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Human immunodeficiency virus (HIV)-infected persons taking highly active antiretroviral therapy (HAART) may have an increased risk for cardiovascular-related events, although the underlying mechanism remains unclear. We tested the hypothesis that carotid arterial stiffness was higher among persons taking HAART compared to HAART-naïve and HIV-uninfected persons.
Between 2004 and 2006, we performed high resolution B-mode ultrasound on 2,789 HIV-infected and HIV-uninfected participants of the Women’s Interagency HIV Study (WIHS; 1865 women) and the Multicenter AIDS Cohort Study (MACS; 924 men) and determined carotid arterial distensibility, a direct measure of carotid arterial stiffness. We used generalized estimating equations to evaluate the association between distensibility and HIV infection, CD4+ cell count, and exposure to HAART adjusted for demographic, behavioral, and clinical characteristics.
In multivariable analysis, distensibility was 4.3% lower (95% confidence interval (CI): -7.4% to -1.1%) among HIV-infected versus uninfected participants. Among HIV-infected participants with fewer than 200 CD4+ cells, distensibility was 10.5% lower (95% CI: -14.5% to -6.2%) than that among HIV-uninfected participants, and this effect did not differ significantly by cohort or race. Concurrent HAART use was independently associated with lower distensibility among MACS participants but not among WIHS participants.
Our finding that advanced HIV-related immunosuppression was associated with increased carotid arterial stiffness independent from the effects of traditional atherosclerosis risk factors suggests that the etiologic mechanism underlying reports of an increased cardiovascular disease risk among HIV-infected individuals might involve HIV-related immunosuppression leading to vascular dysfunction and arterial stiffening.
The benefits of highly active anti-retroviral therapy (HAART) for treating human immunodeficiency virus (HIV) infection are well established,1, 2 but there is mounting evidence that HAART may also be linked to an increased risk of cardiovascular disease. Protease inhibitor (PI)-based HAART appears to be associated with an increased risk of myocardial infarction (MI)3, perhaps from increased levels of atherosclerosis.4 Several risk factors for atherosclerosis including hyperglycemia, diabetes, hypertension, and dyslipidemia have been reported to be more common in persons infected with HIV, especially among those taking PI-containing HAART regimens.4-8 However, the relationship of HIV and HAART with cardiovascular disease is not clear. Data from the Strategies for Management of Antiretroviral Therapy (SMART) trial suggest that in comparison to HIV-infected patients managed using a drug conservation strategy, those receiving uninterrupted antiretroviral treatment had a lower risk of vascular events despite their sustained exposure to HAART.9 Furthermore, a large cohort study recently found that HIV and HAART were not associated with an increased rate of cardiovascular events,10 and carotid intima-media thickness (cIMT) has not consistently been found to be associated with HIV infection or HAART use.11, 12
We previously reported that systolic hypertension, but not diastolic hypertension, was increased with longer HAART exposure 13. These findings suggest that the vascular effect of HAART may involve the “sclerosis” component of atherosclerosis that is measured by the stiffness of the arteries14 which, itself, is only weakly correlated with cIMT15 and can predict cardiovascular events and death.16 The current study of over 2,700 men and women was designed to investigate the association of HIV infection and HAART use with arterial stiffness using measurements of common carotid artery diameter change over the cardiac cycle.
This cross-sectional study included baseline data from HIV-infected and HIV-uninfected participants enrolled in the Carotid Ultrasound Substudy of the Multicenter AIDS Cohort Study (MACS) and the Women’s Interagency HIV Cohort Study (WIHS). MACS and WIHS are ongoing observational HIV cohort studies comprised of 6,972 men and 3,766 women, respectively, in selected metropolitan areas across the United States. Recruitment and follow-up procedures have been reported previously.17-20
Between April 2004 and March 2006, 1,865 women and 924 men received a baseline carotid ultrasound examination.21 Briefly, all WIHS participants who attended a study visit during this time period were eligible for this substudy, while MACS participants attending a study visit during this calendar period were eligible if they were at least 40 years old, weighed less than 300 pounds, and had no prior history of cardiovascular disease. Among WIHS participants, 84 (4.5%) reported a history of MI, congestive heart failure, or stroke. All participants gave written informed consent prior to enrollment, and this study was approved by the Institutional Review Boards at each of the MACS and WIHS institutions.
