Associations between cardiac function and retinal microvascular geometry among Chinese adults

Abnormal retinal microvascular geometry has been associated with cardiac remodeling and heart failure. However, its relation to cardiac function, prior to clinical disease has not been explored. In this cross-sectional study, 50 participants (mean age 62.5 ± 11.7 years) without cardiovascular disease (CVD) were recruited from the Cardiac Ageing Study. Transthoracic echocardiography imaging was performed to measure cardiac function indices, and retinal imaging was used to measure retinal vascular caliber and retinal vascular geometric indices. Multiple linear regressions were applied to examine associations between indices of cardiac function and retinal microvasculature, adjusting for age, sex, body mass index, mean blood pressure and comorbidity (i.e. hypertension, diabetes and dyslipidemia). After adjusting for confounders, each unit decrease in peak systolic septal mitral annular velocity (Septal S′) indicating poorer left function was associated with smaller retinal venular branching angle (β: − 2.69°; 95% CI − 4.92, − 0.46). Furthermore, each unit increase in peak velocity flow in late diastole by atrial contraction (MV A Peak) indicating poorer left atrial function was associated with lower retinal venular fractal dimension (− 0.13Df; − 0.25, − 0.004). Our findings suggested a relationship between poorer cardiac function and suboptimal retinal microvascular geometry, among Chinese without CVD.


Results
The mean (SD) age and male count (%) in our study were 62.54 (11.74) years and 27 (54%), respectively. The majority of participants had reported history of hypertension and dyslipidaemia (Table 1). Compared with patients with E/A ratio > 1.0, those with E/A ratio ≤ 1.0 were older, more likely to have combination of comorbidities and with larger waist-to-hip ratio, higher SBP and lower retinal venular fractal dimension (Table 1). After adjusting for covariates, each unit increase in MV A Peak that indicated poorer left atrium function was associated with lower retinal venular fractal dimension and higher retinal arteriolar curvature tortuosity, respectively (Table 2). Each unit increase in E/E′ lateral that indicated poorer left ventricular function was associated with smaller retinal venular branching angle (β: − 2.69°; 95% CI − 4.92, − 0.46) ( Table 2) Table 2). Examples of retinal imaging showing microvascular geometry differences between patients with poorer and better cardiac function were shown in Figs. 1 and 2. However, no aforementioned cardiac functional indices were associated with retinal vascular calibres (Table S1).

Discussion
Our study observed significant associations between subclinical changes in left ventricular function (i.e. Septal S′, Lateral S′, Lateral E′ and E/E′ lateral) and left atrial function (i.e. MV A Peak) with suboptimal retinal venular geometry (i.e. lower fractal dimension and smaller branching angle), among Chinese subjects without prior history of CVD. Our findings suggested that suboptimal retinal venular geometry could be a potential proxy to mirror cardiac dysfunction.
It is well-known that pathogenic risk factors such as chronic stress and systemic inflammation can lead to adverse alterations in cardiac structure and function (i.e. enlarged volume and reduced cardiac output) 5,6 , which can ultimately progress to CVD such as HF 7 . However, whether there is a parallel microcirculatory disturbance along with the cardiac changes in structure and function has yet to be investigated via conventional techniques. Retinal microvasculature carries substantial information on the general microcirculation. For example, vessel rarefaction and collapse-leading to reduction in vascular fractal dimension-is associated with hypoxia 8 , and increased vessel tortuosity is indicative of vessel wall dysfunction and blood-retina barrier damage 9 , and narrowing venular branching angle is indicative of increased levels of oxidative stress 10 . Our observation suggested that the changes in retina vessels (i.e., arteriolar fractal dimension and venular branching angle reduction) may reflect increased systematic endothelial dysfunction and oxidative stress, resulting in changes in cardiac function. Intriguingly, we found that venular changes, instead of arteriolar changes in the retina, were associated with poorer cardiac function. Our previous investigations have demonstrated that for cardiac structure, changes in retinal arterioles were more sensitive to subclinical alterations than retinal venules 11 . These differential associations may suggest adaptive responses occurring within venules, predating more gross changes within cardiac structure that are reflected later within the retinal arterioles 12 .
Our exploratory study has identified novel associations between cardiac function and retinal venular geometry, capitalizing on detailed cardiovascular and retinal measurements. Nevertheless, we acknowledged the limitations of our study. Firstly, the small sample in this study might have restricted our power to detect more associations with significance. Secondly, while we did not correct our findings for multiple comparisons, we took guidance from other studies that have also similar pre-hypothesized independent variables in their statistical associations 13 . Thirdly, we acknowledged that other variables such as alcohol consumption, socio-economic status, medications, plasma glycosylated hemoglobin (HbA1c) and diabetes duration, which were not available for this study sample, may contribute to residual confounding. Fourthly, this study involved Chinese subjects in an Asian setting for which observed findings might not be generalizable to other ethnic populations. Finally, the cross-sectional nature of our study design precludes inferences about causality. Future larger studies, in other ethnic cohorts, extended over time, may be necessary to verify our findings.
In conclusion, our findings showed that adverse changes in cardiac function might be mirrored by suboptimal retinal vascular geometry. Further longitudinal studies with larger samples with right ventricular and atrial measures are warranted to not only confirm our findings, but also provide more evidence on the underlying pathophysiology for CVD and even HF development.

