The development of sophisticated heart monitoring instruments has made it possible to record every heartbeat during a 24-hour period while a person goes about his normal activities. The recordings of the heartbeats, known as the electrocardiogram or ECG for short, are picked up by wires stuck to the chest and recorded on a cassette in a special recorder worn around the waist. The tape is played back through an analyser that detects abnormal rhythms and possible myocardial ischaemia.
We made a 24-hour recording of the heartbeat of a woman taking part in an experiment to investigate factors affecting heart activity. At 8.30 p.m. there was a change in the shape of part of the ECG, known as the ST segment. Changes in this segment usually indicate myocardial ischaemia. The woman had been asked to keep a 24-hour diary of activities and feelings during the experiment and at around 8.30 p.m. she had recorded a short period of intense emotional arousal soon after she missed her last train home. She reported I was very angry with myself for getting the departure time wrong. I thought it was 8.30, in fact it was 8.15.’ The woman had no diagnosed heart disease nor any indication of heart problems yet at 8.30 p.m. a minor myocardial ischaemic episode occurred. What is more, she was completely unaware of it happening and for this reason such an attack is known as silent myocardial ischaemia.
Many experiments have used this technique to assess the effect of stress on the heart function of normal healthy individuals and it appears that stress can cause abnormal heart rhythms and myocardial ischaemia severe enough to induce a small silent heart attack!