Shocking the heart

The electrical charge that triggers the pump.

Shocking the heart

Just like the family car, your heart needs a spark to start each heartbeat.

Creating the spark | A group of cells at the top of the right atrium, called the sinus node, spontaneously make a little spark known as an action potential. This then gets passed on to all the other heart cells. As it travels through each cell, the wavefront allows calcium ions to enter. These, in turn, trigger the contraction of the muscle proteins which generate the heartbeat. After each beat, the heart rests until the next electrical wave is generated.

While you are sitting quietly reading or watching TV, your heart is constantly doing twice as much work as your leg muscles would when sprinting.
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Defibrillator implanted in the chest. Image: Gregory Marcus, MD, MAS, FACC
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When the electrical system is not functioning well | The heart rate may be too slow, called bradycardia, or too fast, called tachycardia, or even chaotic.

When your electrical system is not coordinated, you may have abnormal heart rhythms, or arrhythmias. Many are harmless and don't need treating.

Others are troublesome and may be fixed by targeting the cause of the problem with radiofrequency energy, known as ablation.

Some are very dangerous and cause sudden cardiac death unless immediately treated.

Atrial fibrillation and flutter are common arrhythmias in older people. If you have atrial fibrillation and flutter, the upper chambers of your heart may beat as often as 300 times a minute - about four times faster than normal - due to rapid or irregular signals from the heart's electrical system.

Mending the broken electrical system | Inventing defibrillators and pacemakers was a huge step forward in saving lives. Sudden cardiac arrest caused by ventricular fibrillation causes half of all cardiac deaths.

Defibrillation, using paddles or electrodes to shock the heart back to normal rhythm, is the only effective treatment. In the 1960s, the electrodes needed to be placed directly on the exposed heart and surgeons carried a scalpel in their pocket for such emergencies. Later, external paddles could be placed on the chest. Now small defibrillators can be put inside the body. The first internal cardiac defibrillator was implanted in New Zealand in 1989.

Pacemakers use electrical impulses to regulate the heart's rhythm. Sid Yarrow, working at Green Lane Hospital's laboratory, built a temporary pacemaker to be placed on the heart during surgery. As technology advanced, pacemakers got smaller and safe to implant. New Zealand's first pacemaker was implanted in 1961.

Current research on dangerous electrical turbulence at the cellular level hopes to discover how and why life-threatening chaotic fibrillation develops in the heart and ultimately prevent this happening.