Coronary Heart Disease: What is it?
Coronary Heart Disease (CHD) describes heart conditions resulting from reduced blood flow to the heart, most commonly due to Atherosclerosis (AT); the presence of fatty atheroma plaques on the arterial intima which partially or entirely occlude the lumen [NHLBI].
Hyperlipidaemia (HL, high blood lipid levels) is accepted as a pivotal risk factor since clinically relevant AT generally will not develop in persons with very low lipid levels, regardless of the presence of other risk factors [Steinberg D, et al.] Typically the result of a high fat diet, HL tends to cluster with other modifiable risk factors including obesity, hypertension, diabetes mellitus and smoking (Figure 1) so is important to recognise that the impact of coexisting risk factors is rather multiplicative than additive (Figure 2).

additive (Figure 2).

Note in Figure 1 that low high-density lipoprotein (HDL) levels are commonly found in CHD sufferers which can be explained by the role of HDL in the transport of lipids to the liver for disposal [Carroll et.al. 2013]

How big is the problem?
CHD is globally the leading cause of death (Figure 3) despite age-adjusted CHD mortality falling in wealthier countries (Figure 4) thanks to advancements in treatment reducing the case fatality rate [Pandya et al. 2013]
In developing countries the prevalence of CHD risk factors is growing due to changing lifestyles but access to treatment remains limited, thus CHD related mortality is rapidly rising [Gaziano et al. 2010]

In response, many countries are investing in the development of cost effective primary and secondary prevention programmes which focus on reducing risk factors via counselling and education, e.g. *Change4Life* campaign, UK.
Secondary prevention includes cardiac rehabilitation (CR) which is usually employed after a cardiovascular event. CR typically involves an initial inpatient phase where tobacco cessation, exercise training, dietary education and therapy are implemented with the aim of enhancing self-efficacy and optimising long term quality of life. [Jolliffe et al.]. Many studies support the cost effectiveness of CR compared with other post-myocardial infarction (MI) treatments, [Shields et al.2018], [Ades P. et al], [Papadakis et al, 2004] but to what extent does exercise contribute to its success.

Is exercise the answer?
In 1973, Morris et al. observed that men who performed vigorous exercise twice a week had a third lesser risk of CHD than their counterparts. The link between exercise and cardiovascular health has long been recognised (Figure 5), but some believe the benefits have been overstated. (Anderson et al)

In 2016 Anderson et al. conducted a meta-analysis involving 63 randomized control trials that compared exercise-based cardiac rehabilitation (EBCR) with a control and had a minimum follow up period of six months. The definition of EBCR was very broad, ranging from ‘supervised inpatient’ to ‘unsupervised home based’ intervention which included some form of exercise training. There are no means to assess whether a participant has adhered to prescribed unsupervised exercise training and no indication is given to the level of exercise imposed so it could be minimal. The CR could also include psychological and educational interventions as it could for the comparator who was also permitted ‘standard medical care’ but this is not defined. Women accounted for <15% of patients which is not representative of the CHD population. The authors acknowledged that the median follow up of twelve months was limited when assessing the effect on mortality and morbidity outcomes.
There was no statistical reduction in total mortality, risk of total MI or total health care costs with EBCR, however risk of admission was reduced compared with usual care (risk rate: 0.82, 95% confidence interval: 0.70, 0.96), as was pooled CV mortality (10.4% to 7.6%; number treated: 37)
This is at odds with a previous meta analyses conducted by one of the authors [Oldridge Neil. et al.1988] which showed a 20-25% reduction in all-cause mortality with exercise-based CR. This could be due to significant advances in the management of CHD, such as the use of statins.
Stewart et al. [2016] investigated the importance of the intensity and duration of exercise interventions for prognosis using 15,486 patients with stable CHD who participated in the STABILITY (Stabilization of Atherosclerotic Plaque by Initiation of Darapladib Therapy) trial. They self-reported their habitual level of PA by questionnaire and were categorised as least, intermediate and most active.
The intermediate and most active tertile had a lower risk of all-cause mortality, cardiovascular mortality and non-cardiovascular mortality compared to the least active tertile, and the most active tertile had a lower risk of MACE. However, there were no differences in the risk of stroke and MI across all tertiles. (Figure 6)
The dose response analysis revealed a curvilinear relationship between PA and mortality with the greatest risk reductions at the beginning of the curve (Figure 7) suggesting that the least active could significantly reduce their risk with a minimal increase in PA. Also, reductions in mortality were greater among CHD patients limited by dyspnoea and those with a high STABILITY CHD risk score, indicating that high risk patients benefit the most from an active lifestyle.

The median 3.7 year follow up for this study was reasonable, however the use of questionnaires was a limitation as people often miscalculate their volume and intensity of PA hence there’s often a large disparity between self-reported and accelerometer measured PA [Dyrstad et al. 2014].
These results have been mirrored by Sattelmair et al [2012] who pooled data from 33 studies of PA and primary prevention of CHD and found a close dose-response relationship between PA and CHD risk and that even people who engage in <550 kcal/week extra PA have a significantly reduced risk.
There is strong evidence to support the dogma that exercise benefits cardiovascular health but also that minimal increases in physical activity, well below current guidelines, can substantially improve CHD risk. Vigorous PA yields the lowest mortality risk but the low volume of the minimal effective dose may encourage people to incorporate feasible PA goals into their daily lives.
There is still much debate regarding the benefits of different modes of exercise which will be examined in more detail on this website.