Complexities in cardiovascular rhythmicity: perspectives on circadian normality, ageing and disease

Biological rhythms exist in organisms at all levels of complexity, in most organs and at myriad time scales. Our own biological rhythms are driven by energy emitted by the sun, interacting via our retinas with brain stem centres, which then send out complex messages designed to synchronize the behaviour of peripheral non-light sensing organs, to ensure optimal physiological responsiveness and performance of the organism based on the time of day. Peripheral organs themselves have autonomous rhythmic behaviours that can act independently from central nervous system control but is entrainable. Dysregulation of biological rhythms either through environment or disease has far-reaching consequences on health that we are only now beginning to appreciate. In this review, we focus on cardiovascular rhythms in health, with ageing and under disease conditions.

Heart rate variability reactivity and new romance: Cause or consequence?

There are documented physiological differences between single and coupled individuals during the "honeymoon period" of nascent romantic relationships. One such difference is in autonomic reactivity, specifically heart rate variability (HRV) reactivity. This finding had previously been interpreted as evidence of a stress buffering effect of relationship formation. The present study explored among university women two competing longitudinal hypotheses conceptualizing differences in HRV reactivity as either a cause or a consequence of romantic relationship formation. Results did not support the hypothesis that HRV reactivity changes as a consequence of beginning a new romantic relationship. Instead, lower HRV reactivity predicted greater relationship formation amongst women with low BMI and higher resting HRV. The functioning of the heart therefore predicted the likelihood that an individual would find love. These interactions may be the result of differing success rates of various mating strategies for women with low and high BMI and HRV.

Heart rate lowering treatment leads to a reduction in vulnerable plaque features in atherosclerotic rabbits

Objective

To investigate the effect of a heart rate (HR) lowering agent (Ivabradine) on features of atherosclerotic plaque vulnerability with magnetic resonance imaging (MRI), ultrasound imaging, and histology.

Approach and results

Atherosclerosis was induced in the abdominal aorta of 19 rabbits. Nine rabbits were treated with Ivabradine (17 mg/kg/day) during the entire study period. At week 14, imaging was performed. Plaque size was quantified on contrast-enhanced T1-weighted MR images. Microvascular flow, density, and permeability was studied with dynamic contrast-enhanced MRI. Plaque biomechanics was studied by measuring the aortic distension with ultrasound. After, animals were sacrificed and histology was performed.

HR was reduced by 16% (p = 0.026) in Ivabradine-treated animals. No differences in absolute and relative vessel wall beat-to-beat distension were found, but due to the reduction in HR, the frequency of the biomechanical load on the plaque was reduced. Plaque size (MR and histology) was similar between groups. Although microvessel density (histology) was similar between groups, AUC and K^trans, indicative for plaque microvasculature flow, density, and permeability, were decreased by 24% (p = 0.029) and 32% (p = 0.037), respectively. Macrophage content (relative RAM11 positive area) was reduced by 44% (p<0.001) on histology in Ivabradine-treated animals.

Conclusions

HR lowering treatment with Ivabradine in an atherosclerotic rabbit model is associated with a reduction in vulnerable plaque features. The current study suggests that HR reduction may be beneficial for inducing or maintaining a more stable plaque phenotype.

Usefulness of physical fitness and the metabolic syndrome to predict vascular disease risk in older Chinese (from the Guangzhou Biobank Cohort Study-Cardiovascular Disease Subcohort [GBCS-CVD])

Physical fitness can independently lower the risk of cardiovascular disease (CVD). We explored the independent and combined associations of physical fitness, measured using the seated at rest heart rate (RHR), and the metabolic syndrome (MS), with CVD risk, as described by an elevated pulse wave velocity (PWV) in older Chinese. Data from 1,996 participants were drawn from the Guangzhou Biobank Cohort Study-Cardiovascular Disease Subcohort. Analysis of variance and logistic regression analysis were used to establish the independent and combined associations of the RHR and the MS with PWV. The RHR was independently associated with an elevated PWV (odds ratio [OR] 1.63, 95% confidence interval [CI] 1.22 to 2.18), as was the MS (OR 2.36, 95% CI 1.76 to 3.17). The participants with a high RHR, but without the MS, had an adjusted OR of 1.63 (95% CI 1.15 to 2.30) for the presence of the CVD proxy. Those with a low RHR and the MS had an adjusted OR of 2.35 (95% CI 1.66 to 3.33). The risk of an elevated PWV increased almost fourfold with both a high RHR and a diagnosis of the MS (OR 3.87, 95% CI 2.39 to 6.28, p = 0.52 for interaction). In conclusion, physical fitness, measured using the RHR, and the MS are independently associated with an elevated PWV, a surrogate marker for CVD. The strength of this association was further increased in the presence of both. These findings confirm the beneficial effects of physical fitness on attenuating the risk of CVD among older Chinese.

Long-term prognostic value of resting heart rate in patients with suspected or proven coronary artery disease

AIMS

Heart rate reduction is the cornerstone of the treatment of angina. The purpose of this study was to explore the prognostic value of heart rate in patients with stable coronary artery disease (CAD).

METHODS AND RESULTS

We assessed the relationship between resting heart rate at baseline and cardiovascular mortality/morbidity, while adjusting for risk factors. A total of 24 913 patients with suspected or proven CAD from the Coronary Artery Surgery Study registry were studied for a median follow-up of 14.7 years. All-cause and cardiovascular mortality and cardiovascular rehospitalizations were increased with increasing heart rate (P<0.0001). Patients with resting heart rate > or =83 bpm at baseline had a significantly higher risk for total mortality [hazard ratio (HR)=1.32, CI 1.19-1.47, P<0.0001] and cardiovascular mortality (HR=1.31, CI 1.15-1.48, P<0.0001) after adjustment for multiple clinical variables when compared with the reference group. When comparing patients with heart rates between 77-82 and > or =83 bpm with patients with a heart rate < or =62 bpm, the HR values for time to first cardiovascular rehospitalization were 1.11 and 1.14, respectively (P<0.001 for both).

CONCLUSION

Resting heart rate is a simple measurement with prognostic implications. High resting heart rate is a predictor for total and cardiovascular mortality independent of other risk factors in patients with CAD.