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Table of Contents
ORIGINAL ARTICLE
Year : 2018  |  Volume : 1  |  Issue : 2  |  Page : 50-55

Aortic pulse wave velocity and augmentation index@75 measured by oscillometric pulse wave analysis in Gujarati nonhypertensives


Department of Physiology, Government Medical College, Bhavnagar, Gujarat, India

Date of Web Publication26-Sep-2018

Correspondence Address:
Jayesh Dalpatbhai Solanki
F1, Shivganga Appartments, Plot No 164, Bhayani Ni Waadi, Opp. Bawaliya Hanuman Temple, Gadhechi Wadlaa Road, Bhavnagar - 364 001, Gujarat
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/VIT.VIT_10_18

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  Abstract 

BACKGROUND: Augmentation indexes (AIxs) and pulse wave velocity (PWV) are arterial stiffness parameters. They can be studied by pulse wave analysis (PWA) noninvasively. Before use, AIx and PWV need normative baseline study to find the predictors.
MATERIALS AND METHODS: We conducted a cross-sectional study in 801 nonhypertensives, aged 15–65 (divided into five subgroups) years. PWA was accomplished by Mobil-o-Graph (IEM, Germany) based on oscillometric principle and cardiovascular parameters reported were further analyzed. Value of P < 0.05 was taken as statistical significance.
RESULTS: There were five age-based subgroups from 15 to 65 years, showing an increase in AIx@75 and PWV with age. Females had significantly higher AIx in male in age groups (31.4 vs. 25.8, 34.95 vs. 27.21, 32.62 vs. 27.61, 34.32 vs. 26.36, 37.09 vs. 29.63; P < 0.05 for all), but PWV was higher in males than females in young age group (15–24 years-4.94 vs. 4.71, 25–34 years-5.28 vs. 5.12, 35–44 years-6.00 vs. 5.84; P < 0.05 in all) and in females than males in older age group (45–54 years-6.88 vs. 6.78,55–65 years-8.13 vs. 8.03). Major positive significant predictors of PWV were age, Body Mass Index, and systolic blood pressure; and of AIx@75 were age, height (negative), heart rate, and pulse pressure. AIx@75 and PWV showed a positive correlation with each other across all age group in either sex except for 55–65 years and 25–34 years age groups.
CONCLUSIONS: Oscillometric measurement of PWV and AIx@75 is feasible in our population. They are dependent on each other and age, totally not dependent on blood pressure and have difference in predictors. This baseline data can be used as a reference for future studies.

Keywords: Augmentation index, nonhypertensive, oscillometric, pulse wave analysis, pulse wave velocity


How to cite this article:
Solanki JD, Mehta HB, Shah CJ. Aortic pulse wave velocity and augmentation index@75 measured by oscillometric pulse wave analysis in Gujarati nonhypertensives. Vasc Invest Ther 2018;1:50-5

How to cite this URL:
Solanki JD, Mehta HB, Shah CJ. Aortic pulse wave velocity and augmentation index@75 measured by oscillometric pulse wave analysis in Gujarati nonhypertensives. Vasc Invest Ther [serial online] 2018 [cited 2018 Oct 15];1:50-5. Available from: http://www.vitonline.org/text.asp?2018/1/2/50/242259


  Introduction Top


Arterial stiffness is a discrete and direct measure of vascular aging,[1] useful as a novel and additive cardiovascular risk factor.[2] Aortic pulse wave velocity (aPWV) and augmentation index at heart rate (HR) 75 (AIx@75) are markers of arterial stiffness.[3] Cuff-based oscillometric pulse wave analysis (PWA) based devices such as Mobil-o-graph has made its noninvasive assessment feasible.[4] aPWV and AIx@75 have predictive values for cardiovascular morbidity and mortality and gaining popularity to seek for beyond brachial blood pressure (bBP) cardiovascular testing.[5] Age, anthropometric measures, ethnicity, and blood pressure are confounders affecting PWV and AIx.[1],[6],[7],[8],[9],[10],[11],[12],[13] Hence, before using these validated and potential tools, normative studies are needed. We aimed to establish reference values of the same in our population.


