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Basic ECG

1. CARDIOLOGY Cardiology l ecture one :- Introduction and Basic ECG Date:- 06 /08/2020 Prepared by Dr Guled Mohamud Nur (GMN@N ) MBBS

2. Introduction The electrocardiogram ( ECG or EKG ) is a special graph that represents the electrical activity of the heart from one instant to the next. Thus , the ECG provides a time-voltage chart of the heartbeat. For many patients, this test is a key component of clinical diagnosis and management in both inpatient and outpatient settings. The device used to obtain and display the conventional ECG is called the electrocardiograph , or ECG machine .

3. Introduction An electrocardiogram ( ECG or EKG ) is a graphic recording of electric potentials generated by the heart. The signals are detected by means of metal electrodes attached to the extremities and chest wall and then are amplified and recorded by the electrocardiograph. The clinical utility of the ECG derives from its immediate availability as a noninvasive , inexpensive, and highly versatile test.

4. Introduction In addition to its use in detecting arrhythmias , conduction disturbances, and myocardial ischemia , An electrocardiography may reveal other findings related to life-threatening metabolic disturbances (e.g., hyperkalemia ) or increased susceptibility to sudden cardiac death (e.g., QT prolongation syndromes ).

5. ESSENTIAL CARDIAC ELECTROPHYSIOLOGY The central function of the heart is to contract rhythmically and pump blood to the lungs for oxygenation and then to pump this oxygen-enriched blood into the general ( systemic ) circulation. The signal for cardiac contraction is the spread of electrical currents through the heart muscle. These currents are produced both by pacemaker cells and specialized conduction tissue within the heart and by the working heart muscle itself.

6. Electrical Activation of the Heart Normally , the cardiac stimulus is generated in the sinoatrial (SA ) node, which is located in the right atrium ( RA ). The stimulus then spreads through the RA and left atrium (LA ). Next , it spreads through the atrioventricular (AV) node and the bundle of His , which compose the AV junction. The stimulus then passes into the left and right ventricles ( LV and RV ) by way of the left and right bundle branches , which are continuations of the bundle of His . Finally, the cardiac stimulus spreads to the ventricular muscle cells through the Purkinje fibers.

7. CARDIAC AUTOMATICITY AND CONDUCTIVITY Automaticity refers to the capacity of certain cardiac cells to function as pacemakers by spontaneously generating electrical impulses. the other major electrical property of the heart is conductivity . The speed with which electrical impulses are conducted through different parts of the heart varies. The conduction is fastest through the Purkinje fibers and slowest through the AV node.

8. CARDIAC AUTOMATICITY AND CONDUCTIVITY

9. Action potential A , Resting cell: Positive ions on the outer surface and negative ions inside equal an electrically balanced or polarized cell. B , Depolarized cell: Negative ions on the outer surface and positive ions inside. C , Repolarization of cell: Positive ions return to the outside.

10. Waves, Intervals, and Segments DEPOLARIZATION AND REPOLARIZATION The more technical term for the cardiac activation process is depolarization . The return of heart muscle cells to their resting state following stimulation (depolarization) is called repolarization. BASIC ECG WAVE FORMS : P, QRS, ST-T, AND U WAVES The basic ECG waves are labeled alphabetically and begin with the P wave: P wave — atrial depolarization ( activation ) QRS complex—ventricular depolarization ( activation ) ST segment, T wave, and U wave—ventricular repolarization ( recovery )

11. ECG GRAPH PAPER The P-QRS-T sequence is usually recorded on special ECG graph paper that is divided into grid-like boxes The horizontally the ECG measures time and duration, One small box = 0.04 sec (40Msec), and One large box = 0.2 sec (200msec) Vertically the ECG graph measures the voltages, or amplitudes, of the ECG waves or deflections. One small square = 1mm (0.1mv ) One large square= 5mm (0.5mv)

12. The basic cardiac cycle (P-QRS-T)

13. P Wave The P wave, which represents atrial depolarization , is a small positive (or negative) deflection before the QRS complex. PR Interval The PR interval is measured from the beginning of the P wave to the beginning of the QRS complex The PR interval represents the time it takes for the stimulus to spread through the atria and pass through the AV junction. In adults the normal PR interval is between 0.12 and 0.2 sec (three to five small box sides).

14. QRS Complex When the initial deflection of the QRS complex is negative (below the baseline), it is called a Q wave . The first positive deflection in the QRS complex is called an R wave . A negative deflection following the R wave is called an S wave . QRS Width (Interval) The QRS width, or interval, represents the time required for a stimulus to spread through the ventricles ( ventricular depolarization ) and is normally about 0.10 sec

15. Measurement of the PR interval and deflections

16. ST Segment The ST segment is that portion of the ECG cycle from the end of the QRS complex to the beginning of the T wave . It represents the beginning of ventricular repolarization . The normal ST segment is usually isoelectric. but it may be slightly elevated or depressed normally ( usually by less than 1 mm ). Some pathologic conditions such as myocardial infarction ( MI) produce characteristic abnormal deviations of the ST segment. T Wave The T wave represents part of ventricular repolarization.

