As the pulse wave travels from the central elastic arteries to the muscular conduit arteries, the amplitude of pressure wave (pulse pressure, PP) is gradually widened. This physiological phenomenon is called pressure amplification [1,2]. There is some evidence that central-to-peripheral blood pressure amplification might be useful for cardiovascular risk stratification [3,4], antihypertensive agent evaluation [5–7], and clinical outcome prediction [8–10]. Currently, the amplification is commonly expressed as the difference of SBP or the difference or ratio of PP between brachial artery and central aorta [1,3,11]. The lack of uniform expression of the formula makes interpretation and interstudy comparisons difficult.
The SphygmoCor (AtCor Medical, Sydney, Australia) and Omron HEM 9000AI (Omron Healthcare, Kyoto, Japan) devices are two of the most widely used devices to perform noninvasive assessment of central hemodynamics. Both devices can obtain radial pulse waveform by applanation tonometry with the calibration of cuff brachial blood pressure and then calculate central blood pressure with each inbuilt algorithm [12,13]. Several studies have validated and compared them in the estimation of central blood pressure against the noninvasive carotid blood pressure or invasive central aortic blood pressure [12,14–17]. However, there is none regarding blood pressure amplification for the two devices. The purpose of the present study is to simultaneously validate the assessment of central-to-brachial pressure amplification derived by the two devices.
The study protocol was approved by the Ethics Committee of Ruijin Hospital, Shanghai Jiaotong University School of Medicine (Shanghai, China). The written informed consent was obtained from all participants. In total, 44 consecutive patients scheduled to undergo elective coronary angiography were screened to enroll 33 eligible patients (age range, 40–85 years) in accordance with the number of participants required by the European Society of Hypertension International Protocol for the validation of blood pressure-measuring devices in adults . Exclusion criteria were as follows: failure to measure central blood pressure by the Omron device (n = 3); arrhythmia (n = 3); severe valvular heart disease (n = 3); heart failure defined as left ventricular ejection fraction less than 50% (n = 2); and significant difference (more than 5 mmHg) in SBP between the two arms (n = 0).
Hypertension was defined as a blood pressure of at least 140 mmHg systolic or 90 mmHg diastolic, or as the use of antihypertensive drugs for controlling blood pressure. Diabetes mellitus was defined as a plasma glucose level of at least 126 mg/dl fasting or 200 mg/dl at 2 h after glucose load, or as the use of antidiabetic drugs or insulin. Dyslipidemia was diagnosed according to the guideline of the National Cholesterol Education Program [Adult Treatment Panel (ATP) III] . Coronary artery disease was diagnosed visually if luminal diameter narrowing was equal to or greater than 50% on at least one of the major epicardial coronary arteries.
Noninvasive assessment of central and brachial blood pressures
Patients lay on the catheterization table with palm up and left forearm on the elbow placement board of the Omron device. Left radial pulse waves and left brachial blood pressures were automatically recorded and measured by the Omron device. Then, the pulse waves at the same sites were manually recorded by the SphygmoCor device. The same brachial blood pressure was used for the calibration of both radial pulse waves. At this sequence, brachial cuff blood pressure was measured three times, and the corresponding central blood pressures estimated with both devices were derived. These three noninvasive blood pressure readings were averaged for the computation of central-to-brachial blood pressure amplification indexes.
Invasive measurement of central and brachial blood pressures
The invasive catheter measurement was performed simultaneously with each of the two noninvasive devices. After a 6 F radial introducer sheath (Glidesheath; Terumo Medical Corporation, Tokyo, Japan), which was 16 cm long and with 2.08 mm of tip inner diameter, was inserted through the right radial artery with its tip at the level of right brachial artery, the brachial blood pressure waves were recorded. Corresponding three pairs of central aortic blood pressures were simultaneously measured at the aortic root with a 6 F diagnostic catheter (Infiniti Angiographic Catheter; Cordis Corporation, Miami, Florida, USA), which was 100 cm long, with 1.45 mm of inner diameter and 0.28 mm of wall thickness. Both the invasive brachial and central blood pressure waves for at least 10 stable beats were recorded with a fluid-filled manometer system (Mac-Lab Hemodynamic Recording System; GE Healthcare, Milwaukee, Wisconsin, USA) and off-line measured. The pressure transducer was zeroed to atmosphere prior to each examination and leveled at the mid-maxillary line till the end of the procedure. The frequency characteristic of the manometric system was confirmed by the flush test with the natural frequency at least 20 Hz and the damping coefficient at least 0.3.