  Materials and Methods Top


Study set up and study subjects

We conducted a cross-sectional study at clinical research laboratory Physiology department of a government medical college attached to tertiary care teaching government hospital from June 18, 2015 to February 3, 2018. The study protocol was first approved by the Institutional Review Board. The sample size was calculated by Raosoft software (Raosoft, Inc., free online software, Seattle, WA, USA). To have 95% confidence level, 5% precision, considering response distribution 50%, a sample size of 801 was adequate for population of the city 6 lakhs in our region. We enrolled, using convenience sampling method, from our institute and community total, 1257 apparently healthy controls with a known history of the hypertension (HTN) and type 2 diabetes. After scrutiny, we finally had 801 individuals considered for the study.

Inclusion and exclusion criteria

We included apparently healthy nonathletic individuals, aged 15–65 years, of either sex, nonsmoking, nonalcoholic, not known for any acute or chronic disease, not taking any medical treatment ready to give written consent. We excluded individuals aged more than 65 years or <15 years, with any acute or chronic cardiovascular diseases, denying written consent, having any disease or drug history, current or ex-smokers or tobacco chewers, trained athletes, individuals using of any alternative system of medicines/lifestyle managements like Yoga and meditation. There were 83 individuals screened to have newly diagnosed HTN who were referred for further diagnosis. We excluded three individuals from study after pulse wave recording due to irregular pulse rhythm. After recording, 14 readings were discarded due to the poor quality of pulse wave recordings. Seven individuals were excluded with arm circumference beyond available cuff size. So, the final sample size was 801.

Subject assessment and definitions

All individuals were interviewed personally in the form of questionnaires including general features, demographic characteristics, and relevant history. Systolic blood pressure (SBP) ≥140 mm of Hg and diastolic blood pressure (DBP) ≥90 mm of Hg or use of anti-hypertensive medication was defined as HTN.

Instrument used–Mobil-o-graph

We used portable, PC attached calibrated and validated [4] instrument Mobil-o-Graph (IEM Gmbh, Stolberg, Germany) owned by Physiology department to record brachial pulse wave. It contained three different sized arm cuffs, connecting tube, recorder, Bluetooth, licensed software, and laptop. It performs PWA based on Oscillometric principle and analysis of pressure pulse wave. Initially, mid-arm circumference of the left arm is measured to choose the BP cuff of appropriate size-small (20–24 cm), medium (24–32 cm) or large (32–38 cm). It is wrapped around left arm and tubing is connected to the recorder device as per standard protocol. As per ARC Solver algorithm, a recording device generates pressure in the cuff by self-inflation and deflation follows it in stepwise manner. If first reading is free of artifact and error, there is a pause of 30 s to follow, after which there is second inflation-deflation. During deflation, the cuff is kept inflated at brachial diastolic pressure for 10 s which allows intermittent flow that produces pressure pulse waves. Brachial arterial pulsation generates the pressure oscillations which are transmitted to blood pressure cuff tied around left arm and measured by the transducer to be fed into the microprocessor. Computerized software records pulse wave of the brachial artery and by validated a generalized transfer factor derives central aortic pulse wave. It further undergoes point-based and area-based analysis by computer to derive various cardiovascular parameters.

Measurement protocol

A blood pressure cuff of appropriate size was chosen based on measured mid-arm circumference and applied to the left arm using standard protocol. All readings were taken after 10 min of rest, in postabsorptive phase with individuals avoiding smoking or alcohol for 12 h before the test, in a calm room avoiding external influences or arm movement. Measurements were taken twice in each individual. Due to objective, algorithm based, validated measurement protocol, there is good inter-or intra-observer reproducibility.[14]

Parameters measured

  1. HR, body mass index (BMI), body surface area
  2. bBP-bSBP, bDBP, brachial PP and brachial mean blood pressure
  3. Arterial stiffness-AIx at HR 75/min (AIx@75), aPWV.