17. QT Interval The QT interval is measured from the beginning of the QRS complex to the end of the T wave It primarily represents the return of stimulated ventricles to their resting state (ventricular repolarization). The normal values for the QT interval depend on the heart rate. As the heart rate increases ( RR interval shortens ), the QT interval normally shortens ; as the heart rate decreases ( RR interval lengthens ), the QT interval lengthens .

18. QT Interval rate-corrected QT or QTc intervals A widely used one ( Bazett formula ) is the square root method, Using the square root method: QTc = QT /√RR Normally the QTc is between about 0.33 sec ( 330 msec ) and about 0.44 sec or ( 440 msec ). The QTc is markedly prolonged , indicating a high risk of sudden cardiac arrest due to torsades de pointes (life threating ventricular arrhythmias).

19. Abnormal QT interval prolongation in a patient taking the drug quinidine. The QT interval ( 0.6 sec ) is markedly prolonged for the heart rate ( 65 beats/min ). Using the “square root ( Bazett ) method”: QTc = QT/√RR = 0.60 sec/√0.92 = 0.63 sec.

20. CALCULATION OF HEART RATE Box Counting Methods 1 (large boxes) The easier way, when the heart rate is regular, is to count the number of large (0.2-sec) boxes between two successive QRS complexes and divide a constant ( 300 ) by this. (The number of large time boxes is divided into 300 because 300 × 0.2 = 60 and the heart rate is calculated in beats per minute, or 60 seconds.)

21. CALCULATION OF HEART RATE Box Counting Methods 2 (small boxes) Count the number of small (0.04 sec) boxes between successive R waves and divide a constant ( 1500 ) by this number. (The constant 1500 is used because 1500 × 0.04 = 60 and the heart rate is being calculated in beats per 60 sec [beats/min].)

22. CALCULATION OF HEART RATE

23. QRS Counting Methods If the heart rate is irregular, the first method will not be accurate. The best method for measuring irregular heart rates with varying R-R intervals is count the number of QRS wave within 30 large boxes (6 second) multiplying 10. By definition, a heart rate exceeding 100 beats/ min is termed a tachycardia , and a heart rate slower than 60 beats/min is called a bradycardia .

24. HEART RATE AND RR INTERVAL: HOW ARE THEY RELATED? Students should know that RR intervals can be converted to heart rate (HR) by the following two simple , equivalent formulas, depending on whether you measure the RR interval in seconds ( sec ) or milliseconds ( msec ): HR in beats/min = 1.0/RR (in sec) × 60 HR in beats/min = 1000/RR (in ms ) × 60

25. ECG LEADS The usual way of recording these voltages from the heart is with the 12 standard ECG leads, the leads can be subdivided into two groups: the six limb ( extremity ) leads and the six chest ( precordial ) leads The six limb leads—I, II, III, aVR , aVL , and aVF — record voltage differences by means of electrodes placed on the extremities .

26. ECG LEADS They can be further divided into two subgroups based on their historical development. three standard bipolar limb leads (I, II, and III) and three augmented unipolar limb leads ( aVR , aVL , and aVF ). The six chest leads—V1, V2, V3, V4, V5, and V6— record voltage differences by means of electrodes placed at various positions on the chest wall.

27. AXIS DEVIATION The mean QRS axis is a basic measurement that should be made on every ECG. In the ECGs of most norma l people this axis lies between –30° and +100° An axis of –30° or more negative is described as left axis deviation (LAD), and one that is +100° or more positive is termed right axis deviation (RAD).

28. AXIS DEVIATION The mean QRS axis is determined by the anatomic position of the heart and the direction in which the stimulus spreads through the ventricles (i.e., the direction of ventricular depolarization).

29. AXIS DEVIATION

30. Clinical Significance RAD , with a mean QRS axis +100° or more , is sometimes seen in the ECGs of normal hearts. However , RVH, myocardial infarction of the lateral wall of the left ventricle, Left posterior fascicular block, RBBB, patients with chronic lung disease (emphysema or chronic bronchitis), and dextrocardia are the causes of RAD.

31. Clinical Significance LAD , with a mean QRS axis of –30° or more , is also seen in several settings. Patients with left ventricular hypertrophy (LVH), Left anterior fascicular block, LBBB , inferior MI and high diaphragm.

32. Reference By Goldberger’s Clinical Electrocardiography A Simplified Approach EIGHTH EDITION