High resolution B-mode ultrasound was used to image the right common carotid artery (CCA), internal carotid artery, and carotid bulb as previously described.22 Sonographers at each of the WIHS and MACS sites were trained at the University of Southern California Atherosclerosis Research Unit Core Imaging and Reading Center (CIRC), and all scans were centrally read at the CIRC. The ultrasound assessment included measurements of the right CCA diameter at systole (DS) and diastole (DD), and pulse pressure (PP) was measured at the brachial artery. Using these measurements, we quantified carotid artery stiffness using the following index of carotid distensibility,
where lower values reflect a stiffer carotid artery. Distensibility was standardized to the units reported by Lage, et al., (10-6 * Newtons-1 * meters2).23
HIV infection was determined via serologic testing on specimens obtained at each MACS or WIHS study visit using enzyme-linked immunosorbent assays and confirmed using Western blot assays.17, 19 Anti-retroviral therapy use was documented via self-report. Following the DHHS/Kaiser Panel24 guidelines, we defined HAART as the reported use of three or more antiretroviral medications, one of which has to be a PI, an NNRTI, one of the NRTIs abacavir or tenofovir, an integrase inhibitor, or an entry inhibitor. HAART exposure was categorized three ways for analysis: taking versus not taking HAART, PI-based versus non-PI-based HAART, and duration of HAART exposure. Other characteristics included in this analysis were demographic/behavioral factors (e.g., age, race, smoking history), clinical factors associated with atherosclerosis (e.g., cholesterol, body mass index), and degree of HIV immunosuppression (e.g., CD4+ cell count).
We characterized the study population using graphical displays of the data and descriptive statistics (e.g., median, inter-quartile range (IQR)), and the characteristics of the MACS and WIHS cohorts were compared using a chi-square test of association. All statistical comparisons of distensibility were performed using generalized estimating equations25 (GEE) to account for clustering within the 10 MACS and WIHS sites. Multiple regression analyses were performed for the entire study population and then separately for the MACS and WIHS cohorts. Distensibility was natural log transformed to account for its skewed distribution and the model results are reported as the percent difference (PD) in distensibility between those with and those without the corresponding covariate. While approximately 90% of participants had complete data for all study variables other than fasting glucose, we used multiple-imputation26 to account for missing data in the multiple regression models. Statistical significance was defined as a two-sided p-value less than 0.05. All statistical analyses were performed using SAS 9.2 (SAS Institute, Cary, NC, USA).
The characteristics of the 2,789 study participants are shown in Table 1; two-thirds were from the WIHS, the median age was 44.3 years (range 20.2-83.0), and 48% were African-American. More than one-half of the participants were overweight or obese (body mass index (BMI) > 25), 21% had low-density lipoprotein-cholesterol (LDL-c) greater than 3.38 mmol/L (130 mg/dl), 30% had high-density lipoprotein-cholesterol (HDL-c) lower than 1.04 mmol/L (40 mg/dl), and 11% had systolic blood pressure (SBP) above 140 mmHg. Diabetes mellitus (DM) was documented in 16% of participants, but only 5% of those who were fasting at the time of the study visit had a fasting glucose level > 6.94 mmol/L (125 mg/dl). Approximately two-thirds (69%) of participants were infected with HIV, among whom 13% had a CD4+ cell count less than 200 (cells/ml) and 70% were taking HAART at the time of the ultrasound examination.
In addition to the gender difference between the MACS and WIHS cohorts, the distributions of all other characteristics included in Table 1 differed significantly between the cohorts except diabetes, fasting glucose, and the duration of HAART exposure among those who were taking HAART at the time of the examination. Compared to HIV-uninfected participants, those infected with HIV were significantly less likely to have education beyond high school, more likely to have a history of injection drug use (IDU), more likely to have a family history of MI, and had significantly lower BMI, LDL-c, and SBP levels (data not shown).
Overall, the median carotid distensibility was 16.8 × 10-6N-1m2, and the relationship between distensibility and age was similar in the two cohorts (Figure 1). In univariate analyses, distensibility was significantly associated with age, race/ethnicity, history of injection drug use, family history of MI, and all the clinical characteristics included in Table 1. Distensibility was also significantly lower among persons infected with HIV and among current HAART users who had initiated HAART more than 5 years earlier.
