MPP2 2025 L02 Electrocardiogram PDF Lecture Notes

Electrocardiogram Lecture 02 Richard Klabunde, PhD Professor of Physiology MU-WCOM 1 Learning objectives 1. Name the different waves, segments, and intervals of an ECG tracing and describe what they represent in terms of cardiac electrica...

Electrocardiogram Lecture 02 Richard Klabunde, PhD Professor of Physiology MU-WCOM 1 Learning objectives 1. Name the different waves, segments, and intervals of an ECG tracing and describe what they represent in terms of cardiac electrical activity 2. Describe the normal placement of electrodes for leads I, II, III, aVL, aVR and aVF and how they are represented by the axial reference system 3. Describe how precordial leads differ from limb leads 4. Determine the mean electrical axis from the standard limb leads 2 Learning resources Klabunde, Cardiovascular Physiology Concepts: 3e, Ch 3: 46-56 Klabunde RE. Cardiac electrophysiology: normal and ischemic ionic currents and the ECG. Adv Physiol Educ 41:29-37, 2017. (A pdf copy is available on Canvas under Week 1 files) Links found on slides See Guided Learning on cvphysiology.com for the electrocardiogram 3 BASIC ELECTROCARDIOGRAM (ECG) 4 What does the ECG measure? The ECG measures potential differences between recording electrodes that are generated by electric currents spreading from the heart throughout the body during depolarization and repolarization Electrodes are placed on the surface of the body in standardized locations 5 Clinical value of ECG Different ECG leads “view” the electrical vectors from different angles, and therefore provide information about electrical activity in different heart regions. The ECG provides information primarily on: ○ Rhythm (rate and synchrony) ○ Conduction ○ Injury currents ○ Muscle mass (e.g., ventricular hypertrophy) 6 Volume conductor principles (Vectors) Instantaneous mean electrical vector + Negative -- + Recording Electrode --- + + + + Instantaneous vectors Positive Recording Electrode (represent electrical dipoles) 7 Volume conductor principles By convention: 1. A wave of depolarization traveling toward a positive electrode records a positive voltage 2. A wave of repolarization traveling toward a positive electrode records a negative voltage 3. Polarities are reversed when waves travel away from the positive electrode 4. Waves traveling perpendicular to the lead axis record no net voltage 5. The magnitude of the voltage is directly related to the tissue mass 6. The instantaneous amplitude of the measured potential depends on the direction of the instantaneous mean electrical vector relative to the recording electrode lead axis 8 Sequence of ventricular depolarization The instantaneous voltage is determined by the direction and magnitude of the mean electrical vector relative to the positive electrode in a particular lead axis Panel A: the vector of septal depolarization moves away from aVL (- mV) and is perpendicular to lead II (0 mV) Panel B: the vector moves toward aVL (+ mV) and lead II (+ mV) Panel C: the vector moves toward aVL (+ mV) and lead II (+ mV) Panel D: the vector moves toward aVL (+ mV) and away from lead II (- mV) Above sequence of ventricular depolarization takes about 100 msec (QRS duration) 9 The QRS complex and mean electrical axis The QRS shape depends on: ○ Sequence (1-4 in figure) and timing of ventricular depolarization abnormal conduction increases QRS duration ○ Ventricular mass hypertrophy increases voltages ○ Relative position of recording electrodes Mean electrical axis is the net vector of the entire sequence of ventricular depolarization 10 Ventricular repolarization – T wave Ventricular repolarization is represented as the T wave following the QRS (red wave) The T wave is normally upright because the wave of repolarization is moving away from the overlying recording electrode This results from the subepicardial cells being the first to repolarize although they were the last to depolarized ○ Normally, subepicardial cells have a shorter AP duration than subendocardial cells; therefore, they repolarize first Klabunde, Cardiovascular Physiology Concepts, 3e 11 ECG tracing – definitions P wave = atrial depolarization (repolarization hidden within QRS) P-R interval = atrial depolarization + AV delay QRS = ventricular depolarization sequence Q-T interval = time from initial depolarization until complete repolarization ST segment = depolarized ventricle (isoelectric) T wave = ventricular repolarization Klabunde, Cardiovascular Physiology Concepts, 3e 12 QRS morphology: naming convention QRS complexes do not necessarily have Q, R and S components By convention: ○ the first positive deflection is an R wave ○ any negative deflection following an R wave is an S wave ○ if the first deflection is negative, it is a Q wave ○ a negative deflection without an R wave is a Q wave (or QS complex) ○ a second positive deflection is Rʹ 13 ECG tracing – normal wave durations and intervals P wave (0.08 – 0.10 s) P-R interval (0.12 – 0.20 s) QRS (0.06 – 0.10 s) QTc interval (≤ 0.44 s)* 𝑄𝑇 * 𝑄𝑇𝑐 = 𝑅𝑅 1 mm = 0.04 sec (= 40 msec) @ 25 mm/sec recording speed 14 Abnormally long P-R intervals indicate impaired conduction within the AV nodal region Termed first degree AV block Can be caused by ○ Excessive vagal stimulation ○ Pharmacologic blockade of -adrenoceptors ○ AV nodal ischemia or disease 15 Altered shape and increased duration of the QRS complex indicate altered conduction within the ventricles Ventricular conduction blocks (e.g., bundle branch block) alter the path of depolarization and increase the time for ventricular depolarization Ventricular rate is not altered by intraventricular conduction blocks unless ectopic beats are triggered Premature ventricular complexes (PVCs) generally have long QRS durations because of altered conduction pathways 16 Heart rate determination 300 150 100 75 60 50 Standard ECG recording paper speed is 25 mm/sec, therefore: 25 𝑚𝑚Τsec 𝑋 60 𝑠𝑒𝑐/𝑚𝑖𝑛 1,500 𝐻𝑅 = # 𝑚𝑚 𝑏𝑒𝑡𝑤𝑒𝑒𝑛 𝑏𝑒𝑎𝑡𝑠 = # 𝑠𝑚𝑎𝑙𝑙 𝑏𝑜𝑥𝑒𝑠 𝑏𝑒𝑡𝑤𝑒𝑒𝑛 𝑏𝑒𝑎𝑡𝑠 Or, count off large boxes between 2 QRS complexes: 300 – 150 – 100 – 75 – 60 – 50 Bradycardia (5 large boxes Tachycardia (>100 beats/min) = 0.2 sec indicates delayed AV nodal conduction. Q2: B ○ QRS complexes are normally ≤0.1 seconds because of rapid conduction via the intraventricular conduction system. If this system becomes impaired (e.g., by ischemia), or is not utilized (because of an ectopic foci), conduction is slowed and the QRS becomes wider. Q3: B ○ If the direction of the time-averaged vector (mean electrical axis) is pointing away from a specific lead, that lead will have a net negative voltage Q4: D ○ Lead II is biphasic and therefore the mean axis is 90 deg from 60 deg (lead II) in the direction of the most positive lead, which is AVF (150 deg); AVL (-30 deg is the most negative lead) 32