European Heart Journal Supplements (2009) 11 (Supplement D), D8–D12 doi:10.1093/eurheartj/sup018 Heart rate and atherosclerosis Jean-Claude Tardif* Department of Medicine, Montreal Heart Institute and University of Montreal, 5000 Belanger Street, Montreal H1T 1C8, Canada KEYWORDS Heart rate; Atherosclerosis; Coronary artery disease; Plaque rupture Epidemiological studies indicate that a lower heart rate (HR) is associated with decreased cardiovascular and allcause mortality.1–4 Clinical trials suggest that HR reduction is an important component of the benefits of beta-blockers in stable angina pectoris, after myocardial infarction and in heart failure.5–12 Pharmacological inhibition of the If current13–15 now provides the opportunity for pure HR reduction, with potential benefits from coronary artery disease to heart failure.16–24 This article summarizes the links between HR and atherosclerosis. Heart rate as a risk factor for cardiovascular disease All-cause and cardiac mortality increased steadily with resting and exercise HR in a prospective study of 5713 healthy men, aged 42–53 years, and followed up for 23 years.1 The relationship was much steeper for sudden cardiac death. Men with a resting HR .75 b.p.m. had a * Corresponding author. Tel: þ1 514 376 3330, Fax: þ1 514 593 2500, E-mail address: [email protected]. relative risk of sudden cardiac death of 3.46 by comparison with men whose HR was ,60 b.p.m., even after adjustment for age, use of tobacco, physical activity, diabetes, body mass index, blood pressure, cholesterol, parental history of sudden death or myocardial infarction, and exercise duration. Heart rate has also been shown to predict mortality in hypertensive populations2,3 and in elderly patients.3 We have reported the results of a study that evaluated the relationship between resting HR and future cardiovascular events in 24 913 patients included in the Coronary Artery Surgery Study registry undergoing coronary arteriography because of suspected or proven coronary artery disease, with a median follow-up of 14.7 years.4 After adjusting the multivariable Cox proportional hazard model for age, sex, diabetes, hypertension, cigarette smoking, left ventricular ejection fraction, number of clinically significant diseased coronary vessels, type of recreational activity, and concomitant treatments (including b-blockers), total mortality was increased in patients with resting HR between 77 and 82 b.p.m. (hazard ratio 1.16; 99% CI, 1.04–1.28) and those 83 b.p.m. (hazard ratio 1.32; CI, 1.19–1.47) when Published on behalf of the European Society of Cardiology. All rights reserved. & The Author 2009. For permissions please email: [email protected] Downloaded from by guest on October 28, 2014 Heart rate (HR) is a potent predictor of major cardiovascular events in both the general population and the patients with various cardiovascular diseases. An increased HR has been shown to be associated with increased progression of coronary atherosclerosis in animal models and patients. A high HR has also been associated with a greatly increased risk of plaque rupture in patients with coronary atherosclerosis. Pure HR reduction has slowed atherosclerosis progression in experimental models. Endothelial function may be an important link between HR and atherosclerosis. An increased HR has been shown experimentally to cause endothelial dysfunction. The protective effect on the endothelium of long-term pure HR reduction with ivabradine that we have shown in a dyslipidaemic mouse model of endothelial dysfunction could provide an important mechanism for the potential vascular benefits of the If current inhibitor ivabradine. These results, in addition to those of the BEAUTIFUL study, constitute a strong rationale for further clinical investigation of the cardioprotective effects of pure HR reduction. HR and atherosclerosis Table 1 Pathophysiological mechanisms relating an increased heart rate and coronary heart disease Greater myocardial oxygen consumption (MVO2) Decreased myocardial perfusion (shortened duration of diastole) Increased severity and progression of coronary atherosclerosis Lesser development of collaterals Increased risk of coronary plaque disruption Increased arterial rigidity Marker and possible mediator of sympathetic overactivity Pathophysiological mechanisms relating heart rate and coronary heart disease The importance of HR in cardiovascular prognosis can be explained by its relationship with major pathophysiological determinants (Table 1). A high HR is a major determinant of myocardial ischaemia, because it leads both to greater myocardial oxygen consumption and decreased myocardial perfusion, the latter because of the shortening in the duration of diastole. The likelihood of the occurrence of an ischaemic episode increases at