The Spark of Life: Electricity in the Human Body (19 page)

Relationship between the ventricular action potential (AP, upper trace), the electrocardiogram (ECG, middle trace), and the contraction of the heart (lower trace). ‘A’ indicates the duration of atrial contraction and ‘V’ that of ventricular contraction. The QT interval reflects the duration of the ventricular action potential.

The ECG is particularly valuable for detecting irregularities in the electrical activity of the heart and for diagnosing their origin. Changes in the amplitude and timing of the various components can indicate clinical problems. A PR interval that is longer than normal, for example, signals a conduction defect between the upper and lower chambers of the heart known as heart block. An inverted T wave is seen following a heart attack. And an increase in the QT interval is associated with an increased risk of sudden cardiac death.

Sick at Heart

Although the sinus node cells of the right atria usually serve as the pacemaker, all heart cells are capable of generating electrical activity spontaneously. This is fortunate as it means the heart does not stop if the sinoatrial node cells cease to function: other cells, which beat with a slower rhythm, take over. These include the atrioventricular node cells that sit between the atria and ventricle, which contract 40 to 60 times a minute, and the cells that form the conduction pathways within the walls of the ventricles (which beat 30 to 40 times per minute). Even the ventricular cells contract spontaneously. The reason the sinus node cells normally set the pacemaker rhythm is simple: their intrinsic rate is the fastest.

If your heart beats too slowly (a condition known as bradycardia), it will be unable to supply blood quickly enough to your tissues and you will feel tired, weak, dizzy, and short of breath. Walking or climbing stairs becomes a struggle. Tachycardia, when the heat beats too rapidly, is also a problem. At resting heart rates of over 100 beats per minute there is insufficient time for the heart to fill fully between contractions, reducing the amount of blood that can be pumped. Consequently, the tissues will again be short of oxygen and you will be permanently exhausted.

An occasional irregular heartbeat is rather common and many people will have experienced the odd missed beat. In fact, the beat is not really missed – it simply feels as if it is. What actually happens is that a beat arrives early and is not detected because the heart is only half full: there is then an unusually long pause until the next beat, which is more obvious because the heart is then over-filled. Such ‘missed beats’ are very common, but despite being rather alarming they are of no importance. Although most happen spontaneously, they can also be trigged by stress or drugs such as caffeine.

The most common type of abnormal heartbeat is atrial fibrillation (AF), which affects around 5 per cent of the population over sixty-five. In this condition, the upper chambers of the heart beat erratically and out of synchrony. This happens if the electrical activity of the sinoatrial node cells is disturbed, or if the spread of electrical excitation through the atria is impaired by tissue damage. If the atria beat asynchronously, their ability to force blood into the ventricles is reduced and cardiac output is compromised, causing the patient to feel faint. It also produces a pulse that appears to flutter erratically. Atrial fibrillation can lead to blood clots, which enhance the risk of a stroke, because a clot may lodge in the blood vessels of the brain, cutting off the blood supply to downstream tissues and causing their death (which is why stroke victims may find they cannot speak, or part of their body is paralysed). Normal cardiac rhythm can sometimes be restored by drugs, or by a mild electric shock (a process known as cardioversion), but if atrial fibrillation persists an artificial pacemaker may be needed.

One of the newer treatments for atrial fibrillation is the removal of a small region of atrial tissue, which blocks the circular pattern of electrical activity that underlies the problem. This is usually very effective and recurrence of atrial fibrillation with this treatment occurs far less frequently than with drugs. It can be carried out using a catheter that is inserted into a vein and threaded through the blood vessels to the correct place in the heart. An energy source, such as high frequency radiowaves, is then transmitted via the catheter to selectively destroy the targeted cells.

A more severe condition is heart block, where damage to the conduction pathways means that passage of the electrical signal from the atria to the ventricles is impaired (note it does not mean that the vessels of the heart are blocked). In total heart block, transmission of the atrial signal is completely prevented. Consequently, the ventricles take over, which means the heart rate may fall as low as 30 beats per minute and the patient will have severe difficulty in exercising. Thus an artificial pacemaker is essential.

The most serious arrhythmia of all is ventricular fibrillation (VF), which is fatal if not corrected. In this condition there is electrical chaos, with many regions in the lower chambers of the heart fighting for control of the rhythm. As a result, the ventricles beat so asynchronously that the whole heart appears to quiver continuously, but never contracts properly. It looks, the great sixteenth-century anatomist Vesalius said, like a writhing bag of worms. No significant cardiac output is possible when this happens, so that the heart soon stops through lack of oxygen and the patient dies within minutes. Even before the heart stops, the brain will have been irreversibly damaged by oxygen deprivation. In such a situation, the only hope is to restore normal rhythm immediately. For this it is necessary to stop the heart by administering an electric shock with a defibrillator and hope that it will revert to normal rhythm when it spontaneously restarts – a bit like pressing the reset button on a computer.

A heart attack results from disruption of the blood supply to the heart and is commonly caused by blockage of one of the coronary arteries. As the tissues downstream of the blockage are deprived of oxygen they start to die. This may trigger ventricular fibrillation because the resulting tissue damage prevents the synchronized spread of electrical signals across the heart. Different groups of heart cells then go their own way and start to beat at different times. As in any society, cooperation between the component parts of the heart is vital for it to work effectively.

Restoring the Rhythm

If the heart beats irregularly, an artificial pacemaker is often used to correct its rhythm. Early pacemakers were large and bulky machines, about the size of a washing machine, and they were supplied with mains electricity. Consequently, the patient could not move around easily. They also had another disadvantage: they stopped when the electricity supply failed. In the 1950s, Dr C. Walton Lillehei was doing pioneering open-heart surgery on ‘blue babies’ at the University of Minnesota. These children were born with a hole between the left and right ventricles, which results in the blood bypassing the lungs, so that oxygen uptake is much reduced. After surgery to repair the hole, some babies suffered from short-term heart block; tissue damage meant that the electrical signals from their sinoatrial node did not reach the ventricles and their hearts failed to beat properly. In such cases, Lillehei used an artificial pacemaker machine until the child’s heart healed. This usually took one or two weeks.

