected wave”.
Fig. 4.2
Reflected waves (a) Generation of a single reflected wave. (b) Backward waves arising and superimposition of forward and backward waves
Now, if we drop a series of stones, at regular intervals of one second, into the center of our basin, we can see that the backward waves to the center superimpose on the centrifugal waves generated directly by the next stone falling into the basin, generating much larger waves (Fig. 4.2b). Therefore, the amplitude of the resultant wave will be defined by the sum of the amplitude of the forward wave and the backward wave.
Let us consider another example that can help explain the idea of wave reflections. Think about ocean waves breaking on the rocks and then going back seawards. These waves add to the next ones, generating higher and higher waves, which are the result of the sum of the waves coming from the ocean and the backward waves coming from the rocks.
The arterial system behaves like any other hydrodynamic circuit in this regard, where the wave generated by the activity of an intermittent pump (heart) travels down a pipe (the aorta, arteries, arterioles, capillaries, etc.). At reflection sites, reflected waves are generated and travel towards the center of the system.
However, in the arterial system, reflected waves arise and travel in a very particular way. The circulatory system has three features: (a) it is a closed circuit, (b) it is small sized, and (c) pressure waves travel quickly, on the order of 4–30 m/s, as we have already seen in the chapter on pulse wave velocity (Chap. 2). This is the reason why wave reflection does not affect the next wave (as is the case for the stone in the basin, or the wave on the rocks), but rather the backward wave superimposes on the same forward wave generating it, therefore, having repercussions for the whole pressure waveform. Figure 4.3 helps us have a better understanding of these important features on the hemodynamics of the cardiovascular system. As a consequence, arterial blood pressure results from the sum of a forward (centrifugal) pressure wave and backward (centripetal) pressure waves.
Fig. 4.3
Superimposition of backward waves on the forward pressure wave in the ascending aorta: illustrative models. Each figure shows the forward (centrifugal) pressure wave (dark gray), the backward (centripetal) pressure wave (light gray, at the bottom of each picture), and the superimposition of both the forward and backward waves. In fact, it is well known that there are a number of reflection sites, but, in this scheme, given for illustrative purpose, only one reflection site has been considered and it is placed at 500 mm from the registration point. The heart rate is 60 beats/min (cardiac cycle = 1000 ms). The backward wave delay time has been considered under different arterial stiffness conditions. (a) In normal viscoelastic properties of the aorta, with a pulse wave velocity (PWV) of 5 m/s, the backward wave goes back to the ascending aorta 200 ms after the beginning of left ventricular ejection. The superimposition with the forward wave occurs in the meso-telesystolic phase and lasts almost for the whole diastolic phase. (b) With a PWV of 10 m/s, the forward and backward waves meet 100 ms after the beginning of the cardiac cycle. (c) In the presence of aortic stiffness, with a PWV of 20 m/s, the two waves meet very early (after 50 ms), and the superimposition with the forward wave occurs in the protomesosystolic phase and lasts almost for the whole systolic phase. The resulting shape of the pulse wave depends on the earliness of the superimposition onto the forward and backward waves
Wave reflection is very important and must be taken into consideration, as it is a proven fact that the magnitude of the reflected wave may be until 80–90 % that of the forward wave.
4.2 Reflection Sites
In the cardiovascular system, there are some well-defined sites from which reflected waves arise:
- 1.
Arterial bifurcations
- 2.
Atherosclerotic plaques, causing arterial narrowing or obstruction
- 3.
Terminal arterioles, which define the systemic vascular resistance
4.2.1 Arterial Bifurcations
Important reflection sites are arterial bifurcations (Fig. 4.4). In the presence of a bifurcation, not only does the forward wave (FW) divide into two single centrifugal waves (FWa and FWb), but it also generates a centripetal backward wave (BW). The amplitude of these waves is related to the angle of bifurcation and to the caliber of the secondary branches, which arise from the main artery.
Fig. 4.4
Reflected wave generated by arterial bifurcation
4.2.2 Atherosclerotic Plaques
Atherosclerotic plaques, causing arterial narrowing or obstruction and segmental alterations of the viscoelastic properties of arteries, are another reflection site (Fig. 4.5), relevant under hemodynamically significant stenosis and multifocal atherosclerotic vascular disease.
Fig. 4.5
Stenosis and arterial thrombotic phenomena generate reflected wave
The forward wave (FW), next to an atheroma and vascular stenosis, divides into two components: a forward component, which continues its journey through the arterial system in a centrifugal way, and a component which, on the contrary, is sent back to the heart (BW) by the endoluminar obstruction.
4.2.3 Systemic Vascular Resistance
It is well known that resistance is directly proportional to viscosity (η) and inversely proportional to the fourth power of the radius (Hagen–Poiseuille law):
Therefore, the resistive properties in the large arteries are insignificant whereas vascular resistance is concentrated at the level of the precapillary arterioles (diameter < 150 μm). The arteriolar system represents a sort of narrowing of the arterial system, and, at this level, a sharp fall in blood pressure values occurs (Fig. 4.6).
Fig. 4.6
Schematization of the cardiovascular system (top). Rapid increase of systemic vascular resistance in precapillary small-diameter arterioles (bottom)
The forward pressure wave, generated by the interaction between the heart and large arteries, travels through the arterial tree and, at the periphery of the cardiovascular system, breaks against the “wall” made up by peripheral vascular resistance. Here, a backward wave is generated, going to the heart and superimposing on the centrifugal (forward) wave.
It is likely that peripheral resistance is the major reflection site, and its importance is also linked to the possibility of changing and modulating reflected waves arising here, by means of pharmacological therapy.
4.3 Reflected Waves and Peripheral Blood Pressure
The superimposition of the forward pressure wave and backward waves is particularly important in the peripheral arteries, following the first bifurcations of the aorta, such as the femoral, brachial and radial arteries, i.e., those arteries where we usually measure blood pressure with traditional sphygmomanometers.
Peripheral arteries are close to the main sites of wave reflection (we are near the rocks on which the waves break), so that the superimposition of the forward and backward waves occurs very early during the systolic phase (Fig. 4.7).
Fig. 4.7
Pressure waveform recorded in peripheral artery in a subject with undamaged viscoelastic properties of the arterial wall. Early superimposition of backward and forward pressure waves
As a consequence, the pressure peak will be strongly affected by backward waves, and the reflected component will strongly affect systolic blood pressure values. Therefore, systolic blood pressure in the peripheral arteries is mainly defined by the presence of wave reflection.
4.4 Reflected Waves and Central Blood Pressure
The pressure waveform always depends on the temporal relationship between the encounter and superimposition of the forward (centrifugal) pressure wave and backward (centripetal) pressure waves.
From the periphery of the arterial system, the backward wave “travels” centripetally towards the heart. In the ascending aorta, its encounter with the forward wave occurs at the end of the systolic phase, and the superimposition of the two waves lasts the whole diastolic phase (Fig. 4.8).
Fig. 4.8
Pressure wave recorded in the ascending aorta in a subject with undamaged viscoelastic properties of the arterial wall. The encounter between backward and forward waves occurs at the end of the systole, and the superimposition of the two waves lasts the whole diastole
This occurs when the viscoelastic properties of large arteries are undamaged. Thus, the central blood pressure waveform shows the following consequences:
The peak systolic blood pressure is not affected by the backward waves, and the backward waves do not change systolic blood pressure values. Therefore, systolic blood pressure is only defined by the forward wave, i.e., by the heart–aorta interaction.
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