The electrocardiogram

The electrocardiogram<img alt="The Electrocardiogram signal between 2 12 lead ECG iäiiiih. t me electroca rdiograph " src="https://knowt-user-attachments.s3.amazonaws.com/0b37404c-7a26-4028-967a-bffccba14bc9.png" /> <ul><li>Conduction through the heart is monitored by recording an electrocardiogram.</li><li>This diagram shows a typical placement of 10 electrodes. Six are positioned on the chest, around the area occupied by the heart. In addition, one is attached to each wrist and ankle. Each of the electrodes is then attached to the electrocardiograph and its output can be recorded on paper or using a computer.</li><li> The output is 12 channels of traces. Each one of these channels is known as a lead. Each lead records the potential difference between 2 of the electrodes.</li><li>Ten electrodes are used to generate 12 leads, or 12 different views.</li><li>When recording from a healthy individual, a characteristic pattern of activity is repeated each time the heart beats.</li><li>The units on the trace are voltage versus time.</li><li>There are five clear deflections in the trace, which are known as P, Q, R, S and T</li></ul><img alt="The Electrocardiogram Records difference between 2 electrodes on surface of the skin Amplitude of signal proportional to mass of cardiac tissue generating it ventricles depolarise QRS compl ex atria depolarise P wave time segment T wave ST segment s QT interval RR interval ventricles repolarise Isoelectric line PR interval " src="https://knowt-user-attachments.s3.amazonaws.com/5b84b30e-d8e8-45e1-88f4-83eac131d3f1.png" /><ul><li>In between beats, when the heart is not generating electrical activity, the difference in voltage between each of the pairs of electrodes is known as the isoelectric line.</li><li>Deflections in the trace are measured relative to the isoelectric line.</li><li>The time between the start of the P wave and the start of the Q wave is known as the PR interval.</li><li>The time between the end of the P wave and the start of the Q wave is known as the PR segment.</li><li>The large event between the start of the Q wave and the end of the S wave is known as the QRS complex.</li><li>From the peak of the Q wave to the end of the T-wave is known as the QT interval.</li><li>the time between the end of the S wave and the start of the T wave is known as the ST segment.</li><li>the RR interval, which is the period between each cardiac cycle.</li><li>The P wave is caused by depolarisation of the atria during the atrial action potential.</li><li>The QRS complex is caused by the ventricles depolarising</li><li>the T wave is caused by the ventricles repolarising at the end of the ventricular action potential.</li><li>The reason is that the ECG detects changes in voltage between the 2 electrodes. During the plateau phase of the action potential there is little change in membrane potential, so the ECG returns to the isoelectric point between depolarisation and repolarisation.</li><li> the QT interval gives a measure of action potential duration.</li><li>the QRS complex is so much bigger than the P wave because the ventricles are much bigger than the atria: The more tissue contributing to a current at any one time, the easier it is to detect. Thus, the amplitude of the ECG signal is proportional to the mass of cardiac tissue from which it originates.</li></ul><img alt="ECG detects direction of current flow 12 lead ECG records informatio 12 different views Of the heart body fluid and tissues " src="https://knowt-user-attachments.s3.amazonaws.com/f6b9cb78-7af6-4faa-ac20-aae8c2a53a42.png" /><ul><li>the ECG also gives positional information by detecting the direction of current flow.</li><li>Current is conducted through the body’s tissues by electrolytes that surround and fill all our cells.</li><li> diagram illustrates two ECG electrodes, detecting electrical currents as they flow through the body as a difference in voltage between them. First consider current approaching the electrodes from the left. As it passes the cathode, it is detected as a change in voltage relative to the anode, so there is a deflection in the ECG trace. As the current proceeds it eventually reaches the anode, creating a difference in voltage between the electrodes that is opposite in direction.</li><li>If instead the current arrives perpendicular to the electrodes and between them, it is detected simultaneously by both electrodes and gives a biphasic deflection.</li><li> By measuring from pairs of electrodes located at different positions on the skin, you therefore get different views, providing information on the direction of current flow through the heart. This is the reason that a 12 lead ECG gives 12 different views.</li></ul> Basic analysis of ECG <img alt="Regular interval between each P wave?

