Diagnosis Center, Hôtel-Dieu Hospital, 1 place du Parvis Notre-Dame, 75181 Paris Cedex 04, France
* Corresponding author. Tel: +33 1 42 34 80 25; fax: +33 1 42 34 86 32. E-mail address: michel.safar{at}htd.ap-hop-paris.fr
This editorial refers to Increased arterial wave reflections predict severe cardiovascular events in patients undergoing percutaneous coronary interventions
by T. Weber et al., on page 2657
Since the last century, studies of pulsatile arterial haemodynamics have shown that the blood pressure (BP) curve should be interpreted as a wave.1 Following ventricular ejection, a shock wave results from the acute interaction between stroke volume and the aortic wall. Then, this resulting forward wave propagates at a given velocity (pulse wave velocity, PWV) along the arterial tree. At any discontinuity of the vascular wall, but mainly at the arteriolar branching points, the wave is reflected and comes back towards the heart at the same PWV. Thus, at each point of arterial circuit, but especially at the site of the thoracic aorta, the BP curve is, in fact, the summation of a forward and a backward waves.
This haemodynamic pattern of the arterial system is quite important to consider because pressure and flow, especially in the larger arteries, are by definition oscillatory.1 By relating only mean values of pressure and flow, as it is usually done in linear models of clinical research, many of the information contained in the wave shapes are lost. The use of Fourier analysis makes possible to study both the mean value of the wave and its shape, which is described by a series of sine waves (harmonics), and characterizes the oscillatory system. Using a digital computer, the technique to derive a series of sine waves from a given pressure or flow signal has been considerably improved in the last years and now becomes easy to perform. As shown extensively by computer studies of the cardiovascular system, about 20 harmonics are sufficient for an adequate re-construction of the totality of the BP curve. Under this condition, a correct description of the curve does not only consider peak-systolic (SBP) and end-diastolic blood pressure (DBP), but rather the mean arterial pressure (MAP) value and the oscillation around the mean, frequently summarized as pulse pressure (PP=SBPDBP).
The modelization of an oscillatory phenomenon is not new, since, during the XVIIth century, Huygens showed that the white light was the summation of the various colours of arc en ciel, each of them corresponding to a given colour and/or wavelength. Post-impressionists painters at the end of the XIXth century widely used such possibilities, also commonly used in physics and mainly in electricity. Perhaps for all these reasons, such kinds of methods were particularly investigated by specialists of haemodynamics in Holland, where the arc en ciel is frequent.2 In medicine, there is a single population of physicians who are reluctant to apply Fourier analysis to cyclic phenomenon. It is the large population of specialists in cardiovascular medicine. In hypertension, for instance, the BP curve is usually described by a single or two points of the curve, SBP and/or DBP, whether the specialist is experimentalist (measure of tail SBP in rodents), physicists (brachial measurement of SBP and/or DBP), or even epidemiologists. There is no guideline in the literature of hypertension,35 indicating that: (i) BP is an oscillatory phenomenon which propagates at a given PWV along the arterial tree and (ii) due to the phenomenon of wave reflections, PP and SBP are constantly higher in peripheral than in central arteries, a finding not observed with MAP and DBP. For instance, the physiological difference of SBP or PP between brachial and central arteries approximate 1214 mmHg, which is quite important by comparison with BP variability, as explored from the BP standard deviation.
Studies of pulsatile arterial haemodynamics rapidly improved the prognosis of cardiovascular surgery1 when the counter-pulsion phenomenon was used. However, exactly the same concepts may be applied widely in pharmacology to large populations of subjects with hypertension and atherosclerosis, mostly in the elderly. More specifically, O'Rourke was one of the first to understand that pulsatile models could be easily applied to CV pathophysiology and that non-invasive devices, as those enabling pulse wave analysis, could be developed to understand some pharmacological applications of nitrates.1 The predictive value of wave reflections in the evaluation of the severity of cardiovascular complications was investigated in patients undergoing percutaneous coronary interventions.6 It was shown that, in such patients, the augmentation index, a parameter indicating clear-cut alterations in amplitude and/or timing of carotid wave reflections and therefore of PP in humans, was independently associated with an increase in the severity of long-term cardiovascular events. Here, our comments should be limited to the interest of such results in cardiovascular medicine and particularly in therapeutics.
Since the early development of anti-hypertensive drug therapy, it is widely accepted that the prevention of stroke is correctly obtained, but that the beneficial effect of drug treatment on coronary events (namely myocardial infarction) is less effective.35 This opinion is largely accepted. Several authors proposed that, independent of BP changes, non-haemodynamic (i.e. biochemical) changes produced by some specific anti-hypertensive agents might be able to further reduce coronary risk. Such a finding was suggested when, in long-term therapeutic trials, the differences in brachial SBP between two drug regimens did not reach the statistical significance and approximated 2 mmHg.7 Others argued that central and brachial SBP measurements differ substantially under drug treatment. For instance, when the angiotensin-converting enzyme-inhibitor perindopril is compared with the beta-blocking agent atenolol, a 4 mmHg difference in brachial SBP between the two drug regimens was shown to approximate a 10 mmHg difference in carotid SBP.8 The same authors showed that, in special populations such as those with high risk of coronary ischaemic disease, central BP was superior to brachial BP to predict cardiovascular risk.9 The findings by Weber et al.6 corroborate such findings and suggest that, in large therapeutic trials, the non-invasive evaluation of pulsatile indices should be an interesting goal enabling to evaluate the severity of cardiovascular risk. Moreover, in this study, the augmentation index, an indice of wave reflection, was even superior to PP in the prediction of CV risk. Thus, therapeutic trials in CV diseases need to determine the most appropriate mechanical factors enabling to evaluate CV risk.
Conflict of interest: none declared.
Footnotes
The opinions expressed in this article are not necessarily those of the Editors of the European Heart Journal or of the European Society of Cardiology.
References
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