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. 2014 Sep 9;16(10):707–712. doi: 10.1111/jch.12396

A New Tonometric Device for Radial Augmentation Index and Subendocardial Viability Ratio: Potential Use in Health Screening

Jing‐Zhi Wang 1,2, Yong‐Liang Zhang 1,✉, Fu‐Song Hu 1,2, Zi‐Jun He 1,2, Xian‐Jun Yang 1, Zu‐Chang Ma 1, Yi‐Ning Sun 1
PMCID: PMC8031812  PMID: 25203355

Abstract

Augmentation index (AIx) and subendocardial viability ratio (SEVR) are widely accepted indices of wave reflection and myocardial oxygen demand relative to supply. This study aimed to validate a new tonometric device (IIM‐2010A) for obtaining AIx and SEVR from radial artery. A total of 68 outpatients (32 men and 36 women) aged 20 to 76 years (44.7±16.6 years) recruited from a health screening center participated in the study. AIx was obtained from radial pressure using the HEM‐9000AI and IIM‐2010A devices, while SEVR was measured from carotid pressure with the tonometric method and from radial pressure by the IIM‐2010A device. In a subgroup of 24 patients, the measurements of AIx and SEVR were repeated after an interval of 10 minutes. The correlation of radial AIx between the IIM‐2010A and HEM‐9000AI devices was highly significant (r=0.956, P<.01). Radial SEVR determined from IIM‐2010A was also highly related to carotid SEVR (r=0.864, P<.01), although the value was about 13.1% lower. There was no statistically significant difference between the repeated measurements of both indices. The lower coefficient of variation (2.9% vs 4.3% for AIx, 3.3% vs 4.1% for SEVR) and higher intraclass correlation coefficient (0.96 vs 0.91 for AIx, 0.93 vs 0.86 for SEVR) of IIM‐2010A confirmed better short‐term reproducibility, compared with the HEM‐9000AI device and carotid tonometry. The new tonometric device IIM‐2010A is effective and reproducible in calculating radial AIx and SEVR and has potential use in health screening.

Scientific interest has focused increasingly on vascular‐ventricular interaction as a result of its preeminent cardiovascular significance.1, 2, 3 Increased artery stiffness with early return of wave reflection causes increased left ventricular afterload, leading to an imbalance between myocardial oxygen demand and supply. Augmentation index (AIx) derived from central artery with invasive aortic catheter or noninvasive carotid tonometry is an index of wave reflection.4, 5 Since neither method is easy to operate, which is necessary for health screening, AIx is calculated from aortic pulse waveforms by means of a validated transfer function with the SphygmoCor device (AtCor Medical, Sydney, New South Wales, Australia).6 Furthermore, AIx has been suggested to be obtained directly from the radial pulse waveform for simplifying the measurement procedure,7 which could provide equivalent information as aortic AIx.8, 9 Several studies have shown that radial AIx was independently predictive of adverse cardiac events.10, 11 The broadly accepted device for calculating radial AIx is the HEM‐9000AI (Omron Healthcare Co Ltd, Kyoto, Japan), which is easy to operate.12, 13

Subendocardial viability ratio (SEVR) derived from aortic pressure is an index of myocardial perfusion relative to cardiac workload.2, 14, 15, 16 SEVR is independently associated with cardiovascular events16, 17 and is suggested for the early detection of individual cardiovascular risk.18, 19 A recent study has shown that SEVR obtained from radial pressure may provide equivalent information as aortic SEVR.20 In that study, aortic SEVR was estimated from transformed aortic pressure using the SphygmoCor device rather than from direct measurement, such as carotid tonometry.

Currently, a new tonometric device is now used in the screening for cardiovascular risk in China, with radial AIx and SEVR as two main indices. This study aimed to validate the IIM‐2010A device (Institute of Intelligent Machines, Hefei, China) by comparing radial AIx with that obtained by the HEM‐9000AI device and radial SEVR with SEVR obtained by carotid tonometry. Additionally, short‐term reproducibility of the two indices obtained from the IIM‐2010A device was investigated.

Methods

Study Patients

The study population consisted of 71 outpatients (33 men and 38 women) from a health screening center in Anhui Provincial Hospital. Clinical data including medical history, smoking status, and medication use were obtained from patient interview. Three patients with insufficient quality of the recorded pulse waveforms were excluded. Finally, a total of 68 individuals (32 men and 36 women) were taken into the study, including 13 smokers, 7 patients with hypertension, 4 patients with diabetes, 2 patients with ischemic heart disease, and 1 patient with cerebrovascular disease, according to clinical data. A declaration of consent was signed by all individuals. This study was reviewed and approved by the local institutional review board.