higher baseline HRs. With a baseline HR less than 60 b.p.m., the likelihood of occurrence of ischaemic episodes with HR acceleration was 8.7%, whereas at rates in excess of 90 b.p.m., the likelihood increased to 18.5%.25 In addition, HR can also directly influence the underlying atherosclerotic process as described below. Heart rate, progression of atherosclerosis, and plaque rupture Experimental and clinical evidence also suggests that sustained elevations in HR may also play a direct role in the pathogenesis of coronary atherosclerosis and its complications. Heart rate was significantly correlated with the severity and progression of atherosclerosis on coronary angiography among men who had developed myocardial infarction at a young age.26 Accelerated atherogenesis resulting from increased HR may be due to both mechanical and metabolic factors. Increased vascular wall stress may contribute to endothelial injury, potentially promoting the complex cascade of events leading to increased atherosclerosis. Experimental data also show that a reduction in HR can delay the progression of coronary atherosclerosis in monkeys.27 Male cynomolgus monkeys subjected to sinus node ablation or those with innately low HRs had significantly less coronary atherosclerosis than animals with higher HRs. These observations are supported by results from the Beta-Blocker CholesterolLowering Asymptomatic Plaque Study randomized trial, which have shown that a b-blocker reduced the rate of progression of carotid intima-media thickness in asymptomatic patients.28 A high HR has also been associated with an increased risk of coronary plaque disruption.29 In this retrospective angiographic study evaluating patients who underwent two coronary angiograms within 6 months, logistic regression analysis identified a positive and independent association between plaque disruption and a mean HR .80 b.p.m. This association again indicates that haemodynamic forces may play a critical role in the process of plaque disruption. A high HR is also strongly associated with increased arterial rigidity, reduced vascular distensibility, and elevated pulse-wave velocity, characteristics that are all associated with an increased risk of myocardial infarction and cardiac death.30 In a retrospective study, a larger number of patients with obstructive coronary artery disease whose HRs were ,50 b.p.m. had developed collateral vessels (potentially decreasing the ischaemic burden) compared with patients with HRs .60 b.p.m.31 The presence of collaterals was independent of the history of angina or the use of b-blockers. A high HR may also reflect an imbalance of the autonomic nervous system and may therefore be a marker of sympathetic overactivity.32 The metabolic syndrome and insulin resistance are also associated with sympathetic overactivity.33,34 Heart rate and endothelial dysfunction A number of studies point to endothelial dysfunction as the missing link between HR and cardiovascular events. The hypothesis is that a higher HR could increase the twisting of large epicardial arteries during systole as well as the number of times per minute that forces are applied to the vascular wall leading to fatigue, causing endothelial damage to these vital arteries, and a simultaneous increase in the probability of atherosclerotic plaque rupture in the coronary arteries, thereby leading to myocardial infarction. Endothelial dysfunction is considered an integral part of the events leading to atherosclerosis initiation and progression and has been shown to be associated with adverse cardiovascular events. We now review the evidence linking HR and endothelial function. Endothelial dysfunction is central to the pathogenesis of atherosclerosis. It is the first step, leading to the Downloaded from by guest on October 28, 2014 compared with the reference quintile (62 b.p.m.). Cardiovascular mortality also increased in the 77–82 b.p.m. (hazard ratio 1.14, CI, 1.00–1.29) and in the 83 b.p.m. (hazard ratio 1.31, CI, 1.15–1.48) groups. The association between HR and total mortality held true in all analysed subgroups: old (.65 years) vs. young, diabetics vs. non-diabetics, hypertensives vs. normotensives, BMI .27 or ,27, those with ejection fraction .50% or ,50%, and patients treated with b-blockers vs. those without such treatment. The predictive power of HR for mortality remained true both in men and in women. A gender-related difference in the association between HR and mortality has been found in some studies in hypertensive subjects2 or in patients with myocardial infarction.5 Data from our study in patients with stable coronary artery disease indicates that a higher HR can also be deleterious in women. D9 D10 J.