ElectroCardioGraphy ECG DR. Yasir Mohsin Khaleel M.B.Ch.B, M.Sc., Ph.D For 2 nd Class Medical Students Mosul College of Medicine Dep. of Medical Physiology

Cardiac Conduction System

In the normal heart during sinus rhythm, impulse formation is initiated in the sinoatrial (SA) node, and then spreads as a wave of depolarization over the atria until it reaches the atrioventricular (AV) node, which is near the junction of the interatrial and interventricular septa. The AV node represents the sole pathway for conducting the impulse from the atria to the ventricles, Its unique purpose is to slow the rate of impulse conduction to give the atria time to finish emptying of blood and the ventricles time to finish filling with blood. This is necessary because mechanical contraction of cardiac muscle is normally slower than the rapid process of electrical depolarization. The common bundle of His next conducts the impulse through the superior ventricular septum and then quickly divides into the right and left bundle branch Cardiac Conduction System

Perhaps because the left ventricle is bigger than the right ventricle, the left bundle splits into two hemibundles, one running anteriorly and superiorly (Anterior Fascicle) and the other running posteriorly and inferiorly (Posterior Fascicle). Finally, the bundle branches divide numerous times into Purkinje fibers, which are the final pathway for conduction of the impulse to ventricular muscle. Once ventricular muscle is stimulated by the impulse traveling down the Purkinje fibers, it depolarizes outwardly from endocardium to epicardium. Electrical impulses are conducted much more rapidly through the specialized conduction system of the heart than through typical cardiac muscle. This allows the electrical impulse to reach almost all ventricular muscles nearly simultaneously, thus, allowing for coordinated contraction of the ventricles.

Muscle Mass of Cardiac Chambers The atria pump blood over only very short distances (across the AV valves into the ventricles); therefore, they are very thin-walled structures with very little muscle mass. Conversely, the right ventricle must pump blood all the way through the lungs, and therefore has a thicker wall and more muscle mass. Finally, the left ventricle pumps blood out to the entire body, and thus has the thickest wall of all the chambers, three to four times thicker than the wall of the right ventricle. The size of an electrical complex on the ECG, is related to how much voltage is generated by depolarization of a given portion of the heart. Thus, the QRS complex is normally larger than the P wave because depolarization of the greater muscle mass of the ventricles generates more voltage than does depolarization of the thinner walls of the atria.

Recording a Wave of Depolarization

The ECG Grid

Electrocardiographic Waveforms

The P wave corresponds to the depolarization of atrial muscle. Because there is relatively little atrial muscle mass, only low voltages are normally produced. P Wave Characteristics: Duration: sec. ( msec) Amplitude: Limb leads < 2.5 mm Morphology: Upright in I, II, aVF, Upright or biphasic in III, aVL, V1,V2 The PR interval corresponds to the time it takes an impulse to travel from the SA node all the way down through the conduction system to the first muscle fibers stimulated in the ventricles, from the onset of atrial depolarization to the onset of ventricular depolarization. Therefore, it is measured from the beginning of the P wave to the beginning of the QRS. Normal duration : sec ( msec)

The QRS is naturally the largest complex on the ECG because it corresponds to depolarization of the ventricles, with their larger muscle mass. Therefore, QRS amplitude may normally reach as high as 25mm or more (five big boxes) in large individuals, or in those with thin chest walls that actually allow the precordial electrodes to be closer to the heart. Normal QRS duration: <0.10 sec Moderate prolongation: <0.12 sec Marked prolongation: ≥0.12 sec QRS Nomenclature 1. The first deflection of the complex is called a Q wave if it is negative. 2. The first positive deflection of the complex is called an R wave. 3. A negative deflection coming after an R wave is called an S wave. 4. Positive deflections coming after the first R wave are labeled R′ (R prime). 5. Negative deflections coming after the first S wave are labeled S′ (S prime).

The ST segment represents the time between the completion of depolarization of the ventricles and the onset of repolarization of the ventricles. It is normally isoelectric, meaning neither positive nor negative, And gently blends into the upslope of the subsequent T wave. The point at which the ST segment takes off from the QRS is called the J point. The ST segment plays a very important role in the diagnosis of ischemic heart Disease.

