The Spark of Life: Electricity in the Human Body

by Ashcroft, Frances

  7

  The Heart of the Matter

  Be still, my heart; thou hast known worse than this.

  Homer

  Early one summer morning, Alex was getting ready for school. Although she was anxious about her exams later that day, she was not unduly stressed and there was nothing that marked the day out as unusual. At least not until she went into the bathroom, reached out to turn on the light – and slid silently unconscious to the floor. Luckily, her mother saw it happen and rushed to the rescue. But this was no simple fainting fit. Alex had a serious heart problem and her increasingly frantic mother was unable to resuscitate her.

  By chance, Alex lived close to a fire station and a local fireman picked up the emergency call. He quickly rushed to the rescue and administered cardiopulmonary resuscitation until the ambulance arrived, thus ensuring that her brain and other tissues were supplied with oxygen even though her heart was not working properly and she was no longer breathing. During the journey to the hospital her heart stopped and was restarted more than once. She was unconscious for seventeen hours but eventually recovered.

  Subsequent analysis revealed that Alex has an abnormality in the electrical activity of her heart that predisposes her to blackouts and sudden cardiac death. It runs in the family. Her grandmother died in her sleep in her twenties, and her father suffered numerous fainting spells as a child and died young, just a year before Alex’s attack. It seems very likely that they carried the same genetic defect that Alex does.

  Alex and her relatives are not alone. Other families have experienced similar tragedies, with one or more children or young adults dying in their sleep, when taking exercise, or when stressed. There are even tales of children suddenly collapsing when reprimanded by their teachers, or while running around in the playground. It is not an exaggeration to say that some children with this condition really have died of fright. Happily, our increased understanding of the electrical activity of the heart means that this disease can now be diagnosed from an electrocardiogram or by a simple genetic test, and it can also be successfully treated.

  The Beat Goes On

  It has been known for centuries that the heart has an intrinsic rhythm and can continue to beat when it is removed from the living animal. One of the first to describe the phenomenon was the great Roman physician Galen, and subsequently many others, including Leonardo da Vinci, reported that the heart moves by itself. William Harvey even showed that when the heart of an eel was cut into ever-smaller parts each individual piece continued to pulsate. This intrinsic activity may have inspired the classical Greek idea that the heart was the seat of the soul. However, the heartbeat has no spiritual origin, but derives instead from electrical events taking place within the cardiac cells themselves.

  In essence, your heart is a pump that is controlled by electricity. Blood enters via the upper chambers (the atria), which contract first and force blood into the much larger lower chambers (the ventricles). The ventricles contract in synchrony about half a second later, the right ventricle pumping blood to the lungs and the left ventricle sending it around the body.

  Non-return valves lie between the upper and lower chambers of the heart so that the blood only flows in one direction; from the atria to the ventricles. Similarly, non-return valves guard the exits from the ventricles into the great vessels. If these valves leak, as can happen with age, then blood is pumped less efficiently, so that the body receives less oxygen and you feel constantly tired. The chambers on the right and left side of the heart are physically separate which ensures that oxygen-rich blood coming from the lungs is not mixed with oxygen-depleted blood returning from the tissues. However, because heart cells are wired together, they contract in synchrony, so that the heart beats as a single organ.

  The electrical system of the heart. The pacemaker cells lie within the sinus node in the wall of the right atrium. The black arrowed lines indicate the bundles of fibres that provide the path along which the electrical signals are conducted to the lower chambers (the ventricles). The two sides of the heart are physically separate but contract together. The pulmonary artery carries blood from the right side of the heart to the lungs. Having picked up oxygen there, blood is returned to the left side of the heart, which then pumps it into the aorta and around the body. The period when the heart is contracted is known as ‘systole’ and the time it is fully relaxed as ‘diastole’.

  Each heartbeat originates in a group of pacemaker cells (known as the sinus node), which lie in the upper right chamber of the heart. These cells generate electrical impulses that are conducted to the rest of the heart along specialized pathways: first to the atrio-ventricular node, which lies at the junction between the right atrium and the ventricles, and then to the walls of the ventricles themselves. The time lag associated with electrical transmission ensures that the electrical signals reach the upper chambers before the lower ones, so that first the atria are triggered to contract, and then the ventricles. The timing of this spread of excitation is crucial for the ability of the heart to serve as a pump. If it is disrupted, the heart no longer beats regularly and its capacity to pump blood is compromised.

  Although the average heart rate at rest is 70 beats a minute (that’s about 100,000 beats a day), it varies widely between individuals. Athletes have much lower resting heart rates, often as little as 40 beats a minute. One of the lowest ever recorded, a mere 28 beats per minute, was that of the cyclist Miguel Indurain, who won the Tour de France five times in a row. By contrast, babies’ hearts beat much faster than adults, speeding along at 130 to 150 beats a minute. It turns out that heart rate varies with body size, so that smaller animals (including babies) have higher resting heart rates: the heart of the tiny shrew races away at 600 beats a minute while that of the elephant can only manage a ponderous 25 beats a minute.

  The Electrocardiogram

  The electrical signals produced by heart cells give rise to tiny fluctuations in the electrical potential at the surface of the body that can be picked up by surface electrodes attached to the skin. This is the basis of the electrocardiogram, commonly abbreviated as ECG (or EKG in the United States).

