In recent years great attention has been given to the role of changes in arterial wall properties and to the increased arterial stiffness in the development of cardiovascular diseases and events. In 2007, the guidelines of the European Society of Hypertension have introduced aortic stiffness as a new parameter of target organ damage, to be evaluated in the stratification of cardiovascular risk . The measurement of arterial stiffness can be performed by non-invasive methods in different sites of the arterial tree [2,3]. Carotido-femoral pulse wave velocity (PWV) is considered as the gold standard measurement of arterial stiffness, since thoracic and abdominal aorta contribute mostly to the reservoir function. In addition, an increase in the carotido-femoral PWV, reflecting a stiffer aorta, has been shown to predict the occurrence of cardiovascular and cerebrovascular events, independently of other traditional cardiovascular risk factors .
PWV is usually measured using the foot-to-foot velocity method. The foot of the wave corresponds to the end of diastole, at the beginning of the steep rise of the waveform. The time of travel from one foot to the next of two forward waveforms over a known distance is defined transit time and the distance covered by the pulse wave is usually assimilated to the surface distance between the recording sites of the two waveforms. Several recommendations have been given to avoid inaccuracies in measuring the distance between the recording sites, since small differences in distance may exert a great influence on the calculated value of PWV. Other important limitations in the measurement of the carotido-femoral PWV may be represented by the recording of femoral pulse waveform in the presence of obesity, diabetes or stenotic lesions and the correct identification of the feet of pulse waveforms, because of the possible interference of the backward wave travelling from reflection sites to the aorta and left ventricle [2,4].
The pressure pulse wave generated by the left ventricle (LV) ejection and travelling forward may encounter several sites of reflection, represented by sites where the arterial properties change (due to vasomotor tone and/or structural changes, including elasticity loss); in addition the architecture (bifurcations or branches origin) or the presence of calcifications are opposed to the propagation of the wave. The presence of different sites of reflection causes the return backward to the heart of several pulse waves. All the reflected waves cannot be identified one by one in the recording of the pressure waveform, but are incorporated in a single component. The point of return of the sum of reflected waves is the inflection point, close after the systolic foot of the pulse waveform. The systolic foot has been chosen as reference point for the measurement of the transit time because it is considered to be free of reflections.
PWV can be also measured locally, on shorter segments of a superficial artery, such as the carotid artery. Carotid PWV cannot be used as an interchangeable predictor with carotid-femoral PWV in high-risk patients, since discrepancies between aortic and carotid stiffness result from different influences of cardiovascular risk factors on both parameters . The carotid artery is of special interest because of its anatomic superficial location and the susceptibility to plaque formation, and, as one of the major arteries providing blood flow to the brain, may have pathophysiological relevance in the development of cerebrovascular disease. For some local arterial stiffness measures, such as distensibility coefficient (relative change in lumen cross-sectional area during systole for a given pressure change), local pulse pressure measurement is necessary; local pulse pressure may be assessed by applanation tonometry on the ipsilateral (immediately after the ultrasound assessment) or simultaneously on the controlateral carotid artery.
No ‘gold standard’ method is available at the present time for the evaluation of local PWV. In vivo, the measurement of local PWV can be obtained with high resolution ultrasound systems operating in multiple M-mode, as reported in the study by Hermeling et al.  published in the present issue of the Journal of Hypertension and in a recent previous paper ; the distance between the piezoelectric elements of an ultrasound probe corresponds to the distance between the measurements sites. The segment length set by the ultrasound probe is fixed and represents an advantage of this approach. Furthermore, using a wall track algorithm, distension waveforms are obtained from the radiofrequency data, with the additional advantage of avoiding the need of measuring pulse pressure locally.
The major limitation of this approach could be represented by the shorter distance between two recording sites that implies a mistake in measurement of the transit time. Hermeling et al.  have previously shown that local PWV can be measured on a phantom with good reproducibility, and have thereafter  assessed PWV in a carotid segment, positioning the ultrasound probe about 3 cm proximal to the carotid bifurcation in vivo, using the systolic foot as the time reference point for measurement of the transit time because is considered to be free of reflections.
However, as shown by Hermeling et al. , the interaction between the incident and reflected waves may impair the correct identification of the systolic foot. The reflected wave(s) origins from sites distal to the carotid bifurcation, in the cerebral circulation, runs backward and can be detected by the inflection point. Among all the 14 simultaneously recorded distension waveforms, the distance between the systolic foot and the inflection point becomes progressively shorter, from the proximal to the distal registration, suggesting that when the recording site is closer to the reflection sites, the inflection point becomes closer to the systolic foot, or may even merge with the systolic foot, affecting the accuracy of transit time measurement.
For this reason Hermeling et al., in their technical report, decided to propose the dicrotic notch (corresponding to the aortic valve closure) as an alternative reference point, comparing the two different time reference points, that is, the systolic foot and the dicrotic notch. Authors suggest that the dicrotic notch is superior to the systolic foot because of a lower intra-subject variability of the measurements obtained with the dicrotic notch as compared with the systolic foot. Authors have, however, excluded a large number of beats, because of poor quality of the waveform recordings (based on correlation coefficient lower than 0.5), and this might have influenced to a great extent the accuracy of the method. The problem of poor quality recordings could be, at least partially, solved by the contemporary view of the B-mode image of the carotid artery segment explored on the ultrasound machine screen, in order to have a real-time feedback about the correct alignment of the probe.
In addition, other evidence supporting the more accurate measure of local PWV at the dicrotic notch are the stronger relation with carotid distension and distensibility coefficient and the statistically significant difference between the two measures of local PWV obtained in younger and older individuals. In older individuals, when compared with younger individuals, the presence of more pronounced structural changes in small cerebral arteries , in addition to carotid atherosclerosis distal to the explored segment, could contribute to the number and extent of reflection sites in the carotid arterial tree, leading to a progressively shorter interval between the inflection point and the systolic foot, resulting in an underestimation of local arterial stiffness.
The influence of the ‘foot-to-foot’ method might have lower relevance for the carotid-femoral PWV measurement, since the distance between the feet and the transit time are much longer as compared with that of local carotid PWV and the inflection point of reflected waves is not so close to the systolic foot, even in the presence of increased stiffness.
A final additional issue of major clinical importance is represented by the possible influence of different measures of blood pressure on the proposed alternative reference point for PWV. The timing of the dicrotic notch corresponds to the aortic valve closure and the pressure at which the dicrotic notch occurs corresponds to mean arterial pressure, whereas the pressure corresponding to the systolic foot is the diastolic pressure. In patients without rhythm disturbances and valvular diseases, aortic dicrotic notch pressure was shown to be mainly related to the steady component of aortic pressure, that is mean blood pressure, at rest and during Valsalva maneuver, in a wide range of aortic mean blood pressure values . Other influences on the dicrotic notch timing, and pressure, could be exerted by more pronounced changes in cardiac contractility or valvular diseases; this point remains to be completely investigated and elucidated.
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