Calculation of pressure amplification
For each patient, we obtained two pairs of averaged central and brachial blood pressures derived from noninvasive and invasive methods, respectively. However, central DBP was not available for the Omron device. According to the data of our invasive catheter measurements, neither mean arterial pressure (−0.5 mmHg, P = 0.71) nor DBP (−0.1 mmHg, P = 0.93) significantly changed from the central aorta to the brachial artery. We, therefore, used the brachial DBP measured oscillometrically as the central one of the Omron device with the assumption of constant DBP between central and peripheral arteries. The central-to-brachial amplification was expressed as the systolic pressure difference (SPD), which was quantified as the difference between brachial (bSBP) and central SBP (cSBP): SPD = bSBP−cSBP; the pulse pressure difference (PPD), which was quantified as the difference between brachial (bPP) and central pulse pressure (cPP): PPD = bPP−cPP; and the pulse pressure ratio (PPR), which was quantified as the ratio of brachial (bPP) and central pulse pressure (cPP): PPR = bPP/cPP.
Statistical analyses were performed using SPSS 19.0 for Windows (SPSS Inc., Chicago, Illinois, USA). The intraclass correlation analysis, the paired Student's t-test, and Bland–Altman plots were used to assess the agreement between the noninvasive and invasive central-to-brachial amplification. We performed single and multiple linear regression to study determinants of the difference in various pressure amplification indices between the noninvasive device and invasive catheter measurements.
Characteristics of the study participants
The characteristics of our study population are presented in Table 1. Of the 33 patients, 21 (63.6%) were men. Mean age (± SD) was 60.1 years (± 8.7), ranging from 45 to 83 years. Mean BMI was 25.5 (± 2.9) kg/m2. The number of current smokers was 12 (36.4%). The number (prevalence) of patients with hypertension, diabetes mellitus, dyslipidemia, and coronary artery disease was 23 (69.7%), eight (24.2%), 14 (42.4%), and 17 (51.5%), respectively. All patients took antihypertensive medication. Twenty (60.6%) took a statin.
Brachial SBP measured with the cuff Omron device was 18.7 mmHg lower than that with the invasive catheter (P < 0.001). Central SBP measured with the SphygmoCor and Omron devices was 14.8 and 2.4 mmHg lower, respectively, than with the invasive catheter (P < 0.001). However, DBP measured with the two noninvasive devices and the invasive catheter was virtually identical at the brachial artery as well as at the central aorta (P ≥ 0.33, Table 1).
Comparison between the noninvasive and invasive central-to-brachial pressure amplification
The central-to-brachial SPD and PP difference and ratio estimated by the Omron device were significantly (P < 0.001) smaller than those measured by the invasive catheter with an average difference of −16.3 mmHg [95% confidence interval (CI), −21.5 to −11.1 mmHg], −16.2 mmHg (95% CI −20.1 to −12.4 mmHg), and −0.26 (95% CI −0.31 to −0.20), respectively. No significant difference was observed in these parameters between the SphygmoCor and the invasive catheter measurements (P ≥ 0.07, Table 2).
The central-to-brachial SPD estimated by either SphygmoCor (r = 0.11; P = 0.56) or Omron (r = 0.15; P = 0.41) was not significantly correlated with that measured by the invasive catheter (Fig. 1). The central-to-brachial PP difference estimated by SphygmoCor (r = 0.38; P = 0.03) but not Omron (r = 0.23; P = 0.20) was significantly correlated with that measured by the invasive catheter (Fig. 2). However, the central-to-brachial PP ratio estimated by both SphygmoCor (r = 0.40; P = 0.02) and Omron (r = 0.53; P = 0.002) was significantly correlated with that measured by the invasive catheter (Fig. 3).
