In the first in a series of understanding ECGs Dominic Cox and Hamish Dougall take you through the basics

Are electrocardiograms (ECGs) difficult? Should we believe the titles of numerous texts written on the subject, which proclaim "ECGs Made Easy," "ECGs Made Simple," or "ECGs So Simple Your Granny's Cat Could Do Them"? We'll let you into a secret. Reading ECGs is not that difficult. A medical student can make the majority of important diagnoses just as well as a cardiologist and probably far better than the average orthopaedic surgeon. (The definition of a double blind study is an orthopaedic surgeon and an ECG.)

At present you may run for cover on seeing an ECG. We hope to show you that there is no need to do this. If you experience a combination of confusion, apathy, and palpitations on hearing strange words, like ECG mapping, signal averaging, and QT dispersion by a consultant cardiologist on a ward round, take heart. Such discussions are intellectual twaddle and completely unnecessary for the majority of practising doctors to understand. All you need is to understand the basics of the ECG and the rest is simple pattern recognition. We hope to show that, with a simple approach to ECGs, you will no longer fear these "squiggly lines" and that they will aid and enhance your understanding of patients on a daily basis in whatever area of medicine you practise.

The engine itself, its control, and power supply

The heart is a simple, robust, and clever machine that beats around 3.2 billion times during an average life. It adapts to its situation by becoming more efficient the more it is used. It never stops (we hope) and it takes quite a battering from our lifestyle. A reasonable analogy in understanding the heart and therefore ECGs is to think of the heart as the three separate and interlocking systems involved in running a motorcar. If you understand the basic function and physiology of the heart and apply this as you learn to read ECGs then the whole subject area should click effortlessly into place in your grey matter.

figure 1	figure 2

The pumps (the engine)

This is what delivers the end product. In this case, blood to vital organs and tissues in the body. Each contraction (ventricular systole) sends blood to the body from the left ventricle (LV) through the aorta and, via a lower pressure system, to the lungs via the pulmonary artery. Because the pulmonary circulation is in such close proximity to the heart, only relatively small pressures are required (15-30 mm Hg) for the right ventricle (RV) to service this. The left ventricle has to create much higher pressures (100-140 mmHg--that is, systolic blood pressure) to circulate blood to the four corners of the body. As such the muscle of the LV is much thicker and necessarily more powerful than that of the RV. From a basic understanding of this simple physiological fact you can then understand why people talk generally about LV failure rather than heart or RV failure, as you can see the obvious systemic impact of a malfunctioning left ventricle. (See figure 1.)

Atrial contraction occurs during ventricular relaxation (diastole) and its sole function is to fill the ventricle with blood in preparation for ventricular contraction. The atria contribute roughly 30% to the filling of the ventricle. The rest occurs passively into the relaxing ventricular chambers after systole. In the absence of coordinated atrial activity--for example, atrial fibrillation--the 70% passive filling may be enough in many individuals to avoid haemodynamic compromise.

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The electrics (the engine's control system)

The electrical conducting system (see figure 2.) of the heart conducts and coordinates the performance of the pumps. When there are short circuits in the system the heart's pumping may become disorganised, which can result in a variety of sequelae, ranging from relatively minor to haemodynamically catastrophic.

The starting point in the control system is the heart's spark plug, the sinoatrial (SA) node. The cells of the SA node discharge spontaneously. The rate of this discharge determines the speed at which the heart beats. An electrical wave stems from this spark plug flooding across the atria. This is seen as a P wave on the ECG and it causes the atrial muscle fibres to contract, thereby pushing blood through the mitral and tricuspid valves into the left and right ventricles respectively. The atrial muscle fibres are separated from those of the ventricle by a fibrous insulating tissue ring. This does not allow the passage of the atrial electrical impulse to continue to the ventricles except at the atrioventricular (AV) node. The AV node funnels this impulse from the atria and leads it to the start of the specialised "electrical wires" in the ventricles. The AV node has a major role in controlling the heart. It delays conduction of the electrical impulse long enough so that the ventricles are filled by atrial contraction before they themselves contract. Once past the AV node the electrical activity passes on to the bundle of His, and is rapidly conducted by branches of this specialised fast conducting tissue through both ventricles. These "wires" are divided into two bundles supplying the left and right ventricles. The left system separates into anterior and posterior fascicles. These fibres allow rapid, ordered, and near synchronous activation of the ventricles.