is atrial rhythm regular? Regular interval between each R wave? - is ventricular rhythm regular? 6.5 0.5s Is P wave precedes every QRS complex? - is AV node conducting every signal? All components of ECG complex present? Heart rate? = 8 boxes R-R interval = 6.5/8 s Rate = I/(R-R interval) = 0.81 s = 0.0135 min = 73.8 b.p.m. Tachycardia Bradycardia Sinus rate > 100 bpm and regular rhythm Sinus rate < 60 bpm and regular rhythm " src="https://knowt-user-attachments.s3.amazonaws.com/a940e6a9-d36a-49ff-aa04-045b6b477888.png" /><ul><li>Look for the P wave<ul><li>If present the atria are depolarising and contracting.</li><li>If absent there is a problem in the atria or sinoatrial node.</li></ul></li><li>Look for the QRS complex.<ul><li>If present the ventricles are depolarising and contracting.</li><li>If absent there is a problem in the ventricles or in the conducting tissues supplying the ventricles.</li></ul></li><li>Measure the heart rate by measuring the R-R interval. It will give the interval between beats in seconds per beat. Convert to the heart rate in beats per minute.<ul><li>Is it normal? Is there tachycardia? Is there bradycardia?</li></ul></li><li>Check the heart rhythm by looking at the P wave and measuring the P-P interval<ul><li>Does it correspond with normal sinus rhythm i.e. a normal heart rate?</li><li>Does P-P interval = R-R interval? If so the atria and ventricles are beating at the same rate. If not conduction is blocked, resulting in the atria and ventricles being excited and beating separately.</li><li>Does every P wave precede each QRS complex by a constant interval? If so action potentials are consistently conducted from the atria, through the atrioventricular node to the ventricles. If not there is a blockage in conduction at the atrioventricular node.</li></ul></li></ul> ECG disturbance due to altered conduction:<ul><li>The conduction of action potentials through the heart can be disrupted in different places, generating arrhythmias with characteristic ECGs.</li><li>Conduction defects are classified as ventricular when they occur within the ventricles and supraventricular when they occur at any level above the ventricles. The tables outline changes to the ECG that are indicative of particular causes of cardiac dysfunction.</li></ul><img alt="ECG feature P wave interval disruption altered shape too long too short Supraventricular indication Signal may have an abnormal origin e.g. the atrioventricular node Conduction delay through the atrioventricular node (atrioventricular block) Sinoatrial node is not the source of the signal ( e.g. Wolff-Parkinson-White syndrome) ECG feature QRS complex interval disruption deep & wide notched prolonged shortened indication Myocardial infarction Right or left bundle block Failure to repolarise quickly enough, thus lengthening the refractory preiod and reducing excitability Repolarisation is too fast, resulting in a short refractory period and increased excitability. " src="https://knowt-user-attachments.s3.amazonaws.com/362d9f56-7f92-4cea-89d8-10880df3072f.png" /><table><thead><tr><th>Heart block:</th><th>Loss of signal conduction at the atrioventricular node</th></tr><tr><th>First degree</th><th>Conduction through AV node is slowed, giving long P-Q interval</th></tr><tr><th>Second degree(partial)</th><th>Some signals not conducted through AV node, giving rise to missed QRS complexes (and ventricular beats)</th></tr><tr><th>Third degree(complete)</th><th>Atrial signals not conducted through the AV node, which becomes an ectopic pacemaker. P waves are disconnected from QRS (or absent)</th></tr></thead></table><img alt="Heart block.png" src="https://knowt-user-attachments.s3.amazonaws.com/28ab526c-2ee5-4c8e-95fd-f3d578330bb8.png" />Myocardial infarction: Loss of blood supply to an area of the heart causes ischaemic damage, resulting in myocardial cell death. This disrupts the normal conduction pathways, which may result in an arrhythmia that can develop into ventricular fibrillation. It usually occurs in the left ventricle.<ul><li>ECG changes depend on the severity and location of ischaemia.</li><li>In general, the more leads on a 12 lead ECG showing changes, the larger the infarct size.</li><li>Typical changes include an abnormal Q wave and heightened T wave. The ST segment, which normally lies flat along the isoelectric line, can be elevated or depressed</li><li><img alt="STEMI.png" src="https://knowt-user-attachments.s3.amazonaws.com/56c289f2-6c49-4b35-81fb-287fc212fb3c.png" /></li><li> This is because the normally co-ordinated action potential plateau is disrupted, with repolarisation and action potential duration varying in the damaged areas.</li><li>This is detected as an abnormal voltage difference between two ECG electrodes. Myocardial infarction with ST segment elevation (STEMI) is generally the worst type</li></ul>