Understanding heart attacks

IN THE last 10 years, new knowledge of the mechanisms of a heart attack have broadened the understanding of what is defined as a heart attack and what causes a heart attack. The Fourth Universal Definition of Myocardial Infarction (heart attack) was released in 2018 and represents the combined opinion of the major cardiology organisations of the world.

I had the privilege of participating in the Second Universal Definition of Myocardial Infarction, which was released in 2007. In the latest Universal definition, myocardial infarctions are divided into five categories.

The typical heart attack - occlusion by blood clot (Type 1 myocardial infarction)

The commonly held belief is that if you have multiple risk factors for heart disease - such as smoking, high cholesterol, diabetes mellitus, high blood pressure and/or a family history of heart disease - you are likely to get a gradual narrowing of the heart arteries resulting from a build-up of cholesterol deposits (plaque) in the walls of the heart arteries. This is a condition also known as coronary artery disease (CAD).

This narrowing may become so severe that the increased pressure and shear stress from the turbulent blood flow across the narrowed segment may result in a tear in the lining of the wall - that is, plaque rupture. This exposes the plaque contents to the blood, thereby triggering the formation of a blood clot.

The occlusion of blood flow to the heart muscle results in a heart attack. This conventional model of a heart attack is encompassed within the first category of heart attack (Type 1 myocardial infarction) in the latest Universal Definition.

It includes spontaneous heart attacks which result from the sudden blockage of the heart arteries following plaque rupture and blood clot formation.

Typically, these patients may have chest pain or unexpected shortness of breath on exertion.

Heart attack in absence of blood clot occlusion (Type 2 myocardial infarction)

Although severe blockage of the heart arteries is present in most of these heart attacks, some studies have shown that there was no evidence of obstructive disease in the heart arteries of up to 20 per cent of patients studied, especially in women. We now know that heart attacks can occur in the absence of plaque rupture and blood clot formation.

Hence, heart attacks can occur even in the absence of risk factors for heart disease, such as smoking, high cholesterol, diabetes, high blood pressure or a family history of heart disease.

The global panel of experts recognised this and has created a separate category - Type 2 myocardial infarction - for heart attacks resulting from an imbalance between oxygen supply and demand to the heart due to causes other than severe narrowing of the heart arteries.

The reduction in blood supply to the heart can result from spasm or spontaneous tear of the heart artery (more common in women), sudden blood loss, severe anaemia, and/or significant drop in blood pressure.

The increased demand can result from a sustained fast abnormal heart rhythm, severe hypertension, or abnormally thickened heart muscle.

Patients who develop this type of heart attack will generally not have the typical chest pain on exertion.

For example, in those with spasm of the heart arteries, there may be central or left-sided chest tightness occurring spontaneously but not specifically related to exertion.

Heart attack with sudden death (Type 3 myocardial infarction)

The third category, Type 3 myocardial infarction, encompasses patients who suffer cardiac death and provides for patients who suffer an acute heart attack and die before further blood tests or other confirmatory tests can be done.

This is mainly for the purposes of classification and has no implications in the clinical setting.

Heart attack related to a heart procedure (Type 4 myocardial infarction)

Type 4 myocardial infarction refers to heart muscle injury related to the procedure of opening heart arteries using balloon dilatation and stents (cylindrical mesh expanded to keep the artery lumen patent), blood clot occlusion of stents, or the narrowing of heart artery segments which were previously treated with balloon dilatation or stent placement or other percutaneous heart procedures.

Hence, the designation of this as a separate category of heart attack means that if you are undergoing a procedure to open up your heart arteries, you should understand the heart attack risks associated with that specific procedure, the potential risk of occlusion of the heart stents by blood clot should you not take your blood thinning medications for the initially prescribed period, and the future risk of the treated heart segment narrowing again. All these are real risks for anyone undergoing procedures to open your heart arteries with balloon dilatation or stenting.

Heart attack associated with heart artery bypass graft surgery (Type 5 myocardial infarction)

As stated in the Fourth Universal Definition of Myocardial Infarction document, injury of the heart muscle, as detected using magnetic resonance imaging (MRI), has been observed in 32 per cent to 44 per cent of patients who have undergone heart artery bypass graft surgery. This heart muscle injury is categorised as Type 5 myocardial infarction

Preventing heart attacks

The diagnosis of heart attack is premised on the presence of acute heart muscle injury, which can be objectively documented with blood markers of muscle injury such as cardiac troponin and the presence of acute heart muscle ischaemia which can be documented with serial electrocardiograms (ECGs), which are recordings of the electrical pattern of the heart.

Cardiac troponin I (cTnI) and T (cTnT) are components of the contractile apparatus of heart muscle cells. Increases in cTnI values are exclusive to the heart. However, cTnT can be elevated when skeletal muscle is injured. Hence, if you have chest tightness

and/or shortness of breath that suggest that you may have a heart attack, measurement of the cardiac troponins and doing an ECG will help the doctor make a diagnosis.

The fact that heart attacks can occur in the absence of significantly blocked heart arteries means that if you have recurrent chest tightness even if your treadmill tests are normal, your doctor may consider further tests. These may include ambulatory or mobile recorders that can allow the capture of ECG patterns during episodes of chest pain and allow early diagnosis of conditions such as spasm of the heart arteries.

Beyond the usual factors for heart disease, stress can also be a cause of heart attack too. The broken heart syndrome - or takotsubo cardiomyopathy - can strike healthy individuals, more than 90 per cent of whom are post-menopausal women.

A sudden surge in stress hormones in response to severe emotional stress can result in intense chest pain. The stress may be precipitated by bereavement, social upheaval, betrayal or the break-up of relationships. It presents like a typical heart attack, but the heart arteries do not show any blockage.

Controlling risk factors, managing stress, maintaining hydration, living a healthy lifestyle and seeking medical attention when you have chest pain will go a long way towards preventing a heart attack.

• This article is produced on alternate Saturdays in collaboration with MWH Heart, Stroke & Cancer Centre