In further analyses, we studied the correlation between the difference and the average of the SPD and PP difference and ratio derived from each of the two noninvasive devices and the invasive catheter measurement. The corresponding correlation coefficients were −0.82 (P < 0.001), −0.69 (P < 0.001), and −0.08 (P = 0.67), respectively, for SphygmoCor and −0.48 (P = 0.005), −0.15 (P = 0.40), and 0.00 (P = 0.99), respectively, for Omron (Figs 1–3).
Determinants of the device-catheter difference in central-to-brachial pressure amplification indices
Because of the appreciable device-catheter difference in central-to-brachial pressure amplification indices, we investigated determinants of the difference. In univariate analyses, the device-catheter difference in central-to-brachial SBP difference and PP difference and ratio was significantly (P ≤ 0.002) and negatively correlated with the level of the invasively measured brachial SBP and PP, respectively, with a correlation coefficient of −0.52, −0.57, and −0.56, respectively, for SphygmoCor and −0.67, −0.72, and −0.42, respectively, for Omron (Fig. 4). However, in multiple linear regression analyses accounting for both the level of the invasively measured brachial SBP and PP and the cuff-catheter difference in brachial SBP and PP, the device-catheter difference in central-to-brachial pressure amplification indices was significantly correlated with the latter (P ≤ 0.04) but not the former (P ≥ 0.08), except for central-to-brachial PP ratio of Omron (P = 0.02, Table 3).
The potential implications of our findings are two-fold. First, if the invasive catheter measurement would be considered as standard, the two noninvasive devices have similar accuracy in the estimation of pressure amplification. However, because of the appreciable difference in absolute values between these two devices, device-specific criteria for diagnosis would be apparently required. Second, among the three pressure amplification indices tested for validity, namely SBP difference and PP difference and ratio, PP ratio is consistent across the invasive and noninvasive measurements. The inaccurate noninvasive measurement of pressure amplification by the two ‘pressure difference’ indices can be largely attributable to the underestimation of brachial SBP and PP by the cuff method.
Our study is the first that has simultaneously investigated the accuracy of two noninvasive devices, SphygmoCor and Omron, against the invasive catheter measurement in the estimation of pressure amplification from the central aorta to the brachial artery. The mean invasive central-to-brachial PP difference (17.9 mmHg) and ratio (1.31) of our study are comparable with the mean values of several previous studies that measured central hemodynamics invasively [14.5 mmHg (range, 12.0  to 16.3  mmHg) and 1.38 (range, 1.31  to 1.46 ), respectively] . Our finding on SphygmoCor is also in line with the results of several previous studies [3–7,13]. In these studies, the mean central-to-brachial pressure amplification ranged from 10  to 18  mmHg for SBP difference, 10  to 20  mmHg for PP difference, and 1.22  to 1.54  for PP ratio. Our finding on Omron is similar to the study of Richardson et al. , which reported that the central-to-brachial PP ratio estimated by Omron was 1.03 ± 0.17 in 33 British patients.
Our finding on the better consistency of PP ratio than the other two pressure amplification indices might be clinically relevant. In a cross-sectional study of 675 Australian patients, PP ratio (P < 0.001), but not the central-to-brachial SBP difference (P = 0.76), was significantly smaller in 453 patients with known or suspected coronary artery disease (n = 229) or diabetes mellitus (n = 224) than 222 healthy individuals (free from known cardiovascular disease with the exception of hypertension) . Similarly, in a large study of 10 613 British patients, PP ratio, but not SBP difference, significantly differed between 4030 patients with hypertension (n = 3420) or cardiovascular disease (n = 610) and 5648 healthy individuals . Nonetheless, the clinical relevance and significance of these three pressure amplification indices should be properly investigated in prospective observational and interventional studies on target organ damage and cardiovascular events.
Calibration of the radial pulse wave with the cuff-measured brachial blood pressure makes it possible to evaluate central hemodynamics noninvasively. However, the cuff method underestimates brachial SBP. As observed in the present study, this underestimation could be as large as 18.7 mmHg of brachial SBP and increased with higher levels of systolic pressure. This underestimation largely accounted for the inaccuracy in the evaluation of pressure amplification. Taking the ratio instead of the difference between the central and brachial pressures significantly improved the evaluation but did not abolish the errors induced by calibration with the cuff pressure.
