Monitoring the Risk of Heart Disease in Patients with Diabetes

Detecting and preventing heart disease in diabetesA disorder caused by insufficient or absent production of the hormone insulin by the pancreas, or because the tissues are resistant to the effects. patients is critical. This article deals with two fundamental questions:

1. What is the best method to spot any potential heart disease?
2. What is the best test to accurately diagnose heart pain in diabetic patients?

Overview - Diabetes and Coronary Artery Disease

Diabetes is reaching epidemicA sudden outbreak of infection that affects a large proportion of a population. proportions in both the Western and developing worlds. Currently it affects nearly 5% of the adult population in the UK. Due to the increasing prevalence of obesityExcess accumulation of fat in the body. the incidenceThe number of new episodes of a condition arising in a certain group of people over a specified period of time. and vascularRelating to blood vessels. complications of type-2 diabetes will continue to rise (The Foresight Report estimates that by 2015 64% of adults aged 21 to 60 in the UK will be obese, and by 2025 this will rise to 83%). Nearly 80% of patients with type 2 diabetes suffer from cardiovascular illness, with coronary artery diseaseNarrowing of the blood vessels supplying the heart muscle, leading to symptoms such as angina and sometimes to a heart attack or myocardial infarction (CAD) representing the major component of this disease. Approximately 10 – 15% of patients admitted to hospital suffering from a heart attackThe death of a section of heart muscle caused by an interruption in its blood supply. Also called a myocardial infarction. (myocardial infarctionDeath of an area of heart muscle due to poor blood supply. This is commonly known as a heart attack.) and 20% of patients dying from coronaryRelating to the arteries supplying the heart itself. heart disease (CHD) have diabetes as a contributing factor. Therefore diabetes is placing a large burden on our health care system, both directly and indirectly. 

Assessing the extent of atherosclerosis and plaque burden

Atherosclerosis is a condition in which the arteries become clogged by fatty deposits that over time harden into plaques. This causes the arteries to become narrower and in turn restricts the flow of bloodA fluid that transports oxygen and other substances through the body, made up of blood cells suspended in a liquid. through them. This narrowing and hardening of the arteries is dangerous for two reasons. Firstly, if the blood flow to an organ is restricted the organ can become damaged and cease to work properly. Secondly, if a plaque bursts it will cause a blood clotBlood that has coagulated, that is, has moved from a liquid to a solid state. that can then block the blood supply to important organs such as the heart, triggering a heart attack or the brain, triggering a strokeAny sudden neurological problem caused by a bleed or a clot in a blood vessel..

Assessing the extent of atherosclerotic plaqueFatty plaques that form in the inner walls of arteries in atherosclerosis. in the arteries has therefore long been the goal of cardiologists as it underlies all coronary artery disease and a large percentage of cardiacRelating to the heart disease. Despite an increasing understanding of the risk factors and mechanisms of atherosclerosis, a large proportion of patients suffering from this condition will unfortunately only be seen by a doctor when actually suffering from a sudden heart attack (myocardial infarction) or sadly, cardiac death. The gold standard for assessing the extent of coronary artery disease remains invasive coronary angiographyA technique for X-ray imaging of the coronary arteries, which involves introducing a catheter through a blood vessel and threading it towards the heart, then injecting a contrast medium to improve visibility. (a specialist x-ray that reveals detailed information about the heart,) which is essentially simply an image of the heart that provides very little information about the plaque itself. Our ability to identify those patients who are not displaying symptoms, but who are actually suffering from coronary artery disease that will progress and lead to reduced blood supple to the heart, or whose lesions will become unstable and possibly burst resulting in heart attack remains limited. However, with the advances in CTThe abbreviation for computed tomography, a scan that generates a series of cross-sectional x-ray images (computerised tomography) technology assessing the total plaque burden itself with information regarding the structure and components of the plaque is now becoming possible. Diagnostic tests that could be used to screen for individuals needing more intensive management would significantly increase the detection and prevention of coronary artery disease. This is particularly important for patients with type 2 diabetes as they are at higher risk compared to the general population.

Screening for coronary artery disease and the limitations with current screening methods

There are a number of screening methods for the diagnosisThe process of determining which condition a patient may have. of coronary artery disease and the main ones are as follows:

Coronary Artery Calcium Scoring

All the office based screening calculations assess a patient’s 10-year risk of developing coronary heart disease. Over the last few years, coronary artery calciumAn element that forms the structure of bones and teeth and is essential to many of the body's functions. (CAC) imaging, where a cardiac CT scan reveals information about the extent of hardened plaque in the coronary arteries, has been increasingly used to further assess a patient’s risk, particularly those classed in the intermediate risk category.  However, there is a poor correlation between CAC and the degree of narrowing of the coronary blood vessels and its ability to predict which patients will go on to suffer a medical event, although there has been a large amount of data demonstrating its utility in predicting cardiovascular survival over 7 – 10 years. Patients who are asymptomatic (not displaying symptoms) who are assigned to the high-risk group with office-based risk assessment calculations will benefit from intensive risk modification. Patients in the low risk group are advised to follow a healthy lifestyle and their risk factors should be treated as and when diagnosed according to current guidelines. Patients in the intermediate-risk group pose a clinical dilemma and it is this group of patients that benefits most from CAC scoring. The management strategy for patients who are classified into the intermediate risk category remains predominantly subjective and therefore using the CAC score gives us the tool to try and re-classify some of these patients into higher or lower risk groups, making management decisions easier and more evidence based.

