Precision, accuracy and added value of central pressure measurement

The interest in measurement of central pressure is a hot subject at the present time. Indeed, as the geometry of the arterial tree and its mechanical properties are altering the pressure generated by the left ventricle, this has important consequences for measurement of blood pressure. The conventional site of blood pressure measurement is the upper arm. Blood pressure measured at the brachial artery site does not represent accurately the pressure applied on organs suffering from increased blood pressure, such as the brain, the kidney and the left ventricle. Because of anatomical proximity (for left ventricle), and because the kidney and the brain have torrential blood flow exposing them directly to the pulsatility of central pressure, it is believed that pressure measured centrally, that is close to the root of the aorta may be more relevant in terms of associated risk. The main difficulty with central pressure is that it is not accessible by direct measurement outside the catheterization room but it is rather estimated from peripheral waveforms. Although there is compelling evidence for added predicted risk for aortic stiffness (measured by carotid to femoral pulse wave velocity) [1–3], evidence is scarcer for central pressure, likely because central pressure is estimated, not directly measured [4]. This is the principal interest of the article from Kips et al. in the present issue of the Journal of Hypertension, which carefully investigates the systematic differences between two popular devices estimating central pressure, the SphygmoCor system, first introduced in the market and making use of a transfer function to synthesize aortic pressure from radial tonometry, and the Omron HEM 9000 AI, which uses intelligent multisensor radial tonometry to extract central pressure from the late systolic peak (using a dedicated algorithm and an additional linear equation) [5]. The authors chose to compare these two devices to calibrate carotid waveforms obtained from carotid tonometry. This choice was a pragmatic one, as there is no noninvasive gold standard and as invasive measurements in the catheterization room are not practical and also not immune from big drifts. The advantage of direct carotid tonometry is that it makes no assumptions above calibration to brachial pressure (common to all techniques). Carotid tonometry has been validated against invasive measurements, showing a small systematic difference of 1.8 mmHg, which the authors correct for [6]. Omron and SphygmoCor devices are very well correlated with r-values above 0.99, and SD of difference of 4.3 mmHg, which suggests that both devices might be interchangeable if correctly calibrated. Calibration is where the problem stands in obtaining good estimates of central pressure, from what we already know to be reliable wave recordings. The authors strictly complied with the manufacturers’ recommendations. Doing that, the Omron device provided systematic overestimation of central SBP by 11.7 mmHg (5.5) and SphygmoCor underestimation by 7.1 mmHg (5.0). This is true for values of blood pressure up to about 175 mmHg, the difference between devices and reference being larger above this value. No significant drift is apparent on Bland–Altman plots. As values of central systolic blood pressures were virtually identical with both devices when analyzed by the same algorithm (the SphygmoCor software), differences are related to algorithm, not to the shape of the pressure wave. Agreement between devices is less clear for augmentation index and identification of late systolic peak. This study is essential because it asks the question of accuracy (ability of a measurement device to provide values as close as possible to the gold standard) and precision (the dispersion of values in repeated measurements). Although precision (also known as ‘repeatability’) was not directly addressed here (but the excellent agreement between the devices implies excellent precision), the authors demonstrated that precise devices may not be accurate. Precise devices are crucial for clinical research and intervention trials. It has been possible to discriminate antihypertensive treatments for their capacity to decrease central pressure despite similar effect on brachial pressure [7,8] by using SphygmoCor and to find important associations with pathological conditions using the Omron device [9]. For wider clinical applications, it is mandatory to have accurate devices. It is relatively easy to make these devices accurate by modifying the algorithms so that they provide absolute true values. As reference values for central pressure will soon be provided for different techniques using statistical normalization of blood pressure values, it is mandatory that manufacturers take responsibility and make the necessary adjustments to make their devices both precise and accurate for central blood pressure. Another element for choosing the best technique would be to have large-scale estimates of feasibility of measurement, together with repeatability estimates in real life. This point is not directly addressed here. There are still important differences for augmentation index between devices that need to be addressed if this parameter is to become popular. In any case, improvement should also come from better assessment of brachial pressure.