Electrocardiography (ECG) Physiology 2023 PDF

Electrocardiography (ECG) Physiology 2023 Learning outcomes I At the end of the lecture students will be able to • Describe the way the electrocardiogram (ECG) is recorded • Describe the standards that are used for recording a 12-lead ECG • Compare the various waveforms that are generated when rec...

Electrocardiography (ECG) Physiology 2023 Learning outcomes I At the end of the lecture students will be able to • Describe the way the electrocardiogram (ECG) is recorded • Describe the standards that are used for recording a 12-lead ECG • Compare the various waveforms that are generated when recording electrocardiograms with the standard limb leads, augmented limb leads, and precordial leads • State the relationship between electrical events of cardiac excitation and the generation of the various waveforms, intervals, and segments that can be observed on ECG 2 Learning outcomes II • Calculate heart rate by using ECG data • Explain how the electrical axis of the heart can be calculated by using ECG data recorded from limb leads • Calculate mean electrical axis of QRS complex under different conditions 3 Electrocardiography & Electrocardiogram (ECG) The ECG (EKG) is a recording of electrical fluctuations that represent sum of the electrical activity generated by the myocardial fibers during depolarization and repolarization. (Not contractions) These changes are detectable on the surface of the body because body fluids can conduct electrical currents 4 There are two major components of an ECG: waves and segments. Waves appear as deflections above or below the baseline. Segments are sections of baseline between two waves. Intervals are combinations of waves and segments. 5 6 • ECG provides information on heart rate and rhythm, conduction velocity, and even the condition of tissues in the heart. • The interpretation of an ECG begins with the following questions: – What is the heart rate? – Is the rhythm of the heartbeat regular (that is, occurs at regular intervals) or irregular? – Are all normal waves present in recognizable form? – Is there one QRS complex for each P wave? • The more difficult aspects of interpreting an ECG include looking for subtle changes, such as alterations in the shape, timing, or duration of various waves or segments 7 From: Centennial of the string galvanometer and the electrocardiogram J Am Coll Cardiol. 2000;36(6):1737-1745. doi:10.1016/S0735-1097(00)00976-1 A. Einthoven in Leiden with the original string galvanometer. B. Original string galvanometer. From left to right: arc lamp, water-cooled electromagnet, independent timer, projecting lenses and the falling-glass-plate camera. (Reprinted from Snellen HS, University of Leiden School of Medicine). C, E. Recording leads I and II. (Einthoven W. Arch International Physiol 1906;4:132). D. An ECG recorded by Einthoven in 1903–4. (Einthoven W. Arch International Physiol 1906;4:132). Date of download: 1/7/2015 Copyright © The American College of Cardiology. All rights reserved. 8 ECG Recording 9 ECG Recording • The ECG can be recorded by using an active or exploring electrode connected to an indifferent electrode at zero potential (unipolar recordings) or by using two active electrodes (bipolar recordings). The Limb Leads (Bipolar) Chest leads (unipolar) 10 The Limb Leads In the augmented limb leads, the negative electrodes are a combination of two electrodes allowing an amplification of the signal recorded by the positive electrode. (Compared to measurement against Wilson’s central terminal) These electrode pairs are used to determine the heart vector in the frontal plane 11 Precordial (Chest) Leads http://www.nottingham.ac.uk Standard position of positive electrodes: V1 - the fourth intercostal space to the right of the sternum V2 - the fourth intercostal space to the left of the sternum. V3 - directly between leads V2 and V4 (in the middle) V4 - the fifth intercostal space in the midclavicular line V5 - horizontally with V4 in the anterior axillary line V6 - horizontally with V4 and V5 in the midaxillary line. The activity of the right ventricle and atria is better visible on these leads The activity of the left ventricle is better visualized on these leads 12 Unipolar leads 13 Limb electrodes are relocated to the trunk https://ecgwaves.com/topic/ekg-ecg-leads-electrodes-systems-limb-chest-precordial/ aVL aVR I II III aVF Different leads "view" the heart from different angles. 16 Vectoral Representation A vector is an entity that has both magnitude and direction. At any given moment during cardiac cycle a vector may represent the net electrical activity seen by a lead. A vector is commonly visualized as an arrow: • The length of the shaft represents the magnitude of the electrical current • The orientation of the arrow represents the direction of the current flow • The tip of the arrow represents the positive pole of the electrical current • The tail of the arrow represents the negative pole of the electrical current Einthoven's Triangle •The three standard limb leads (I, II, III) can be transported into an equilateral triangle called the Einthoven's triangle Walter Einthoven is the father of the modern ECG . He named the parts of the ECG and created “Einthoven’s triangle,”. The choice of P, using letters from the second half of the alphabet