Several clinical epidemiological studies have suggested the relevance of the assessment of central blood pressure by means of noninvasive methods for a better evaluation of cardiovascular risk [1–3]. In particular, attention has been focused on the phenomenon of pulse pressure amplification in the clinical evaluation of cardiovascular patients [4–6]. Applanation tonometry is currently considered the reference method for noninvasive evaluation of central blood pressure and central pulse wave analysis [7,8].
Even if transcutaneous arterial tonometry is not yet considered to be one of the diagnostic tests recommended for routine arterial hypertension management, all the same, it represents a useful tool for more in-depth investigation of vascular changes in a number of clinical conditions characterizing daily practice. In particular, it is useful in conditions in which an accurate central pulse wave analysis is required, for an indirect estimate of myocardial function or cardiac work. Therefore, it is necessary that central blood pressure values recorded with this method are reliable and that parameters describing central arterial pressure waveform faithfully reflect the corresponding ones invasively recorded in the ascending aorta. The study by Hirata et al. , published in this issue of the Journal of Hypertension, focuses on a particular aspect concerning the position of the patient during acquisition of the central arterial pressure waveform, which appears to be an important requirement to optimize standardization of applanation tonometry in clinical practice. As a matter of fact, some tonometric devices are used with the patient in sitting position, whereas other instruments require the patient to lie down in a supine position during arterial pressure waveform recording.
Hirata et al.  have shown that the data acquired when the patient is sitting or lying down are comparable and similarly reliable. These results are particularly significant, especially in those conditions in which patient's clinical conditions determine the position in which the test has to be performed. The unavoidable orthopnoea characterizing cardiac patients during acute heart failure and the debilitating musculoskeletal disorders of some elderly individuals are just examples of conditions in which a sitting or semi-sitting position of the patient is required.
In their study, Hirata et al.  measured central blood pressure by applanation tonometry, using the main direct and indirect methods at present recommended, with different calibration systems. While discussing their results on the effects of patients’ position, the authors claim that indirect methods based on transfer function analysis or on use of the second systolic peak of a peripheral pulse wave provide a more accurate assessment of central blood pressure than ‘direct’ methods based on carotid tonometry , a conclusion not supported by any of their findings, however. Indeed, the ‘direct method’ based on central blood pressure estimates from noninvasive carotid arterial recordings has been repeatedly shown to provide pressure wave forms comparable with those invasively obtained in the ascending aorta [10,11]. Chen et al.  comparing carotid tonometry versus the invasive method concluded that this technique may be a valuable tool not only for waveform analysis at the carotid artery level but also as a surrogate for central aortic waveform analysis. Nevertheless, absolute measurements of carotid arterial pressure with a tonometer are likely to be unreliable ; thus, a calibration of carotid pressure wave is always required. Actually, the main limitation of applanation tonometry is its inability to accurately provide absolute values of arterial pressure, that is arterial tonometry can define pulse pressure values, but it is unable to provide accurate values of diastolic (D) and systolic (S) blood pressure (BP). However, carotid pressure waveform may be effectively calibrated to brachial blood pressure measured by a traditional, validated sphygmomanometer. The calibration method using brachial mean BP and DBP has been shown to yield accurate central pressure estimates, as it is not influenced by the brachial-to-radial pressure amplification phenomenon . This algorithm of calibration starts from the concept that mean arterial pressure remains constant from aorta to peripheral arteries, as does DBP (which weakly tends to decrease from the centre to the periphery). Thanks to this type of calibration, direct tonometry at the carotid artery level may indeed be considered an easy and reproducible approach to estimate central BP . Moreover, calibration of carotid pulse wave guarantees that BP values derived from arterial tonometry are free from the possible errors coming from a number of factors that may affect pulse wave recording, such as the pressure the operator may exert with the probe on the sensor when the pressure signal is recorded, and anatomical features such as the variable depth of the artery. Likewise, the beat-to-beat analysis of pulse waves avoids all errors related to instability in the signal offset secondary to respiratory movements of the patient. Certainly, a short period of training is necessary to learn this technique, but common carotid artery is generally superficial and well accessible and can easily be flattened against the neck anatomical structures located at its back (Fig. 1). Moreover, carotid tonometry is a test that is well tolerated by the patient, and the theoretical risk of stimulating carotid baroreceptors through neck compression is very small (in our experience, we have had only two cases of asymptomatic bradycardia in very old patients, over 4000 investigated individuals). An invasive study performed by Van Bortel et al.  found systolic central BP estimated by carotid tonometry to be only 1.8 mmHg higher than central invasive aortic pressure. All these considerations thus support the conclusion that central BP estimates based on carotid tonometry may be used as a reference method , at variance from what claimed by Hirata et al. .
With the ‘Indirect Method’, tonometry is performed in the radial artery. Central pressure waveforms and ascending aorta pressure values are defined, starting from the radial pulse wave, by means of a transfer function or taking into account the late systolic shoulder of the radial pressure waveform. The radial pulse waveform is calibrated using SBP and DBP values, measured by a traditional sphygmomanometer on the brachial artery. The main technical issues regarding this indirect method are related to the relative difficulty in recording and analysing radial arterial pressure waves and to the calibration of pressure signal . The demonstration by several studies of a significant difference between BP values at radial and brachial artery levels has in fact questioned the reliability of radial pulse wave calibration from brachial SBP and DBP values [12,14].
Theoretically, the most reliable method to derive calibrated central pressure values from analysis of radial or carotid waveforms should be based on brachial pulse waveform recordings, calibrated by SBP and DBP measured through a validated sphygmomanometer at the same brachial artery level. The mean arterial pressure so calculated from the integral of the brachial pressure waveform can then be used, together with the corresponding DBP, to calibrate pulse waveforms recorded elsewhere.
Surprisingly, Hirata et al. , following recent claims by Adji and O’Rourke , do not support this hypothesis of waveform calibration and suggest that use of applanation tonometry cannot be performed accurately at the brachial site because the brachial artery lies beneath the stiff bicipital aponeurosis, it is not supported by bone and thus cannot be reliably flattened under the sensor. Actually, with a standard procedure, the inter and intraobserver reproducibility of pulse waveforms recorded at brachial site is weak. However, the reliability of brachial pulse wave recordings may significantly increase by placing index and middle fingers against the lateral wall of the brachial artery aimed at preventing its displacement by the probe and at allowing its adequate recording of pulse waveforms (Fig. 2).
In conclusion, the study proposed by Hirata, by allowing for different individuals’ position during test, may contribute to a larger diffusion of arterial tonometry as a method of clinical investigation, by emphasizing the importance of methodological aspects [4,16]. On the contrary, the authors’ claims on what the best approach to pulse waveform calibration might be appear to require further investigation by studies carefully considering a number of critical aspects, including not only difficulties in waveform calibration but also the reliability of transfer functions, when considering arterial tonometry implementation in clinical practice.
Conflicts of interest
P.S. is consultant for DiaTecne s.r.l., manufacturer of a pulse wave analysis system.
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