HIV infection remained significantly associated with lower distensibility (PD -4.3, 95% CI -7.4 to -1.1) in the multiple regression analysis (Table 2). The effect of HIV was greater in the MACS (PD -5.5) than in the WIHS (PD -1.9), but the difference between the cohorts was not statistically significant (p-value for interaction 0.24). Other characteristics that were independently associated with lower distensibility in this model were older age, African-American and Hispanic race/ethnicity, higher BMI, higher LDL-c, lower HDL-c, and higher SBP. We then added covariates for the use of blood pressure medications and cholesterol lowering medications to the regression models, but these cofactors were not independently associated with distensibility (data not shown) and their inclusion did not alter any inferences drawn from the results in Table 2.
To examine the association between HIV-related immunosuppression and distensibility, we stratified the HIV infected subgroup by CD4+ cell count. HIV-infected participants with fewer than 200 CD4+ cells had significantly less distensible carotid arteries (PD -10.5, 95% CI -14.5 to -6.2) (Table 3, model 1) compared to those who were not infected with HIV, and the association between advanced HIV-related immunosuppression and distensibility did not differ significantly between MACS (PD -6.9) and WIHS (PD -9.2) (p-value for interaction 0.46) or between African-Americans (PD -9.9) and non-African-Americans (PD -9.5) (p-value for interaction 0.42).
Among the 1931 HIV-infected participants, distensibility was significantly lower among those taking HAART in the MACS (PD -4.2, 95% CI -7.0 to -1.4), but not in the WIHS or in the overall model (Table 3, model 2). Among the 1317 HAART users, distensibility was also significantly lower in MACS participants who had been taking HAART for more than 5 years (PD -4.6, 95% CI -8.5 to -0.6), but duration of HAART use was not significantly associated with distensibility in the WIHS or overall models (Table 3, model 3). Finally, distensibility was similar among participants taking PI-based and non-PI-based HAART regimens (Table 3, model 4).
In this study, the largest study of the association between HIV disease and arterial stiffness performed to date, carotid arterial distensibility was significantly lower in HIV-infected versus HIV-uninfected participants after adjusting for the demographic, behavioral, and clinical characteristics of the study population. Distensibility was lowest among HIV-infected participants with advanced HIV-related immunosuppression, and this was true for both MACS and WIHS participants and for both African Americans and non-African Americans. Distensibility was also significantly lower among HIV-infected HAART users in MACS, especially among men who had initiated HAART more than 5 years earlier, but it was not associated with HAART use in the WIHS nor did it differ between those taking PI-containing versus non-PI-containing HAART regimens in either cohort. Thus, our study provides stronger support for an effect of HIV-related immunosuppression, compared to that of HAART, on carotid distensiblity.
Much of the research linking HIV disease with cardiovascular disease has focused on the effects of HIV therapy on metabolic-related factors such as hyperglycemia, insulin resistance, dyslipidemia, and fat redistribution which contribute to arterial disease 8, 27, 28 and may account for some of the increased risk of cardiovascular disease observed among HIV-infected persons taking HAART.4, 29, 30 However, if these factors were to mediate the association between HIV disease and atherosclerosis, then we would have expected that adjusting for them would nullify the association between HIV and distensibility in our analysis; this was not the case. Moreover, BMI, LDL-c, HDL-c, and SBP were all independently associated with distensibility in the expected direction which supports the validity of our results.
