The rhythm of the heart is dictated by a fascinating electrical, chemical and muscular timekeeping mechanism. This too can go wrong, producing disorders of rhythm or arrhythmias. The pathways along which the signal from the pacemaker cells is sent must be kept open and clear, since otherwise the message may become garbled. If the route is diseased this is exactly what happens: the muscle cells receive instructions to beat either too quickly (tachycardia) or too slowly (bradycardia). In addition, irregularities such as extra beats and atrial fibrillation can occur. Some can be controlled by a regime of drugs, while others benefit from an artificial pacemaker – still others can be prevented altogether.
If, for some reason, the sino-atrial node fails, any other part of the heart can take over its function and keep the heart contracting – albeit at a slower rate. This is seen dramatically in a condition called atrio-ventricular block where, although the heartbeat originates normally in the right atrium, it cannot be conducted down the Bundle of His and so the ventricles produce their own ‘node’. The condition stabilizes with the atria beating at their usual 60 to 70 beats per minute and the ventricles beating at a much slower rate, say 40 to 50 beats per minute. Since the ventricular contractions produce the pulse which is felt at the wrist or neck, this condition can be detected by a slowing of the pulse-rate. If the slowing is severe enough to cause symptoms such as blackouts, the ventricles will require help to speed them up a little, and a pacemaker will probably be inserted.
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