is a mathematical 18 convention Medical Physiology, 2e, Elsevier 2012 Einthoven's Law: Lead II = Lead I + Lead III 19 Cardiac Vector • The propagation of action potential in the heart cells is in a certain direction: vectoral • The common vector of depolarization waves is called cardiac vector (axis). Calculation of mean QRS vector. In each lead, distances equal to the height of the R wave minus the height of the largest negative deflection in the QRS complex are measured 20 21 II Reflection of depolarization waves to standart limb lead II Depolarization moving toward an active electrode (+) in a volume conductor produces a positive deflection (R wave) Depolarization moving in the opposite direction produces a negative deflection (S wave) 22 Biopac Student Lab Lesson 6 – ECG II Introduction Repolarization moving opposite direction to active electrode produce pozitive deflection. (e.g., T wave) 24 Atrial depolarization and generation of P wave At each point along the electrical activity, a small charge separation exists in the extracellular fluid between polarized membranes (positive outside) and depolarized membranes (negative outside). Thus, the wavefront may be thought of as a series of individual electrical dipoles (regions of charge separation). Einthoven’s law : at any instant the potential in lead II is equal to sum of the potentials in leads I and III 25 Ventricular depolarization and the generation of the QRS complex • the initial ventricular depolarization usually occurs on the left side of the intraventricular septum àa negative component on lead I, a small negative component on lead II, and a positive component on lead III During ventricular depolarization the number of individual dipoles is greatest and/or their orientation is most similar à large net cardiac dipole à R wave 26 Ventricular depolarization and the generation of the QRS complex Late activation of posterobasal portion of the left ventricule During the ST segment, all ventricular muscle cells are depolarizedà no electrical activity moving through the heart tissue 27 1)atria begin to depolarize 2) atria depolarize 3)ventricles begin to depolarize at apex; atria repolarize 4)ventricles depolarize 5) ventricles begin to repolarize at apex 6) ventricles repolarize 28 Normal • What is the heart rate? • Is the rhythm of the heartbeat Fast Slow regular (that is, occurs at regular intervals) or irregular? • Are all normal waves present in recognizable form? • Is there one QRS complex for each P wave? Irregular 29 Analyze the following ECG traces Proficiency in ECG interpretation http://ecg.bidmc.harvard.edu/maven/mavenmain.asp 30 31 Amplute: 1 mV = 10 small square = 10 mm and paper speed: 25 mm/sec. Heart rate can be calculated : distance between two QRS complex (e.g.0.84 sec) heart rate per minute (60 / 0.84 = 71) Amplitude time 32 33 Boron, W. F & Boulpaep, E. L.; Medical Physiology 3rd edt. 2017 The P-wave duration -how long atrial depolarization takes. The PR interval indicates how long it takes the action potential to conduct through the AV node before activating the ventricles. The QRS duration - the wave of depolarization to spread throughout the ventricles. The QT interval indicates how long the ventricles remain depolarized and is thus a rough measure of the duration of the overall “ventricular” action potential. The QT segment gets shorter as the heart rate increases, which reflects the shorter action potentials that are observed at high rates. aVL aVR I II III aVF 35 Plotting the mean electrical axis of the ventricles from two electrocardiographic leads (leads I and III). 36 Biopac Student Lab Lesson 6 – ECG II Introduction Left axis deviation in a hypertensive heart (hypertrophic left ventricle) 38 COMMON MISCONCEPTIONS ABOUT THE ECG • The PR interval is NOT measured from the P wave to the R wave. It is defined to be the time from the beginning of the P-wave to the beginning of the QRS complex. • The QT interval is NOTmeasured from the Q-wave to the T-wave. It is defined as the time from the beginning of the QRS complex to the end of the T-wave. • The P-wave (QRS complex) is NOT generated by the contraction of the atria (ventricles). It is generated by electrical activity (more specifically depolarization or activation) of the muscle. • Purkinje fibre cells are NOT nerve cells. Rather, they are specialized cardiac muscle cells. The sinoatrial node, atrioventricular node, bundle of His, and bundle branches are also made up of specialized cardiac muscle cells. http://www.medicine.mcgill.ca/physio/vlab/cardio/ECGbasics.htm a wave - atrial contraction c wave- ventricles begin to contract v wave slow flow of blood into the atria 40 https://www.clinicalkey.com/#!/browse/book/3-s2.0C20170004883?indexOverride=GLOBAL 41 • ECG provides information on heart rate and rhythm, conduction velocity, and even the condition of tissues in the heart. • The interpretation of an ECG begins with the following questions: – What is the heart rate? – Is the rhythm of the heartbeat regular (that is, occurs at regular intervals) or irregular? – Are all normal waves present in recognizable form? – Is there one QRS complex for each P wave? • The more difficult aspects of interpreting an ECG include looking for subtle changes, such as alterations in the shape, timing, or duration of various waves or segments 42