-C. Tardif Figure 1 Ivabradine prevents endothelial dysfunction associated with dyslipidaemia in mice. Adapted from Drouin et al.35 Pure heart rate reduction to preserve endothelial function We performed a study in dyslipidaemic mice to document the effects of pure heart reduction on endothelial function.35 The If current inhibitor ivabradine was chosen because it reduces HR in mice independently of sympathetic activation, and it does not affect blood pressure, myocardial contractility, or intracardial conduction.36,37 Endothelial vasodilator capacity was used as the means to show preservation of endothelial function. The experiments were conducted in dyslipidaemic mice expressing the human apoprotein-B 100 as they develop changes in endothelial-dependent arterial dilation.38,39 Dyslipidaemic mice were assigned to 3 months of treatment with ivabradine, metoprolol, or no treatment. The outcomes in terms of vessel dilation in renal and posterior communicating cerebral arteries were compared between these groups and those of wild-type C57BI/6 mice. Throughout the experiments, it was found that HR remained stable in wild-type mice whereas it increased in untreated dyslipidaemic mice. The use of ivabradine reduced HR in dyslipidaemic mice by 17%. Endothelium- dependent dilation in response to acetylcholine was decreased in untreated dyslipidaemic mice compared with those treated with ivabradine, which maintained maximal dilation (Figure 1). The use of ivabradine completely prevented the impaired dilator response to acetylcholine in dyslipidaemic mice. The use of the antioxidant N-acetylcysteine fully restored dilation in response to acetylcholine in dyslipidaemic mice, whereas it did not affect the response to acetylcholine in wild-type mice or in mice treated with ivabradine.39– 41 This shows that the endothelial dysfunction in dyslipidaemic mice is caused in large part by oxidative stress, which was not increased in wild-type mice and dyslipidaemic mice treated with ivabradine. Therefore, ivabradine protected the treated dyslipidaemic mice against oxidative stress. Given that ivabradine has no direct antioxidant effects, the protection it afforded might be due to alternative mechanisms, such as improvement of the shear stress-dependent stimulation of the endothelium, which favours endothelial nitric oxide synthase and/or prevents nitric oxide or hydrogen peroxide degradation, or decreased mechanical fatigue of the arterial wall associated with pure heart reduction. The use of L-NNA, a nitric oxide synthase inhibitor, reduced vasodilation in wild-type and in dyslipidaemic mice treated with ivabradine. Since ivabradine has no direct vascular effect, this finding suggests that chronic pure heart reduction preserved the nitric oxide pathway for vasodilation. Endothelium-dependent dilation of cerebral arteries induced by acetylcholine was impaired in dyslipidaemic mice compared with wild-type mice. The use of ivabradine also completely prevented this impairment of vasodilatory capacity in cerebral arteries. Treating dyslipidaemic mice with metoprolol reduced HR to the same extent as ivabradine. In contrast, ivabradine provided superior preservation of endothelial function in renal and cerebral arteries in dyslipidaemic mice compared with metoprolol. In renal arteries, metoprolol did Downloaded from by guest on October 28, 2014 formation of fatty streaks, in a series of events that could eventually lead to the formation of atherosclerotic plaques and thrombus. Endothelial dysfunction allows lipoproteins to enter the intima and be modified in situ by oxidation and glycation. These events will exacerbate endothelial dysfunction and promote macrophage adhesion to the endothelium and migration into the intima. Subsequently, the generation of extracellular matrix will promote the formation of a fibrofatty lesion, the atherosclerotic plaque. Under conditions of haemodynamic stress and degradation of the extracellular matrix, the plaque can rupture and promote the formation of an intra-luminal thrombus leading to an acute coronary syndrome. HR and atherosclerosis Figure 2 Metoprolol does not prevent cerebral endothelial dysfunction associated with dyslipidaemia. Adapted from Drouin et al.35 Conclusion An increased HR has been shown to be associated with greater atherosclerosis progression and a higher risk of coronary plaque rupture. Pure HR reduction has slowed atherosclerosis progression and improved endothelial function in experimental models. These results, in addition to those of the BEAUTIFUL study, constitute a strong rationale for further clinical investigation of the cardioprotective effects of pure HR reduction. Funding Dr Tardif has received honoraria from Servier. Conflict of interest: none declared. References 1. Jouven X, Empana JP, Schwartz PJ, Desnos M, Courbon D, Ducimetiere P. 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