ECG 1 KTN 2023 PDF

The Electrocardiogram (ECG): origins and components Dr K. Thaxter Nesbeth Physiology Section Department of Basic Medical Sciences  The Electrocardiogram (ECG) Objectives: At the end of this lecture, you will be able to: ▪ Describe the origins of the electrocardiogram. ▪ Expla...

The Electrocardiogram (ECG): origins and components Dr K. Thaxter Nesbeth Physiology Section Department of Basic Medical Sciences  The Electrocardiogram (ECG) Objectives: At the end of this lecture, you will be able to: ▪ Describe the origins of the electrocardiogram. ▪ Explain the parts of the electrocardiogram in relationship to the electrical activity of the heart.  A Brief introduction to ECG ECG (EKG) Measurement and recording of the electrical activity of the heart Via electrodes placed on the skin in specific locations  Electrocardiogram Waves recorded on international standard ECG paper  Electrocardiogram (ECG/EKG) recording of electrical activity of the heart using electrodes on the surface https://www.youtube.c om/watch?v=RYZ4daFw Ma8 Components of the ECG Atrial depolarisation, initiated by the SA node, causes the P wave Components of the ECG With atrial depolarisation, complete, the impulse is delayed at the AV node – causing isoelectric PR segment Components of the ECG Ventricular depolarisation begins at the septum, spreads to the apex and ventricular muscle, causing the QRS complex.  Ventricular Depolarisation 1st: Interventricular septum, wave of depolarization LEFT TO RIGHT – Q wave 2nd: wave of depolarization from interventricular septum to right and left ventricle via apex – R wave 3rd: Depolarisation of posterior part of muscular left ventricle – S wave Components of the ECG Ventricular depolarisation is complete Components of the ECG Ventricular repolarisation begins at the apex, causing the T wave Components of the ECG Ventricular repolarisation is complete  Where is Atrial Repolarization? Wave is hidden as it takes place at the same time as ventricular depolarization! Small atrial muscle mass produces small potential that is ‘overshadowed’ by the large QRS complex  Components of the ECG Wave: Deflection from isoelectric baseline Segment: Region between two waves Interval: Duration of time that includes ONE or more waves AND often a segment  P Wave Amplitude 0.05- 0.25mV Duration 0.06-0.10 seconds Waveform is upright and slightly asymmetrical 15  PR Interval Duration 0.12-0.20 seconds  QRS Complex Amplitude 0.5- 3.0mV Duration 0.06-0.10 seconds  ST Segment and T Wave ST segment is isoelectric line that follows QRS complex T wave is larger, slightly asymmetrical waveform that follows ST segment J point is start of ST segment  QT Interval Distance from onset of QRS complex until end of T wave – Measures time of ventricular depolarization and repolarization Has normal duration of 0.36 to 0.44 seconds  Small upright seen U Wave following the T wave, but before the next P wave Repolarisation of papillary muscles / Purkinje fibers Voltage low U wave often goes unnoticed Intervals and segments of the ECG  Recording the Electrocardiogram Video demonstrates practical aspects of performing an ECG https://www.youtube.com/watch?v=1k4B_fIX_ t0  Measuring the ECG: Electrodes form Leads Depolarisation produces a negative wave (outside of the cell becomes more negative) Wave is detected by positive ECG electrode Electrodes are positioned to create specific ‘Leads’ for a standard ECG Form an imaginary line in the body along which the electrical signals are measured ECG leads are either bipolar or unipolar  ECG Leads Each lead provides a different view of the heart: – Electrodes are placed on chest, arms and legs – Sites vary depending on which view of the heart's electrical activity is being assessed Direction (up, or down) of waveforms depend on whether they move toward or away from the positive electrode 24 ECG Leads  https://www.youtube.com/watch?v=xIZQRj kwV9Q Deflections explained – stop at 3:30sec for now…  Limb Leads 1: Bipolar Leads Record the flow of the electrical impulse between two (one is positive, the other is negative) selected electrodes Includes I, II and III G  Bipolar Leads Record difference in electrical potential between a positive and negative electrode Uses a third electrode called a ground (RL) Remember! = Leads I, II and III are bipolar G Bipolar leads  Limb Leads 2: Unipolar Leads Use only one positive electrode and a reference point calculated by the ECG machine Includes