  August Waller’s pet dog Jimmie was the most popular personage at the annual conversazione of the Royal Society at Burlington House. This scientific party for both scientists and the general public is still held and it traditionally includes many demonstrations. Jimmie stands sedately with his left paws in a conductive salt solution, which is connected to an Einthoven string galvanometer (the large box on the left) that measures his every heartbeat. The string is illuminated by limelight so that its shadow is projected on a sheet, and it vibrates with the bulldog’s heartbeat. The experiment is not painful, as many of the audience discovered when they volunteered to take Jimmie’s place. August Waller is seen on the far left.

  The electrical activity of the heart was first recorded by Augustus Waller in 1887 both in himself and in his pet dog Jimmie. His demonstration of the method at the annual conversazione of the Royal Society of London in 1909, which was open to the public, was reported in the Illustrated London News. It triggered a storm of protest in Parliament, with Mr Ellis Griffith, the MP for Anglesey, demanding to know if the Cruelty to Animals Act of 1876 had been contravened. The Times stated the Secretary of State, one Mr Gladstone,1 replied, ‘I understand that the dog stood for some time in water, to which sodium chloride had been added, or, in other words, a little common salt. If my hon[ourable] friend has ever paddled in the sea, he will have understood the sensation. (Laughter.) The dog – a finely developed bulldog – was neither tied nor muzzled. He wore a leather collar ornamented with brass studs [this had been referred to by Mr. Griffith in far more emotive terms as “a leather strap with sharp nails [. . .] secured around the dog’s neck”]. Had the experiment been painful, the pain no doubt would have been more immediately felt by those nearest the dog. (Laughter.) There was no sign of this.’ He might have added that after Jimmie had shown the way, the ladies in the audience queued up to h
ave their heartbeats recorded, by dipping their hands in pots of salt solution and ‘their hearts were in every case much steadier than Jimmie’s’. As this story also shows, the English concern about animal experimentation has a long history.

  Waller’s early recordings were of poor quality and unsuitable for clinical purposes, and he is reputed to have said that he did not imagine electrocardiography was likely to find any very extensive use in the hospital and that, at most, it might be ‘of rare and occasional use to afford a concrete graphic record of some rare anomaly of cardiac action’. But technical innovations meant that by the 1920s it was routinely used to diagnose heart problems, and it remains an important clinical tool today.

  The key was the development of very sensitive instruments, capable of detecting the tiny electric currents produced on the surface of the body when the heart beats. The pioneer in this field was Willem Einthoven, who won the Nobel Prize in 1924 for his invention of the string galvanometer. It consisted of a fine glass fibre, coated with silver to ensure that it could conduct current, which was suspended between two very strong electromagnets. When a current passed though the filament (the ‘string’ of the galvanometer), the electromagnetic field caused it to move. The greater the current, the more the filament was displaced. This tiny movement was detected by shining a light on the fibre, and the shadow it cast was recorded on a moving photographic plate. All that was needed was to connect the conductive filament to the body. This was done by attaching wires to each end of the filament and immersing the other end of the wires in pots of salt solution. Dipping the hands and feet in the solution completed the electrical connection between the ‘string’ and the skin. Current from the heart, picked up via the surface of the body, was then able to influence the movement of the filament.

  The original string galvanometer was huge. It weighed several tons, took five people to operate and required constant running water to cool the electromagnets. The glass filament, however, had to be very light and thin. It was made by melting quartz glass in a crucible. This was then drawn out into a fine filament by a most unusual means – one more reminiscent of a Boy Scout improvisation than the conventional image of a scientific experiment. The molten glass was attached to an arrow that was then shot across the room, dragging the filament with it and stretching the glass into a very fine ‘string’. This was then coated with silver to make it electrically conductive. Safety considerations would undoubtedly ban this experiment today, but fortunately we now have other methods to record tiny currents.

  Early photographs show Einthoven sitting with both hands and his left foot (trouser leg carefully rolled up) in separate tubs of conductive salt solution, which were wired up to the monitoring equipment. Today, conducting jelly is used to attach the recording electrodes, one on each arm and one on the left leg. The equipment has also got a lot smaller. Einthoven’s original machine occupied two rooms, but nowadays portable twenty-four-hour heart monitors are available that can be worn as the patient goes about their normal life.

  The ECG simply reflects the sum of the electrical signals from individual heart cells and it provides a very good, non-invasive indicator of their function. Each ECG complex consists of an initial bump known as the ‘P wave’, followed by a much larger and sharper bipolar peak known as the ‘QRS complex’ and then, two to three hundred milliseconds later, by the smaller and slower ‘T wave’. The P wave corresponds to the electrical activity of the atrial cells, while the QRS and T waves reflect the beginning and end of the electrical impulse (the action potential) in the ventricular cells. Since these electrical signals drive muscle contraction, the P wave also signifies the contraction of the atria and the interval between the QRS and T waves indicates the duration of ventricular contraction. The delay between the P and Q waves is due to the length of time it takes for the electrical signal to spread from the atria to the ventricles, whereas the interval between the Q and T waves reflects the duration of the ventricular action potential. Why Einthoven should have chosen to name the ECG peaks after the middle letters of the alphabet remains something of a mystery.

  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.