Because of the awareness of the underestimation of brachial SBP by the cuff method, the inventor of the Omron device accounted for this underestimation in the inbuilt algorithm . In doing so, it diminishes not only the difference between the noninvasively and invasively measured central SBPs [14,17] but also the difference between central and brachial SBPs, that is, pressure amplification. The observation of the modest or no difference in SBP or PP between the central aorta and the brachial artery is, therefore, inherent with the inbuilt algorithm of the Omron device . In clinical practice, brachial blood pressure is usually only measured with the cuff. The commonly held view is that blood pressure amplifies from the central aorta to the brachial artery [1,22]. Thus, ‘nil’ pressure amplification would confuse the users of the Omron device, although pressure amplification indices derived by both devices similarly correlated with the invasive catheter measurements.
Our study has to be interpreted within the context of its strengths and limitations. A major strength of our study was that we simultaneously studied two noninvasive devices against the invasive catheter measurement and evaluated the accuracy of three different pressure amplification indices. However, the absence of untreated normotensive individuals served as a control group is a limitation of our study. In addition, both noninvasive devices assumed no pressure amplification from the brachial-to-radial artery  and calibrated the radial pulse waveform with the oscillometrically measured blood pressure in the brachial artery. This methodological issue is still highly debated [24,25]. Finally, the Omron device used in our study did not provide central DBP. Our calculation of pressure amplification indices for the Omron device had to rely on the assumption of equivalent DBP at the central aorta and the brachial artery.
In conclusion, if the invasive catheter measurement would be considered as standard, the two noninvasive devices have similar accuracy in the estimation of pressure amplification, but apparently require device-specific criteria for diagnosis. PP ratio seems to be a more consistent measure of central-to-brachial pressure amplification. One of the major implications of our study is that the scientific community has to act on the choice of central hemodynamic parameters. We should probably focus on the evaluation of central-to-brachial pressure amplification as expressed by PP ratio, and hence simplify the whole issue of central hemodynamics. In this case, the age of central hemodynamics might soon come.
The authors gratefully acknowledge the participation of all study participants, and the technical assistance of Miss Jie Wang.
The study was partially supported by a grant from Omron Healthcare (China), which is a branch of the manufacturer of the Omron HEM-9000AI device, Omron Healthcare, Kyoto, Japan.
Conflicts of interest
J-G.W reports receiving funding from the National Natural Science Foundation of China (grants 30871360 and 81170245) and the Ministry of Science and Technology (grant 2006BAI01A03 and a grant for China-European Union Collaborations 1012), Beijing, China, the Shanghai Commissions of Science and Technology (grant 07JC14047) and Education (grant 07ZZ32), the Shanghai Bureau of Health (XBR2011004), and the European Union (grants LSHM-CT-2006–037093 and HEALTH-F4–2007-201550) and consulting and lecture fees from GSK, Omron, Pfizer, Sankyo, Sanofi-Aventis, and Servier. Y. L. reports receiving funding from the National Natural Science Foundation of China (grant 30871081), and the Ministry of Education (NCET-09-0544), Beijing, China, the Shanghai Commissions of Science and Technology (11QH1402000) and Education (08SG20) and Shanghai Jiaotong University School of Medicine (a grant of distinguished Young Investigators). The other authors declare no conflicts of interest.
Reviewers’ Summary Evaluations Reviewer 1
This is a relatively unique three-way study comparing two devices for estimating central BP, both with each other and with directly measured central and brachial pressures. The presence of systematic differences between the two devices and between the individual devices and direct measurement is highlighted, implying that device-specific normal value seem to be required. A major strength of this paper is the authors’ prevision of adequate fundemental data to enable the reader to form their own assessment regarding the implications of the study on specific device use.
This study addresses the clinical application of noninvasive estimation of central aortic pressure from the peripheral pulse and the associated problems related to calibration. The study is of potential importance given the emerging interest in the development of device technologies for noninvasive estimation of central aortic pressure. However, the suggestion of device-specific comparisons presents limitations, indicating the need for standardization of methods to compare device algorithms as well as calibration procedures.
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Keywords:© 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins
applanation tonometry; catheterization; central aortic blood pressure; pressure amplification