Arterial stiffness is associated with impaired endothelial function,31 and it has been proposed that HIV may affect the vasculature directly via endothelial damage,32 promoting arterial stiffness33-35 and contributing to atherogenesis.36 Consistent with this hypothesis, HIV infection was associated with increased arterial stiffness among children.37 The ACTG 5152s study group reported that initiating antiretroviral treatment improves endothelial reactivity, with a direct correlation between the improvement in vascular function and the viral and immunologic response to therapy.38 Data from the SMART trial suggest that, in terms of vascular disease, the risks associated with occasional adverse metabolic effects of antiretroviral therapies may be outweighed by the benefits of maintaining good immunologic function.9 A prior study from our group found that advanced HIV-related immunosuppression was independently associated with an increased prevalence of carotid lesions but not with cIMT.21 Together with our results, these findings suggest that the etiologic pathway connecting HIV infection to arterial stiffness may directly involve HIV-related immunosuppression rather than, or in addition to, antiretroviral therapy and atherosclerosis risk factors. However, caution is urged when interpreting our data because CD4+ cell count was a marker of the immunological status at the time of the ultrasound evaluation while arterial stiffness may stem from multiple processes operating over many years.39
Our finding that HAART was associated with increased carotid artery stiffness in the MACS and that stiffness increased with prolonged HAART use is consistent with our previous MACS study13 in which men who had prolonged exposure to HAART had a higher prevalence of systolic hypertension. It is unclear, however, why this association existed only among the MACS participants in the current study. While the effect of antiretroviral therapies on the vasculature might differ between men and women, it is also possible that differences in the course or treatment of HIV infection between the MACS and WIHS cohorts or, more generally, between men and women40 may have led to these results. Nevertheless, in comparison to HIV-related immunosuppression, HAART was not as strongly or consistently associated with increased carotid stiffness.
The strengths of the current study include the large sample size, being nested within two ongoing HIV cohorts, and the analysis of standardized data from ten different sites. The primary limitation of our study was its cross-sectional design that made it impossible for us to tease apart the etiological importance of HIV-related immunosuppression versus therapy, to describe changes in arterial stiffness across time, or to evaluate the effect of increased stiffness on cardiovascular outcomes. Finally, the unique demographic and clinical characteristics of the MACS and WIHS participants must be considered when interpreting the results from this study.
HIV-related immunosuppression was associated with increased carotid stiffness among both MACS and WIHS participants, while HAART use was associated with carotid stiffness among the MACS participants only. The mechanisms underlying these findings remain unknown, but these results suggest that the etiologic pathways may be independent from traditional atherosclerosis risk factors. Our study contributes to a growing volume of research supporting the hypothesis that HIV-related immunosuppression impacts the risk of cardiovascular disease and underscores the need for further exploration of cardiovascular disease risk among persons infected with HIV.
Data in this manuscript were collected by the Multicenter AIDS Cohort Study (MACS) and the Women’s Interagency HIV Study (WIHS) Collaborative Study Group with centers (Principal Investigators) located at the following institutions:
MACS centers: The Johns Hopkins Bloomberg School of Public Health (Joseph B. Margolick, Lisa P. Jacobson), Howard Brown Health Center, Feinberg School of Medicine, Northwestern University, and Cook County Bureau of Health Services (John P. Phair, Steven M. Wolinsky), University of California, Los Angeles (Roger Detels), and University of Pittsburgh (Charles R. Rinaldo).
WIHS centers: New York City/Bronx Consortium (Kathryn Anastos); Brooklyn, NY (Howard Minkoff); Washington DC, Metropolitan Consortium (Mary Young); The Connie Wofsy Study Consortium of Northern California (Ruth Greenblatt); Los Angeles County/Southern California Consortium (Alexandra Levine); Chicago Consortium (Mardge Cohen); Data Coordinating Center (Stephen Gange).
Funding Sources: The MACS is funded by the National Institute of Allergy and Infectious Diseases, with additional supplemental funding from the National Cancer Institute. UO1-AI-35042, 5-MO1-RR-00052 (GCRC), UO1-AI-35043, UO1-AI-35039, UO1-AI-35040, UO1-AI-35041.
The WIHS is funded by the National Institute of Allergy and Infectious Diseases (UO1-AI-35004, UO1-AI-31834, UO1-AI-34994, UO1-AI-34989, UO1-AI-34993, and UO1-AI-42590) and by the Eunice Kennedy Shriver National Institute of Child Health and Human Development (UO1-HD-32632). The study is co- funded by the National Cancer Institute, the National Institute on Drug Abuse, and the National Institute on Deafness and Other Communication Disorders. Funding is also provided by the National Center for Research Resources (UCSF-CTSI Grant Number UL1 RR024131).
This project was also funded, in part, by grants from the National Heart, Lung, and Blood Institute (1R01HL083760-04 and 5R01HL095140-03) to RK.
Conflicts of interest/disclosures: J.P. is a consultant to Pfizer. We have no other disclosures.