leads aVR, aVL, aVF, (unipolar limb leads) – and V1 through V6 (chest leads)  Augmented Limb Leads aVR, augmented lead toward the right (arm) aVL, augmented lead toward the left (arm) aVF, augmented lead toward the foot Reference point Limb Electrode Locations RA electrode – positioned anywhere on the right arm or below the right clavicle in the midclavicular line LA electrode – positioned anywhere on the left arm or below the left clavicle in the midclavicular line LL electrode – positioned anywhere on the left leg or left midclavicular line below the last palpable rib RL electrode – positioned on the right leg or right midclavicular line below the last palpable rib  Comparing limb leads Only 1 electrode placed on each limb: provides information from both bipolar (I,II,III) and augmented limb leads (aVR, aVL, aVF)  Bipolar Leads Axes of these 3 leads form a triangle around the heart Einthoven’s triangle Used for calculations when interpreting ECGs G  Chest Leads Positioned in order across the chest  The Precordial (Chest) Leads:V1 through V6 Chest leads – - starting over the right atrium with V1, and placed in a semi-circle of positions leftwards, to the left side of the left ventricle  The Precordial (Chest) Leads Chest Electrode Placement V1: Fourth intercostal space to the right of the sternum. V2: Fourth intercostal space to the Left of the sternum. V3: Directly between leads V2 and V4. V4: Fifth intercostal space at midclavicular line. V5: Level with V4 at left anterior axillary line. V6: Level with V5 at left midaxillary line. (Directly under midpoint of armpit)  12 Lead ECG gives 3-Dimensional view of heart 9 Electrodes (and ground) on the extremities and chest wall view the heart’s electrical activity from the frontal and horizontal planes – Provides a cross- sectional view of the heart  Frontal Plane Leads that view this plane: limb leads Include leads I, II, III, aVR, aVL and aVF Provide inferior, superior, and lateral views of heart  Horizontal Plane Precordial or chest leads Include leads V1, V2, V3, V4, V5, and V6 Provide anterior, and lateral views of heart  Standard 12-Lead ECG Usually performed when person is resting in supine position. Composed of three bipolar limb leads: I, II, and III; three augmented voltage leads: aVR, aVL, aVF; and six chest or precordial leads: V1 – V6. All limb leads lie in frontal plane. Chest leads circle heart in (horizontal) transverse plane.  Views of heart from 12-Lead ECG Each lead provides a different electrical angle or picture of the heart. Anterior view of heart: V1 – V4. Lateral view of heart: I, aVL, V5 and V6. Inferior view of heart: II, III, and aVF.  ECG videos https://www.youtu Video be.com/watch?v=1 demonstrates k4B_fIX_t0 practical aspects of performing an ECG https://www.youtu Video explains the be.com/watch?v=xI ECG waves, and ZQRjkwV9Q how they appear on different leads  ECG Paper Each small square equals – 0.04 sec in duration and 0.1 mV in amplitude Five small squares (1 large box) equals – 0.20 seconds in duration  Artifact Markings on ECG tracing that are not a product of heart’s electrical activity – Patient movement is among its many causes – Can mimic life- threatening dysrhythmias  Summary Graphic record or tracing is called an electrocardiogram while the machine that produces it is called the electrocardiograph Electrodes placed on patient’s skin detect heart’s electrical activity Abnormalities in cardiac rate and/or rhythm are called dysrhythmias  Summary Each lead provides a different view of heart. Impulses traveling toward a positive electrode are recorded on ECG as upward deflections. Impulses traveling away from a positive electrode or toward a negative electrode are recorded as downward deflections.  Summary Limb leads are produced by placing electrodes on right arm (RA), left arm (LA) and left leg (LL). – Include leads I, II, III, augmented vector right (aVR), augmented vector left (aVL) and augmented vector foot (aVF). Precordial leads include leads V1, V2, V3, V4, V5, and V6.  Summary ECG paper consists of horizontal and vertical lines that form a grid. Horizontal measurements used to determine heart rate and duration of various waveforms, segments and intervals. Vertically on ECG paper, distance between lines, or boxes, represents amplitude in millimeters (mm) or electrical voltage in millivolts (mV).  Summary Calibration Artifact is markings on ECG tracing that have no relationship to electrical activity of the heart.