What the ECG is About

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What the ECG is about



‘ECG’ stands for electrocardiogram, or electrocardiograph. In some countries, the abbreviation used is ‘EKG’. Remember:




WHAT TO EXPECT FROM THE ECG


Clinical diagnosis depends mainly on a patient’s history, and to a lesser extent on the physical examination. The ECG can provide evidence to support a diagnosis, and in some cases it is crucial for patient management. It is, however, important to see the ECG as a tool, and not as an end in itself.


The ECG is essential for the diagnosis, and therefore the management, of abnormal cardiac rhythms. It helps with the diagnosis of the cause of chest pain, and the proper use of early intervention in myocardial infarction depends upon it. It can help with the diagnosis of the cause of dizziness, syncope and breathlessness.


With practice, interpreting the ECG is a matter of pattern recognition. However, the ECG can be analysed from first principles if a few simple rules and basic facts are remembered. This chapter is about these rules and facts.



THE ELECTRICITY OF THE HEART


The contraction of any muscle is associated with electrical changes called ‘depolarization’, and these changes can be detected by electrodes attached to the surface of the body. Since all muscular contraction will be detected, the electrical changes associated with contraction of the heart muscle will only be clear if the patient is fully relaxed and no skeletal muscles are contracting.


Although the heart has four chambers, from the electrical point of view it can be thought of as having only two, because the two atria contract together (‘depolarization’), and then the two ventricles contract together.



THE WIRING DIAGRAM OF THE HEART


The electrical discharge for each cardiac cycle normally starts in a special area of the right atrium called the ‘sinoatrial (SA) node’ ( Fig. 1.1). Depolarization then spreads through the atrial muscle fibres. There is a delay while depolarization spreads through another special area in the atrium, the ‘atrioventricular node’ (also called the ‘AV node’, or sometimes just ‘the node’). Thereafter, the depolarization wave travels very rapidly down specialized conduction tissue, the ‘bundle of His’, which divides in the septum between the ventricles into right and left bundle branches. The left bundle branch itself divides into two. Within the mass of ventricular muscle, conduction spreads somewhat more slowly, through specialized tissue called ‘Purkinje fibres’.





THE DIFFERENT PARTS OF THE ECG


The muscle mass of the atria is small compared with that of the ventricles, and so the electrical change accompanying the contraction of the atria is small. Contraction of the atria is associated with the ECG wave called ‘P’ ( Fig. 1.2). The ventricular mass is large, and so there is a large deflection of the ECG when the ventricles are depolarized: this is called the ‘QRS’ complex. The ‘T’ wave of the ECG is associated with the return of the ventricular mass to its resting electrical state (‘repolarization’).



The letters P, Q, R, S and T were selected in the early days of ECG history, and were chosen arbitrarily. The P Q, R, S and T deflections are all called waves; the Q, R and S waves together make up a complex; and the interval between the S wave and the beginning of the T wave is called the ST ‘segment’.


In some ECGs an extra wave can be seen on the end of the T wave, and this is called a U wave. Its origin is uncertain, though it may represent repolarization of the papillary muscles. If a U wave follows a normally shaped T wave, it can be assumed to be normal. If it follows a flattened T wave, it may be pathological (see Ch. 4).


The different parts of the QRS complex are labelled as shown in Figure 1.3. If the first deflection is downward, it is called a Q wave ( Fig. 1.3a). An upward deflection is called an R wave, regardless of whether it is preceded by a Q wave or not ( Figs 1.3b and 1.3c). Any deflection below the baseline following an R wave is called an S wave, regardless of whether there is a preceding Q wave ( Figs 1.3d and 1.3e).




TIMES AND SPEEDS


ECG machines record changes in electrical activity by drawing a trace on a moving paper strip. ECG machines run at a standard rate of 25 mm/s and use paper with standard-sized squares. Each large square (5 mm) represents 0.2 second (s), i.e. 200 milliseconds (ms) ( Fig. 1.4). Therefore, there are five large squares per second, and 300 per minute. So an ECG event, such as a QRS complex, occurring once per large square is occurring at a rate of 300/min. The heart rate can be calculated rapidly by remembering the sequence in Table 1.1.




Just as the length of paper between R waves gives the heart rate, so the distance between the different parts of the P-QRS-T complex shows the time taken for conduction of the electrical discharge to spread through the different parts of the heart.


The PR interval is measured from the beginning of the P wave to the beginning of the QRS complex, and it is the time taken for excitation to spread from the SA node, through the atrial muscle and the AV node, down the bundle of His and into the ventricular muscle. Logically, it should be called the PQ interval, but common usage is ‘PR interval’ ( Fig. 1.5).



The normal PR interval is 120–220 ms, represented by 3–5 small squares. Most of this time is taken up by delay in the AV node ( Fig. 1.6).



If the PR interval is very short, either the atria have been depolarized from close to the AV node, or there is abnormally fast conduction from the atria to the ventricles.


The duration of the QRS complex shows how long excitation takes to spread through the ventricles. The QRS complex duration is normally 120 ms (represented by three small squares) or less, but any abnormality of conduction takes longer, and causes widened QRS complexes ( Fig. 1.7). Remember that the QRS complex represents depolarization, not contraction, of the ventricles – contraction is proceeding during the ECG’s ST segment.



The QT interval varies with the heart rate. It is prolonged in patients with some electrolyte abnormalities, and more importantly it is prolonged by some drugs. A prolonged QT interval (greater than 450 ms) may lead to ventricular tachycardia.




THE ECG – ELECTRICAL PICTURES


The word ‘lead’ sometimes causes confusion. Sometimes it is used to mean the pieces of wire that connect the patient to the ECG recorder. Properly, a lead is an electrical picture of the heart.


The electrical signal from the heart is detected at the surface of the body through electrodes, which are joined to the ECG recorder by wires. One electrode is attached to each limb, and six to the front of the chest.


The ECG recorder compares the electrical activity detected in the different electrodes, and the electrical picture so obtained is called a ‘lead’. The different comparisons ‘look at’ the heart from different directions. For example, when the recorder is set to ‘lead I’ it is comparing the electrical events detected by the electrodes attached to the right and left arms. Each lead gives a different view of the electrical activity of the heart, and so a different ECG pattern. Strictly, each ECG pattern should be called ‘lead …’, but often the word ‘lead’ is omitted.


The ECG is made up of 12 characteristic views of the heart, six obtained from the ‘limb’ leads (I, II, III, VR, VL, VF) and six from the ‘chest’ leads (Vi-V6). It is not necessary to remember how the leads (or views of the heart) are derived by the recorder, but for those who like to know how it works, see Table 1.2. The electrode attached to the right leg is used as an earth, and does not contribute to any lead.




THE 12-LEAD ECG


ECG interpretation is easy if you remember the directions from which the various leads look at the heart. The six ‘standard’ leads, which are recorded from the electrodes attached to the limbs, can be thought of as looking at the heart in a vertical plane (i.e. from the sides or the feet) ( Fig. 1.9).



Leads I, II and VL look at the left lateral surface of the heart, leads III and VF at the inferior surface, and lead VR looks at the right atrium.


The six V leads (V1-V6) look at the heart in a horizontal plane, from the front and the left side. Thus, leads V1 and V2 look at the right ventricle, V3 and V4 look at the septum between the ventricles and the anterior wall of the left ventricle, and V5 and V6 look at the anterior and lateral walls of the left ventricle ( Fig. 1.10).


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Jul 26, 2016 | Posted by in CARDIOLOGY | Comments Off on What the ECG is About

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