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The pipes (the fuel)

Without the fuel the car will not run. The heart requires its own fuel and nutrient supply to enable it to function. The blood pumping through the chambers themselves does not really fulfil this need. The heart therefore has its own blood supply that comes off the root of the aorta just after it leaves the heart. There are two principal systems (right and left coronary systems) that run over the external surface of the heart muscle and give the heart nutrients and oxygen. Simplistically, the right coronary artery (RCA) usually supplies the inferior or underside of the heart. It also supplies most of the fuel for the electrical centres of the heart (SA and AV nodes). (See figure 3.)

The left coronary artery (LCA) starts with left main stem (LMS), and divides into the left anterior descending (LAD) branch down the front of the heart and the circumflex branch around the left side of the heart. The LCA supplies the bulk of the muscle of the LV and ventricular septum. (See figure 4.)

figure 8	figure 9	figure 10

From this simple understanding of the structure of the heart you can work out quite a lot of clinically important information. For example, if there is a blockage of the RCA then the changes on the ECG will be seen on the inferior surface of the heart. Because the RCA supplies the SA and AV nodes then clearly you can get problems with the formation and passage of electrical information in the heart. There may be less damage to the left ventricle if your RCA is blocked as it supplies less of the total muscle. On the other hand, if you have a blockage within the LCA there tends to be greater damage to the left ventricle, particularly if the important anterior descending artery is involved.

Understanding the squiggly lines

Now that we have discussed some of the background information relevant to making ECG reading easy we will move on to look at some of the basic concepts to understanding the squiggly lines seen on the ECG.

To extend the analogy of the motorcar, the ECG is a simple recording of its performance: several microphones placed strategically around the race track to detect the sound. As we are actually recording electricity our "microphone" needs to compare the electricity passing it with a reference point. This is electrical earth for the unipolar leads. The earth lead is customarily placed on the right leg of the patient.

The nine leads (unipolar) shown (figure 5) essentially act as "microphones" that detect the electrical sound as it approaches. V1-6 are the precordial (chest) leads that are essentially all at roughly the same level on a horizontal plane--that is, right/left and anterior/posterior--and are therefore best at picking up "sounds" in this axis and are relatively poor at giving useful information in the vertical plane (inferior/superior). Leads aVR, aVL, and aVF are limb leads attached to the right arm, left arm, and left leg respectively. (The earth lead is attached to the right leg.) They detect "sound" in essentially the same way as the precordial leads but this time are best at differentiating right/left and inferior/superior signals. An additional fact about these three leads, that you really do not need to know or understand, is that they are "augmented" by recording between one limb and the other two--this does not affect how the ECG looks but makes the size of the waves about 50% larger.

The heart is the centre of an electrical field that it generates. The field diminishes rapidly with distance from its centre. However, once the distance is greater than 15 cm, the decrement is insignificant. Therefore all electrodes placed more than 15 cm from the heart can be considered to be equidistant from the heart, even if physically they are not. (See figure 6.)

The three "standard" limb leads I, II, and III were used before the nine unipolar leads were developed. These compare differences in electrical signals in two limbs. Like the unipolar limb leads these are best at looking at differentiating right/left and inferior/superior "sounds." The term "vectors" will bring back distant and perhaps painful memories of school maths so we will not labour the point that essentially leads I, II, and III are cardiac vectors. They are not true recordings but calculated recordings. Thus the three limb leads are each used twice: firstly, to record directly as a "microphone" and, secondly, to compare cardiac electrical signals between limbs. Thus six records are obtained from these three limb leads, and another six records come from the chest lead--the 12 lead ECG. (See figure 7.)

figure 7

Finally, we will show you three key pieces of information that will enable you to understand ECGs with respect to the different leads.

1) An electrical impulse travelling directly towards a sensor will have the maximal positive deflection. (See figure 8a.)

An electrical impulse travelling directly away from a sensor will have a maximal negative deflection. (See figure 8b.)

An impulse travelling half way between these--that is, one that travels at 90^o will show a biphasic deflection as initially it is coming towards you, then moving away. (See figure 8c.)

2) The limb leads and the six recordings made from them are in a vertical plane through the body. They are good at looking at cardiac electrical features in this plane. For example, lead II is commonly used to look at the heart's rhythm. It lies in the same direction as electricity flowing from the SA to the AV node and thus this will be emphasised in this lead. (See figure 9.)

3) The chest leads look at the front of the heart in a horizontal plane. Thus they will be useful in looking for heart attacks affecting the front wall of the heart. (See figure 10.)

In the next article we will show how you can work out what a P wave or QRS complex would look like on an ECG and begin to look at simple rhythm recognition. We would like to emphasise, however, that you treat patients and not ECGs.

Dominic Cox specialist registrar in cardiology, Newcastle upon Tyne