AIx@75 – It is derived from augmentation pressure (AP) and PP of a pulse wave. PP is the difference between systolic and diastolic blood pressure. Pulse wave is a summation of forward wave (producing first systolic peak) and the reflected wave (producing second peak). Increase in amplitude of pulse wave due to reflection of pulse wave is known as pulse augmentation and its contribution to the pulse wave amplitude is known as AP. Moreover, percentage of pulse wave amplitude due to AP is known as AIx. AIx = AP/PP × 100. AIx is dependent on HR so it is corrected for the same and Mobil-o-graph gives it at HR 75, final parameter being AIx@75. AIx@75 is a measure of peripheral arterial stiffness. Stiffer the peripheral arteries, early and augmented will be reflection from periphery. It increases AP and AIx which is a measure of extra afterload that inflicts ventricle.

aPWV – It is the speed at which pulse wave travels in aortic wall. IT is calculated by point based analysis of aortic pulse wave derived from recorded brachial pulse waves. It gives a measure of central arterial stiffness. Most other PWA methods uses regional arterial stiffness like brachial-ankle PWV, but Mobil-o-graph gives aortic stiffness which is the most direct parameters affecting ventricular functioning. With vascular aging (progeria), there is reduced aortic compliance that is measured as increased stiffness giving raised PWV.

Statistical analysis

The data were entered to Excel spread sheet and descriptive analysis was expressed as mean ± standard deviation until specifically indicated. All calculations were done by GraphPad InStat 3 software (demo version free software of GraphPad Software, Inc. California, USA) and MedCalc Statistical Software version 16.4.3 (MedCalc Software bvba, Ostend, Belgium; https://www.medcalc.org; 2016). We calculated the statistical significance of differences of various numerical parameters between various groups by Mann–Whitney test or unpaired Student t-test for two groups and by ANOVA test for more than two groups. Spearman's correlation test was used for correlation between parameters-parametric or nonparametric. Multiple linear regression was used to find major and significant predictors of main study outcomes-PWV and AIx@75. Statistical significance level was accepted at P < 0.05.


  Results Top


We studied PWV and AIx@75 of 801 nonhypertensive individuals (470 males and 331 females) in subgroups based on gender and age-15–24, 25–34, 35–44, 45–54, and 55–65. Parameters were showing increasing trends with age more in PWV than AIx, more in females than males for AIx@75 and in both genders for PWV. Across all age group, females had significantly raised AIx@75 compared to males. However, for PWV, males had higher values than female in young subgroups only and this trend was reversed in older subgroups [Table 1].
Table 1: Age- and gender-wise distribution of aortic pulse wave velocity and augmentation index at heart rate 75 (mean and standard deviation) in study group (n=801)

Click here to view


By multiple linear regression models, we tested predictors of PWV and AIx@75 (dependent parameters) from independent parameters. Age, BMI, and SBP were the major positive predictors of PWV. Age, HR, and PP were positive and height was negative predictors of AIx@75 [Table 2].
Table 2: Predictors of aortic pulse wave velocity and augmentation index at heart rate 75 by multiple linear regressions in study group

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We studied correlation between AIx@75 and PWV in whole group and in age- and gender-based subgroups. There was significant and positive correlation between PWV and AIx@75 in general except age group 25–34 and 55–65 years [Table 3].
Table 3: Correlation between aortic pulse wave velocity and augmentation index at heart rate 75in age- and gender-wise study subgroup

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  Discussion Top


Cardiovascular health is the determinant of overall health and well being.[1] Arterial stiffness is a parameter determining vascular health and a predictor of cardiovascular disease.[1] It fulfils the criteria of a biomarker, being a measure of vascular progeria-early vascular aging.[15] aPWV and AIx at HR 75 (AIx@75) are two such variables which are direct measures giving inference beyond routine bBP.[16] Before using these validated, novel tools for epidemiological studies, it is important to set normative data. We did the same using Mobil-o-graph working on oscillometric principle in our population excluding the presence of cardiovascular disease, smoking, and diabetes.[1] No data were available of normal values of PWV and AIx@75 in the population studied so we used individuals of our region, tested by same study protocol.

Age was found to be the most significant parameter affecting PWV and AIx, more so with PWV. Values showed a trend of increase across all age group decades from 15 to 65 years, in line with most of the previous studies.[1],[6],[9],[10],[11],[12],[13],[16],[17] Across all age groups, aPWV showed more significant increase than AIx@75. It can be explained by altered elastin to collagen ratio with aging in aorta with reduction in compliance and giving stiffness.[18] This is in contradiction to few studies showing that in young and middle aged individuals (like our population) aPWV is better marker of arterial aging than AIx@75, which is rather better index of the same in the elderly population. One study, in contradiction to ours, showed that AIx increases more steeply with age than PWV in younger individual [19] and PWV changes markedly with age in older individuals. This can be due to the method used by previous studies which did not measure aortic PWV and the ethnicity of study individuals. Hence, age should be accounted as the prime factor while interpreting results of these two parameters.