Myocardial Perfusion Imaging (MPI)

Myocardial perfusion imaging is a scanning procedure that is able to illustrate the function of the heart muscleTissue made up of cells that can contract to bring about movement., or myocardium. There is also an extensive body of evidence that has accumulated over the last 2 to 3 decades regarding the usefulness of myocardial perfusion scintigraphy (MPS), which allows us to see how well blood is reaching the heart muscle through the coronary arteries, in the diagnosis and short term prognosisAn assessment of the likely progress of a condition. of coronary artery disease. The limitation of this technique arises from the fact that it is necessary for there to be significant (>50%) narrowing of blood vessels causing relative discrepancies in blood flow to the heart. A normal MPS does not provide any information regarding the extent of the plaque burden of the coronary arteries per se.

Despite these advances nearly 50% of patients with coronary artery disease will not receive medical attention before suffering a heart attack that may result in death. This is probably attributable to the fact that nearly two thirds of culprit plaque lesions that are responsible for acute coronary syndromes/death are not known to cause any significant coronary narrowing (>50%) and are not identifiable by current screening techniques.

CAC and MPI

Clinical studies have shown that combining Coronary Artery Calcium Imaging and Myocardial Perfusion Scanning provides an excellent method of detecting those at high risk of cardiac events (eg. death, heart attacks etc).

CT coronary angiography

Intravascular ultrasonographyThe technique of using high-frequency sound waves to produce internal images of the body. (IVUS), which is an ultrasoundA diagnostic method in which very high frequency sound waves are passed into the body and the reflective echoes analysed to build a picture of the internal organs – or of the foetus in the uterus. imaging technique that is used to visualise deep structures of the body, is the current gold standard for the assessment and characterisation of atherosclerotic plaque burden and structure. As the risk associated with conventional coronary angiography is small yet tangible (0.1-0.2%), there has been a steady push to find a non-invasiveAny test or technique that does not involve penetration of the skin. The term 'non-invasive' may also describe tumours that do not invade surrounding tissues. imaging technique that is capable of visualising the coronary vessel wall and sub-clinical atherosclerotic plaque lesions more effectively.

CTCA Technique

CT coronary angiography (CTCA) requires the injection of an iodine-based substance into the patient through a large veinA blood vessel that carries blood towards the heart. and then x-ray type images are taken. The actual scan time is less than 15 seconds in greater than 90% of patients. The image quality can be improved with the use of a special spray that is administered under the tongue approximately five minutes prior to the scan and also beta-blockersA group of drugs that block beta-receptors to slow the heart rate, or constrict the airways and arteries. to get a heart rate ideally of less than 65 beats per minute.

Clinical role of CT angiography

Contrast-enhanced coronary angiography is a relatively new application for CT scanners. Unlike the two-dimensional image of conventional invasive angiography, this technique also allows us to visualise the vessel wall and the plaque itself, and has the potential for plaque characterisation, bringing us a step closer to identifying the characteristics of a vulnerable plaque (plaque proneLying face-downwards. to rupture), which is the ‘holy grail’ of cardiology.

In order to get high quality diagnostic images of the heart in a reproducible manner, newer scanners such as the Dual-source CT scanner or 320 slice scanner, are better suited for imaging the coronary arteries and are superior to the 64-slice CT scanners. Though CTCA is able to identify the sub-clinical plaque, we are still at the early stages of research into plaque structure and its clinical relevance. Through a combination of Dual-source CT-coronary angiography (DS-CTCA) and the measurement of specific biomarkersA substance that can be measured to help healthcare professionals to assess normal processes, disease processes or a person's response to treatment., which are biochemical substances produced by the body that allow us to measure the progress of a disease, we are able to assess not only the total coronary artery disease plaque burden, but also the ‘stability’ of the plaque and possibly to identify those people who are at higher risk of rapid progression of the disease. We will shortly begin recruiting for a further trial in our centre that will focus on the early detection of coronary heart disease and the evaluation of the progression of the disease in asymptomatic high risk diabetic patients using a combination of DS-CTCA and novel biomarkers. 

NICE Guidelines for chest pain

The new NICE (National Institute for Clinical Excellence) Guidelines relating to “Chest Pain” (April 2010) has provided clear cut guidelines regarding the use of cardiac CT (both calcium imaging and CT coronary angiography) in combination with Myocardial Perfusion Scanning when required. We are at present conducting a large clinical trial to assess the clinical and cost-effectiveness of Dual-source CT coronary angiography in patients from 3 NHS hospitals referred to the chest pain clinics. These patients are recruited at the NHS site and then sent to the Wellington Hospital for CTCA and MPI (If needed). The DS-CTCA is combined with Myocardial Perfusion scanning when intermediate narrowing of the coronary arteries is noted on CTCA to guide management strategies. 

Conclusion

In summary, diabetic patients have a 4-fold increase in the chance of developing silent coronary artery disease compared to the general population. By 2020 there will be over 400 million diabetics worldwide, 70-80% of whom will suffer cardiovascular problems and coronary artery disease will become the number one killer worldwide (according to the World Health Organisation). It is therefore imperative that methods of early detection of coronary artery disease are established to combat this growing threat.