Measurements

Measurements were made in a temperature‐controlled (22±2°C) room between 8:30 am and 11:00 am. All measurements were obtained after the patient had fasted for 12 hours. The patients were advised to refrain from alcohol for 24 hours and from tea, coffee, and smoking for 8 hours before examination. Each patient rested for at least 10 minutes. Records of radial pulse waveforms with the HEM‐9000AI and IIM‐2010A devices were conducted in the sitting position, while records of carotid pulse waveforms with a tonometric method were in obtained in the supine position for obtaining stable signals. All measurements were performed by a trained operator and repeated for 24 of the 68 patients after an interval of 10 minutes.

Measurement of Brachial Blood Pressure

Systolic blood pressure (SBP) and diastolic blood pressure (DBP) were measured in the left arm with an oscillometric method using a HEM 7012 device (Omron Healthcare Co Ltd). The mean of two recordings was used in the study unless the values differed by more than 5 mm Hg when a third measurement was made, and the mean of the two closest values was used. Mean arterial pressure (MAP) was calculated as (SBP‐DBP)/3+DBP.

Measurement of Carotid Pressure Waveform

Carotid pulse waveforms were measured using applanation tonometry. A high‐fidelity handheld probe (Millar Instruments, Houston, TX) was applied to the skin overlying the left carotid artery, with which a good‐quality pulse waveform could be obtained and then calibrated according to the measured DBP and MAP. Carotid pressure upstroke and incisura were detected using the second derivative. Systolic pressure‐time integral (SPTI) is directly related to myocardial oxygen demand and estimated as the area under the systolic portion of the carotid pressure curve, while diastolic pressure‐time integral (DPTI) is related to myocardial oxygen supply and estimated as the area under the diastolic portion of the carotid pressure curve. Carotid SEVR was calculated as the ratio of DPTI to SPTI (Figure 1a).16

Figure 1.

Figure 1

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Calculation of augmentation index (AIx) and subendocardial viability ratio (SEVR) from (a) carotid and (b) radial pressure waveform: AIx=(SBP2‐DBP)/(SBP‐DBP); SEVR=DPTI/SPTI; SBP, systolic blood pressure; SBP2, blood pressure at the shoulder point; DBP, diastolic blood pressure; SPTI, systolic pressure‐time integral; DPTI, diastolic pressure‐time interval.

Measurement of Radial Pressure Waveform

Applanation tonometry was performed on the left radial artery using HEM‐9000AI (Omron Healthcare Co Ltd) and IIM‐2010A (Institute of Intelligent Machines, Hefei, China) devices successively. Radial pulse waveforms were automatically modulated in order to obtain an optimal waveform, which was then calibrated using the measured brachial SBP and DBP. Pressure upstroke, peak, shoulder point, and incisura of the averaged waveform were detected using the algorithms based on the multidimensional derivatives. The main differences between the two devices were as follows: First, an array of 40 micropiezoresistive transducers was incorporated in a probe in the HEM‐9000AI device, whereas in the IIM‐2010A device, a high‐fidelity applanation tonometer was fixed in a watch (Figure 2). Second, the fourth derivative of radial pressure was obtained with numerical differentiation to extract the shoulder point for radial AIx in the HEM‐9000AI, whereas in the IIM‐2010A, it was calculated with an algorithm based on the B‐spline wavelet, which has been described in a recent study.21 Moreover, radial SEVR can be calculated as DPTI/SPTI from radial pressure by the IIM‐2010A, which is not measured by the HEM‐9000AI (Figure 1b).7, 20

Figure 2.

Figure 2

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Measurements of the radial pulse wave were performed by (a) HEM‐9000AI and (b) IIM‐2010A.

Statistical Analysis

All data are given as mean±standard deviation. The correlation coefficient was defined as r according to Spearman correlation test. Differences between the measurements were tested using paired t tests and Bland‐Altman plots. The short‐term reproducibility of the three devices was likewise calculated according to r, as well as coefficient of variation (CV), intraclass correlation coefficients (ICCs), and Bland‐Altman plots. In a Bland‐Altman plot, the differences between two measurements per patient are plotted against the means of two measurements per patient. The 95% confidence interval (CI) of the mean difference should include zero to exclude systematic differences. The limits of agreement (LOA) indicate the range between successive measurements in a patient without real change. Statistical significance was set a priori at P<.05.