The T wave corresponds to repolarization of the ventricles. It is normally inscribed in the same direction as the predominant deflection of the QRS, and has less amplitude than the QRS. Abnormalities of the T wave predominantly take the form of inversion. It may also take the form of very large or small amplitude. Direction, normally it is: -Upright in I,II,V3-V6 -Inverted in aVR, V1 -Upright, flat or biphasic in III, aVL, aVF, V1, V2 Amplitude: -Limb leads < 6mm -Chest leads =< 10mm Tall T-wave : > 6 mm in limb leads, > 10mm in chest leads

The QT interval is measured from the beginning of the QRS to the end of The T wave, and normal intervals vary with heart rate and the person’s sex. The primary potential abnormality of the QT interval is prolongation, reflecting delays in ventricular repolarization. QTc the corrected QT interval,calculated as QT÷ square root of the preceding RR interval (sec). Bazett’s formula. The RR interval, or the duration of one cardiac cycle, is a measure of heart rate. Therefore, when the heart rate is 60 beats/min, and the RR interval is 1 second, the QTc equals the measured QT. When the heart rate is greater than 60 beats/min and the RR interval is less than 1 second, the QTc will be greater than the measured QT. Normal QTc : sec for HR of bpm Prolonged QTc: =>0.44 sec, Short QTc: < 0.35 for HR bpm The normal QT should be < 50% of the RR interval.

The 12 Leads ECG = The 12 Views of the Heart

The Six Limb Leads The limb leads view the heart in a vertical plane called the frontal plane. The frontal plane can be envisioned as a giant circle superimposed on the patient's body. This circle is then marked off in degrees. The limb leads view electrical forces (waves of depolarization and repolarization) moving up, down, left and right through this circle.

The three standard limb leads are defined as follows: Lead I is created by making the left arm positive and the right arm negative. Its angle of orientation is 0°. ( RA LA ) Lead II is created by making the left leg positive and the right arm negative. Its angle of orientation is 60°.( RA LL) Lead III is created by making the left leg positive and the left arm negative. Its angle of orientation is 120°. ( LA LL )

The three augmented limb leads They are called augmented leads because the ECG machinery must amplify the tracings to get an adequate recording. Lead aVL is created by making the left arm positive and the other limbs negative. Its angle of orientation is -30°. Lead aVR is created by making the right arm positive and the other limbs negative. Its angle of orientation is -150°. Lead aVF is created by making the left leg positive and the other limbs negative. Its angle of orientation is +90°.

Limb Leads Views

The Six Precordial ( Chest ) Leads The six precordial leads, or chest leads, are even easier to understand. They are arranged across the chest in a horizontal plane. Whereas the leads of the frontal plane ( limb leads ) view electrical forces moving up and down and left and right, the precordial leads record forces moving anteriorly and posteriorly, ( Transverse or horizontal plane ). V1—right sternal border, 4th interspace V2—left sternal border, 4th interspace V3—midway between V2 and V4 V4—midclavicular line, 5th interspace V5—anterior axillary line, 5th interspace V6—midaxillary line, 5th interspace

Note that the right ventricle lies anteriorly and medially within the body cavity, and the left ventricle lies posteriorly and laterally. Lead V1 lies directly over the right ventricle, V2 and V3 over the interventricular septum, V4 over the apex of the left ventricle, and V5 and V6 over the lateral left ventricle.

The 12 leads

LeadsLook “ View “ V1,V2, V3, V4Anterior I, aVL, V5, V6Left lateral II, III, aVFInferior aVR, V1Right atrium, Right Vent.

R WAVE PROGRESSION

QRS Axis- Electrical Axis of The Heart

12-lead ECG Interpretation 1- Origin of the Rhythm 2- Heart Rate 3- P-wave 4- PR interval 5- QRS interval 6- QT interval 7- QRS axis 8- QRS voltage 9- R wave progression in chest leads 10- Any abnormal Q-wave 11- ST segment 12- T wave

Origin of the Rhythm: P:QSR relation P wave precedes each QRS and the P waves are of normal shape and positive in lead I & II = Sinus rhythm

Heart Rate: 1- Regular Rhythm: Each large square= 0.20 sec. ; Each small square= 0.04 sec 1 minute= 300 large square = 1500 small square Atrial rate=300/ number of large boxes between 2 P waves Ventricular rate= 300/ number of large boxes between 2 R waves OR Atrial rate=1500/ number of small boxes between 2 P waves Ventricular rate= 1500/ number of small boxes between 2 R waves 2- Irregular or slow rhythm: Identify 6 second tracing strip ( 30 large boxes ), count the number of QSR complexes and multiply by 10 = BPM ( vent rate) For Atrial rate: count the number of P waves in this 6 sec strip and multiply by 10 = BPM (atrial rate)

Five Steps for 12-Lead ECG interpretation 1.) Rate and rhythm 2.) Intervals duration 3.)Axis determination 4.) Morphology: waves analysis 5.) Ischemia, Injury, Infarct