Materials and methods
This study conducted a cross-sectional analysis between cardiac function and retinal microvascular geometry among Chinese (Han ethnicity) subjects (n = 50), who were recruited from the Cardiac Ageing Study (CAS) 14 and also underwent retinal examination. Briefly, CAS is a prospective study initiated in 2014 that examines characteristics and determinants of cardiovascular function of community adults aged 38 years and above. CAS excluded subjects who had self-reported history of physician-diagnosed cardiovascular disease (such as coronary heart disease, atrial fibrillation and stroke) or cancer, and required all participants to complete a standardized questionnaire collecting personal medical history and coronary risk variables. Such details have been described in prior publications 14 . The study complied with the Declaration of Helsinki. All participants agreed and signed the written informed consent upon enrolment. The SingHealth Centralised Institutional Review Board (2014/628/C) approved the study protocol.
Transthoracic echocardiography (ALOKA α10 with a 3.5 MHz probe, Hitachi Medical, Wallington, CT, USA) included 2-D, M-mode, pulse Doppler and tissue Doppler imaging was performed to evaluate the cardiac function, namely Peak systolic septal mitral annular velocity (Septal S′), Peak systolic lateral annulus velocity (Lateral S′), Peak early diastolic lateral annulus velocity (Lateral E′), peak velocity flow in early diastole E (MV E peak), Table 1. Characteristics of our Chinese elderly participants. Mean (SD) are presented for continuous variables and N (%) are presented for non-continuous variables. BMI body mass index, SBP systolic blood pressure, DBP diastolic blood pressure, MAP mean arterial blood pressure, LVEF left ventricular ejection fraction, MV E peak peak velocity flow in early diastole, MV A peak peak velocity flow in late diastole by atrial contraction, E/A ratio peak velocity flow in early diastole/peak velocity flow in late diastole by atrial contraction, Septal S′ peak systolic septal mitral annular velocity, Septal E′ peak early diastolic septal mitral annular velocity, Septal A′ septal mitral annular velocity during atrial contraction, Lateral S′ peak systolic lateral annulus velocity, Lateral E′ peak early diastolic lateral annulus velocity, Lateral A′ lateral annulus velocity during atrial contraction, E/E′ septal ratio of mitral peak velocity flow in early diastole to peak early diastolic septal mitral annular velocity, E/E′ lateral ratio of mitral peak velocity flow in early diastole to peak early diastolic lateral annulus velocity, E/E′ average the ratio of MV E peak and average of Septal E′ and Lateral E′, CRAE central retinal arteriolar equivalent, CRVE central retinal venular equivalent, DF-a fractal dimension-arteriole, DF-v fractal dimensionvenule, CT-a curvature tortuosity-arteriole, CT-v curvature tortuosity-venule, BA-a branching angle-arteriole, BA-v branching angle-venule.   At the end of each examination, all measurements among three cardiac cycles were averaged and adjusted with the interbeat interval by the same echocardiographer. Retinal vascular imaging was performed (Canon CR-1, 40D SLR digital retinal camera backing, Canon Inc., Tokyo, Japan) and accessed (Singapore I Vessel Assessment (SIVA) version 3.0, Singapore Eye Research Institute, Singapore) to obtain retinal vascular parameters including calibre, branching angle, curvature tortuosity and fractal dimension, according to a standard protocol described elsewhere 16 . The same grader reanalyzed the vessels in 10% of the total retinal images, and intragrader correlation coefficient is consistently > 80% across all retinal vascular geometric indices. Covariates including age, sex, mean arteriolar pressure (MAP), body mass index (BMI, calculated as weight in kilograms divided by the square of height in meters) and history of comorbidity (i.e. dyslipidaemia, hypertension, diabetes mellitus) were collected during clinical visit interview.
Mean and standard deviation (SD) and counts and percentages were used to describe all variables. Characteristics of groups according to E/A ratios (> 1.0 vs. ≤ 1.0; E/A ratio of ≤ 1.0 is the reflective of impaired myocardial relaxation 17 ) were compared. Multiple linear regressions were conducted to assess the associations between cardiac function and retinal vascular measures. Three models were applied: Model 1, adjusted for age and sex; Model 2, Model 1 and additionally adjusting for MAP and BMI; Model 3, Model 2 and additionally adjusting for history of comorbidity. For all analyses, we defined a significant p-value (two-tailed) as 0.05. We performed all statistical analyses using IBM SPSS software version 23.0 (SPSS, IBM, Chicago, USA). Table 2. Associations between retinal vascular geometric parameters and cardiac functional indices. Model 1 adjusted for age and sex. Model 2 adjusted for age, sex, MAP and BMI. Model 3 adjusted for age, sex, MAP, BMI and combination of co-morbidity. Beta and 95% CI are presented in the table, and values with statistically significance (p < 0.05) are highlighted in boldface font. MV E Peak peak velocity flow in early diastole, MV A peak peak velocity flow in late diastole by atrial contraction, E/E′ septal ratio of mitral peak velocity flow in early diastole to peak early diastolic septal mitral annular velocity, E/E′ lateral: ratio of mitral peak velocity flow in early diastole to peak early diastolic lateral annulus velocity, E/E′ average: the ratio of MV E Peak and average of Septal E′ and Lateral E′, LVEF left ventricular ejection fraction, E/A ratio peak velocity flow in early diastole/peak velocity flow in late diastole by atrial contraction, Septal S′ peak systolic septal mitral annular velocity, Septal E′ peak early diastolic septal mitral annular velocity, Septal A′ septal mitral annular velocity during atrial contraction, Lateral S′ peak systolic lateral annulus velocity, Lateral E′ peak early diastolic lateral annulus velocity, Lateral A′ lateral annulus velocity during atrial contraction, DF-a fractal dimension-arteriole, DF-v fractal dimension-venule, CT-a curvature tortuosity-arteriole, CT-v curvature tortuosity-venule, BA-a branching angle-arteriole, BA-v branching angle-venule.