Females had significantly higher AIx@75 than males across all age groups. This is in line with previous studies.[8],[12],[13],[19] This can be explained by shorter height in females and males and due to effect of sex hormones. In contrast to AIx@75, aPWV was significantly higher in males than females, in accordance with other researchers [6],[11] and in contrast to few.[10],[20] However, this was true with age group 15–45 years only and then, it was reversed with higher aPWV in females than males. PWV shows different behavior before and after menopause [21] and it indicates that in postmenopausal age group (45–65 years) the protective effect of estrogens gets vanished and the female advantage for protection against stiffness turns into female disadvantage. This fact is also supported by other publications.[7],[22] Hence, in our population with age group 15–65 years, proximal aortic stiffness (indicated by aPWV) is more in male than female while systemic arterial stiffness (indicated by AIx@75) has a different pattern in reproductive and postmenopausal age groups.

Apart from age and gender, aPWV was significantly predicted by BMI and SBP; and AIx@75 was significantly predicted by height, HR and PP. These are supported by previous studies.[6],[10],[11],[13],[17],[21] Stiffness increases with obesity,[23] so BMI is a predictor of aPWV. Similarly, SBP, an indicator of decreased compliance,[6] was positively affecting aPWV in study individuals. Shorter height gives larger reflection, so height was a predictor of AIx@75.[12] Tachycardia produces early wave reflection, and it was affecting the measure of the same-AIx@75 even after correction for HR. Wide PP is indicative of an increased reflection of wave from the periphery and a positive predictor of AIx@75.

PWV and AIx correlated with each other with varying significance and age group difference. It was significant overall and in the age group 15–24, 35–44, and 45–54. However, in the age group 25–34 (where cardiovascular aging is relatively accelerated) and 55–65 (where vascular aging accelerates toward diastolic HTN) they were not significant. There are some differences between two.[24] Before 60 years, there is accelerated AIx due to early wave reflection and unchanged incident wave; and after 60 years, there is accelerated PWV due to amplified incident wave.[13] However, our observations are in line with Mitchell et al.[25] who found no change in wave reflection up to 60 years. AIx and PWV cannot be used interchangeably as evidenced by a study showing the different effect of nitroglycerine on both.[26] Even in our study apart from age, predictors were not similar for both. PWV is true indicator of aortic stiffness while AIx is more an indicator of wave reflection than stiffness.[26] Hence, measurement of both offers an advantage which possible with precision by PWA.

Arterial stiffness is the earliest manifestation of anatomical or physiological vessel wall disease.[27] It develops in arteries where atherosclerosis is low and even in population with low risk of atherosclerosis.[27] It has prognostic implications with reference to cardiovascular events in diagnosed individuals,[28] relation with HTN;[29] and is important for risk stratification.[15] With aging from 20 to 80 years, bBP increases only 25% while aPWV increases 200% and elastic Young modulus increases by 1000%.[10] Normotensive individuals with increased arterial stiffness are at risk of future cardiovascular events.[30] Our previous study based on the same instrument has shown increased stiffness indices in young first degree relatives of diabetic patients.[31] In this regards, if optimally utilised, objective, discrete tools such as aPWV and AIx are superior to subjective, indirect parameter-bBP. With the availability of oscillometric devices and further studies, it will be established further as a potential tool to use.

There were few limitations of study like exclusion of the elderly, smokers, diabetic, and hypertensives; Cross-section nature with no follow-up; lack of biochemical investigations; and dependence of results on generalized transfer factor for central pulse wave derivations.


  Conclusions Top


It is feasible to assess arterial stiffness parameters PWV and AIx@75 by oscillometric PWA. Both correlates with each other, not totally dependent on blood pressure, with different predictors except age. PWV and AIx@75 are novel, objective, discrete parameters, and their baseline data can be used for future studies in our population.

Acknowledgment

We are thankful to Physiology department of our medical college for giving the facilities available in the department and to volunteers for participation in this study.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
  References Top

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    Tables

  [Table 1], [Table 2], [Table 3]



 

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