Each heartbeat is produced by a vigorous wave of contraction of heart muscle which sweeps over the entire heart. This contraction requires energy, provided by a steady supply of blood delivered by the coronary arteries. A heart attack occurs when one of these arteries is suddenly blocked by a blood clot. The precise sequence of events has now been followed in detail when patients are under close observation in an intensive care unit. Blocking a coronary artery immediately deprives a wedge of heart muscle of its blood supply. It is known from studies in animals that this produces a vigorous discharge in the fine nerve fibres that supply the heart and deliver the messages to the spinal cord. The intensity of the volley of nerve impulses rises in seconds and then falls off in minutes.
The immediate reaction of patients is surprisingly diffuse. They report a deep stomach discomfort but also a feeling that something is very, very wrong. I have spoken to two friends who are pain doctors who have had the modern operation in which a balloon is pushed into one coronary artery to widen it. They both report no pain to their surprise but an awful feeling of intense terror of impending death. The fine nerve fibres from the heart do not connect to nerve cells that permit precise localization, but they do set off a general feeling of anguish, nausea, sweating and breathlessness.
This is just the beginning for the heart-attack patient because the arriving volley of nerve impulses in the spinal cord sets up a slowly spreading excitation of nearby nerve cells that normally react to the chest, arms and neck. As this 'bush fire' spreads, the patient reports something large slowly expanding in the chest, reaching the surface of the chest like a giant clamp, and spreading down one arm, usually the left, and up into the neck with agonizing pain. Of course, at this stage the progress of the pain may be brought under control by the use of analgesics.
Here we have looked at the natural history of a very specific pain, angina pectoris, in which no pain is located in the heart itself, all the pain being referred to structures whose reporting nerve cells lie close to the cells receiving from the heart's nerve fibres. Angina may occur repeatedly in less dramatic circumstances in which the coronary arteries are partially clogged. Here the blood supply to the heart is adequate at rest but, when the patient begins to exercise, the heart has to beat more frequently and more strongly. The narrowed arteries cannot supply enough blood for the increased demand, the nerve fibres in the heart signal that the oxygen supply is inadequate, and the patient feels the typical pains of angina until he rests or takes medicine to boost the heart's blood supply.
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