Unfortunately, a major power blackout in Minneapolis in October 1957 resulted in the death of one of the ‘blue babies’. Infuriated, Lillehei contacted Medtronic, the electronics company that made the machines, and asked for something that ran on batteries. He was in for a surprise, for in less than a month their engineer Earl Bakken returned with an artificial pacemaker that did indeed run on batteries – but it was now shrunk to the size of a sandwich. Transistorized circuits were the key to this miniaturization.

Bakken wrote in his autobiography,
One Man’s Full Life
, ‘Back at the garage, I dug out a back issue of
Popular Electronics
magazine in which I recalled seeing a circuit for an electronic, transistorized metronome. The circuit transmitted clicks through a loudspeaker; the rate of the clicks could be adjusted to fit the music. I simply modified that circuit and placed it, without the loudspeaker, in a four-inch-square, inch-and-a-half-thick metal box with terminals and switches on the outside – and that, as they say, was that.’ He had intended his prototype as an experimental device for testing on animals and was stunned to discover, when he visited the hospital the next day, that it was already being used on patients. Lillehei calmly informed him that as the device worked he didn’t want to waste a minute before using it to help save patients’ lives. It was so successful that similar pacemakers were soon introduced throughout the world, and Medtronic became a major supplier.

Just one year later, the first implantable pacemaker was used, in a forty-three-year-old Swedish patient called Arne Larsson. Arne suffered from complete heart block and his death seemed inevitable. His wife, however, had other ideas. She had heard of experiments being carried out on dogs at the Karolinska Hospital in Stockholm and decided that the technology might save her husband. Apparently she was extremely persuasive because she convinced the surgeon Åke Senning and the engineer Rune Elmqvist to help. Rune built the pacemaker in his kitchen. It failed within three hours of implantation, so Arne was given another one the next morning, which lasted just a few weeks. These failures did not put him off, however, and he eventually received twenty-six different pacemakers. The pacemaker enabled him to lead an essentially normal life – and he made good use of it, acting as a patient advisor and advocate for pacemakers throughout the world. He died forty-three years after receiving his first pacemaker, at the age of eighty-six, his bravery and willingness to act as a human guinea pig having doubled his lifespan.

The concept of the artificial pacemaker is very simple. The pacemaker supplies a small electric current which substitutes for the heart’s own. This is achieved by inserting a wire into the right ventricle of the heart. It is usually threaded into place through one of the great veins, but in some cases the chest is opened and the wire placed directly on the heart’s surface. The lead is then connected to the pacemaker, which applies small electric shocks to drive the heart at the right rate. The pacemaker also contains a battery and, sometimes, electronic circuits that can sense the patient’s own heart rhythm and adjust it as needed. Once it is clear the device is working, it is implanted in the chest (usually in front of the shoulder), between the muscle and the subcutaneous fat. The first pacemaker Arne received was the size of a hockey puck, but today they can be as small as a ten-pence piece. They need replacing every five to ten years, depending on how long the battery lasts. As electromagnetic interference can cause pacemakers to malfunction, people with pacemakers must avoid high magnetic fields, cellphones and electronic equipment that generates stray electric fields.

Packer Whackers

Everyone is familiar with the typical emergency-room drama in which the patient is surrounded by a throng of medical staff, desperately working to save their life. Suddenly, the regular beep of the heart monitor ceases, the normal ECG vanishes to be replaced by a flat line, and someone screams ‘Arrest!’ Controlled panic ensues. Within seconds, large paddles are slapped onto the patient’s chest and with a warning cry of ‘Clear!’ an electric shock is administered. The patient’s chest jerks violently, the heart is restarted and the heart monitor springs into action once again.

But this dramatic scene is far from accurate. There is usually no jerking of the patient in response to the electric shock – this is mere poetic licence. More significantly, in real life an electric shock is not used to restart a patient’s heart. Dramatic resuscitations are commonplace in modern medicine, but they do not occur in patients whose hearts have stopped, but rather in those whose hearts are fibrillating – whose ventricles are beating so asynchronously that the heart is reduced to a twitching lump of flesh that is quite unable to pump blood. And the electric current is not used to start the heart, but to stop it. As previously mentioned, the hope is that when the heart spontaneously restarts, the natural pacemaker cells in the sinus node will take over and the normal rhythm will be restored.

It is possible the popular misconception has arisen from the use of the term ‘cardiac arrest’. This does not mean, as might be surmised, that the heart has stopped contracting and is totally still. Rather it refers to the fact that blood flow is arrested. Although individual heart cells continue to contract, they fail to do so in synchrony so that the heart is no longer an effective pump. Within a few minutes the brain dies because of lack of oxygen, and eventually the heart itself ceases to beat for the same reason. Unless the victim suffers a cardiac arrest in hospital, cardiopulmonary resuscitation is required to keep them alive until a defibrillator arrives. Artificial respiration is carried out and the heart is manually compressed by pumping the chest with the heels of the hands, forcing blood out of the heart and around the body. The right speed is vital – too fast and the heart has insufficient time to refill between compressions, too slow and the tissues suffer from lack of oxygen. A rate of 100 compressions per minute is just right. Serendipitously, the Bee Gees’ hit song ‘Staying Alive’ has almost exactly the right rhythm and has been used as a training aid for doctors. Although it also has a near-perfect beat, ‘Another One Bites the Dust’, by Queen, seems rather less appropriate.