Results

The physiological characteristics of the outpatients are listed in Table 1. The mean age of the study population was 45 years (aged 20 to 76 years).

Table 1.

Physiological Characteristics of the Study Population (N=68)

Variables Mean ± SD Range
Age, y 44.7 ± 16.3 20–76
Height, cm 165.6 ± 8.2 150–183
Weight, kg 61.3 ± 9.2 40–78
BMI, kg/m2 22.4 ± 2.9 16–29
HR, beats per min 67.8 ± 8.2 49–92
SBP, mm Hg 121.9 ± 12.7 98–157
DBP, mm Hg 77.8 ± 7.5 59–94
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Abbreviations: BMI, body mass index; DBP, diastolic blood pressure; HR, heart rate; SBP, systolic blood pressure; SD, standard deviation.

Comparison of AIx and SEVR Measurements

The measurements of AIx and SEVR are presented in Table 2. Radial AIx and SEVR measured by the IIM‐2010A device were lower than radial AIx measured by the HEM‐9000AI device and carotid SEVR by tonometric method, respectively. As shown in Figure 3a, AIx obtained using the IIM‐2010A and that using the HEM‐9000AI reveal a highly significant correlation (r=0.956, P<.01). The mean difference between the two measurements was −1.3% (95% CI, −2.3% to −0.3%) and the LOA was −9.5% to 6.9% (Figure 3b). Radial SEVR obtained by the IIM‐2010A was also highly correlated with carotid SEVR (r=0.864, P<.01; Figure 3c). The mean difference was −13.1% (95% CI, −15.8% to −10.4%).

Table 2.

AI and SEVR of the Study Population (N=68)

Variables Carotid Tonometry HEM‐9000AI IIM‐2010A
AI, % 75.7 ± 13.9a 74.4 ± 14.1
SEVR, % 136.8 ± 19.4a 123.7 ± 21.7
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Abbreviations: AI, augmentation index; SEVR, subendocardial viability ratio. aCompared with the measurements by IIM‐2010A (P<.05).

Figure 3.

Figure 3

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Comparison of (a) and (b) the measurements of radial augmentation index (AIx) estimated by HEM‐9000AI and IIM‐2010A, and (c) the measurements of carotid subendocardial viability ratio (SEVR) estimated by carotid tonometry and radial SEVR by IIM‐2010A.

Short‐Term Reproducibility of the Measurements of AI and SEVR

Characteristics indicating the variability and reproducibility of AIx and SEVR are presented in Table 3. The paired t tests showed no statistically significant difference between the repeated measurements made by the three devices. As shown in Figure 4, the mean differences were 0.2% (95% CI, −2.1% to 2.5%), 0.5% (95% CI, −1% to 2%), 0.5% (95% CI, −3.9% to 4.9%), and 0.1% (95% CI, −3.5% to 3.6%) for the HEM‐9000AI AIx, IIM‐2010A AIx, carotid SEVR, and radial SEVR, respectively. This meant that neither of these differences was significantly different from zero. The LOAs were −10.8% to 11.2%, −6.7% to 7.7%, −20.5% to 21.5%, and −16.7% to 16.9%, respectively.

Table 3.

Short‐Term Reproducibility of Measurements of AIx and SEVR

Variables r ICC CV, %
HEM‐9000AI AIx 0.906 0.91 4.3
IIM‐2010A AIx 0.963 0.96 2.9
Carotid SEVR 0.860 0.86 4.1
Radial SEVR 0.934 0.93 3.3
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Abbreviations: AIx, augmentation index; CV, coefficient of variation; ICC, intraclass correlation coefficient; SEVR, subendocardial viability ratio.

Figure 4.

Figure 4

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Bland‐Altman plots of (a) and (b) augmentation index (AIx) and (c) and (d) subendocardial viability ratio (SEVR) for short‐term reproducibility using HEM‐9000AI, carotid tonometry, and IIM‐2010A. Dashed lines are ±2 standard deviations of the mean difference.

Discussion

This is the first study that validated the new tonometric device IIM‐2010A for radial AIx and SEVR, which is used for health screening in China. Radial AIx obtained from the HEM‐9000AI device has been validated against central AIx obtained not only from a transformed aortic pulse waveform with the SphygmoCor device but also from carotid tonometry.8, 22 Its clinical prognostic significance has been reported in several studies.11, 23, 24, 25 In a recent study, determination of radial SEVR from the IIM‐2010A was validated by comparing it with aortic SEVR obtained from the SphygmoCor device rather than direct data.20 Therefore, we investigated the validation of IIM‐2010A for radial AIx and SEVR with HEM‐9000AI and carotid tonometry as reference devices.

