Cardiac sudden death often results from a rhythm disorder
FROM time to time in any medical practice, you hear about patients who unexpectedly ‘‘ drop dead’’. While the great likelihood is they had a cardiac cause of their sudden death, the commonly ascribed cause of a ‘‘ massive heart attack’’ reveals some confusion about what this frequently used term actually means.
Outside the context of sudden death, people often use the words heart attack to refer to an episode of severe chest pain, perhaps radiating to the jaw or arm, often accompanied by sweating, breathlessness, faintness and generally feeling awful. This basically correlates with the medical term myocardial infarction — myocardial meaning heart muscle, and infarction meaning tissue that has died as a result of lost blood supply.
It is the most serious manifestation of ischaemic heart disease, ischaemia meaning underperfusion with blood. The underperfusion comes about from atherosclerosis — cholesterol-rich atheromatous plaques building up on the lining of the coronary arteries and clogging them up.
Myocardial infarction typically occurs when these plaques rupture, exposing the underlying material to the blood and triggering the formation of a clot that completely blocks off the artery. The resulting prolonged deprivation of oxygen and other nutrients to the surrounding heart muscle causes death of the affected fibres, which are eventually replaced by scar tissue that (unlike muscle) cannot contract.
Depending on how much muscle tissue is damaged, the pumping ability of the heart may be affected. If reduced seriously enough, this will result in heart failure, a condition where the heart is unable to pump enough blood to adequately meet the needs of the rest of the body.
Lesser degrees of ischaemia occur during an attack of angina. Angina occurs when there is a reversible mismatch between the heart muscle’s demand for blood and the artery’s capacity to supply it. Typically this happens when the heart works harder during exercise, but the narrowed artery is unable to carry any more blood. The affected muscle experiences a relative lack of oxygen, resulting in similar pain, but does not die and recovers with rest.
In real life, of course, things are more complicated. Not all myocardial infarcts are painful, and it is not always possible to tell if someone is having bad angina or an infarct — we often need blood tests and an ECG (electrocardiogram, a readout of the heart’s electrical activity over a period of time).
But while infarcts can cause heart failure, and sudden death is very much associated with ischaemic heart disease, death is not usually the direct result of widespread ischaemic muscle damage. Rather it results from a problem of cardiac rhythm.
The heart is unique among muscles in that it generates its own rhythmic contractions. While heart rate is influenced by nerve activity and hormones like adrenaline, the heart can beat by itself. It has its own pacemaker, the sino-atrial node, and a network of specialised fibres for spreading impulses throughout the heart in a way that results in the synchronised contraction needed for effective pumping.
A heart beating in this normal way is said to be in sinus rhyhthm, usually at a rate between 60 and 100 beats per minute at rest.
There are numerous disorders of cardiac rhythm, the most serious of which is called ventricular fibrillation. In this state the muscle of the main pumping chambers of the heart twitches rapidly and lacks co-ordination, and effective circulation ceases abruptly. The patient collapses, loses consciousness, and without intervention brain death ensues shortly after. Often, but not invariably associated with ischaemic heart disease, this is the mechanism of most cardiac sudden death.
There are many other, less lethal, rhythm disturbances where the heart beats too fast, too slowly, or with less co-ordination. They may cause blackouts, weakness or palpitations. Sometimes you may not have symptoms and the most significant problem is not cardiac dysfunction, but the associated risk of stroke. So along with your blood pressure, it’s always worth getting your pulse checked for rate and rhythm, and having an annual electrocardiograph. Simon Cowap is a GP practising in Newtown, Sydney