In the present study, the correlation of radial AIx determined by IIM‐2010A and HEM‐9000AI devices was highly significant. It was mainly the result of the same calculation method and similar algorithm detecting feature points for AIx. The mean difference of the two devices was about 1.3%. It may be because of the sequence of the measurements, using HEM‐9000AI ahead of IIM‐2010A. However, it was quite small and with little clinical significance. The variation of the difference (standard deviation=4.1%) was also very small. Meanwhile, we found a less powerful (although highly significant) correlation between radial SEVR measured by the IIM‐2010A and carotid SEVR. An approximately 13.1% lower value of radial SEVR in the study was consistent with a recent report.20 Therefore, for radial AIx and SEVR, the IIM‐2010A device proved to be accurate in Chinese patients.

All three devices had good short‐term reproducibility. The mean differences (95%CI) of all the repeated measurements obtained with these devices included zero. The LOA of the HEM‐9000AI and IIM‐2010A devices for radial AIx exceeded 6%, while the LOA of carotid and radial SEVR exceeded 16%. Although there was no consensus about the size of a minimally clinically relevant difference, these LOAs were larger than what we would consider clinically relevant. Compared with the variance and reproducibility of AIx measurements, lower CV, higher ICC, and less narrowed LOA, there seemed to be less variation with the IIM‐2010A. One possible explanation was the technical difference in detecting the shoulder point of radial pulse waveform, which was the key point for calculating AIx. As described in our recent study,21 with the IIM‐2010A, the fourth derivative of radial pulse was calculated by the new algorithm based on the B‐spline wavelet, with better accuracy and anti‐noise capability, while with the HEM‐9000AI, numerical differentiation was used, which might amplify noise, especially at high frequencies.26 Less variation has also been shown in radial SEVR measured by the IIM‐2010A compared with carotid SEVR. The reason might be that the radial pulse waveform was easier to measure than the carotid pulse waveform.

In regard to the function of these devices, both the IIM‐2010A and HEM‐9000AI devices are used for health screening, such as in community hospitals. However, the IIM‐2010A can determine radial AIx and SEVR simultaneously, which the HEM‐9000AI cannot. Moreover, the simple method for tonometer fixation with the IIM‐2010A is more suitable for health screening. Although the SphygmoCor device can also measure both parameters with the use of a transfer function, the probe is handheld, which requires a more trained clinician for a good‐quality pulse waveform. In a recent study, the Goan was validated to derive aortic AIx and SEVR from a transformed aortic pressure.6 However, it has also been shown that the individual transfer function of different devices (SphygmoCor vs Goan) could have affected the absolute value of these indices.

Study Limitations

Possible limitations in the methodology of this study should be emphasized. First, the study population was relatively small and young, with 45 years as the mean age. With aging, the shoulder point of the pulse waveform might be obvious and differently extracted by the HEM‐9000AI and IIM‐2010A devices,27 leading to lower correlation. However, good agreement between them showed that the IIM‐2010A provided a simple approach for calculation of AIx. Second, the study population was mainly patients without cardiovascular disease or drug treatment, although with some cardiovascular risk factors. Pharmacologic response might influence the accuracy and reproducibility of measurements.28 For further investigations, patients in various clinical settings should be included for its wide implication. Additionally, because vascular‐ventricular interaction is an important area for future evaluation, and the influence of age and sex on it is complicated,29, 30 we will further investigate the postmenopausal effect on radial SEVR in the elderly and the relationship between it and arterial stiffness indices.

Conclusions

There were highly significant correlations between radial AIx obtained by the IIM‐2010A and by the HEM‐9000AI, as well as between radial SEVR and SEVR obtained by carotid tonometry. In addition, it had good short‐term reproducibility. The new tonometric device IIM‐2010A is effective and reproducible in calculating radial AIx and SEVR, and has potential use in health screening. The next step of validation of the IIM‐2010A could be the generation of clinical evidence that IIM‐2010A does similarly or better in terms of prognostic value than the reference devices.

Acknowledgments and disclosures

This work was supported by grant 61301059 from the National Natural Science Foundation of China and 2013BAH14F01 from the National Science and Technology Pillar Program.

J Clin Hypertens (Greenwich). 2014;16:707–712. DOI: 10.1111/jch.12396. © 2014 Wiley Periodicals, Inc.

Jing‐Zhi Wang and Yong‐Liang Zhang contributed equally to this work.

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