Cardiovascular Physiology (ECG) - Ross University PDF

CARDIOVASCULAR PHYSIOLOGY 4. Electrocardiogram (ECG) Andre Azevedo, DVM, MSc Assistant Professor of Veterinary Physiology [email protected] Learning objectives for this lecture Know the major indications for an E...

CARDIOVASCULAR PHYSIOLOGY 4. Electrocardiogram (ECG) Andre Azevedo, DVM, MSc Assistant Professor of Veterinary Physiology [email protected] Learning objectives for this lecture Know the major indications for an ECG Know the ECG terminology Understand the 3 rules of thumb for how ECG records depol and repol waves List the ECG waves and describe their significance List the most important segments/intervals and their significance Clinical correlation lead aVF – 50 mm/s lead II – 50 mm/s lead II – 25 mm/s lead II – 50 mm/s lead II – 25 mm/s lead II – 25 mm/s The ECG In 1893, Willem Einthoven introduced the term “ELECTROCARDIOGRAM” At a meeting of the Dutch Medical Association. Einthoven W: Nieuwe methoden voor clinisch onderzoek [New methods for clinical investigation]. Ned T Geneesk 29 II: 263-286, 1893 In 1895, Dr. Einthoven using an improved electrometer and a correction formula, distinguished five deflections, which he named: P, Q, R, S, and T Willem Einthoven  The ECG The ECG is a graphical record of electric potentials generated by the heart muscle during each cardiac cycle These potentials are detected on the surface of the body using electrodes attached to the limbs and chest wall The ECG machine amplifies the potentials – evaluates rhythm and conduction Displayed on a special graph paper in voltage and time Indications: to characterize arrhythmias and conduction disturbances supportive evidence for pericardial effusion, chamber enlargement, and electrolyte disturbances to monitor the heart rhythm under anesthesia OBS: ECG does not record cardiac mechanical activity (contractility) and may be normal even in the face of advanced cardiovascular disease The ECG https://breadnervet.com/services/holter-monitor/ https://bvna.org.uk/blog/ecg-interpretation-by-charlotte-pace/ https://vetmed.iastate.edu/story/cool-tech-vets-and-pets-gd  ECG Terminology WAVEFORM COMPLEX Movement away from the baseline in either a positive or Group of waveforms negative direction. HEART RATE Number of heartbeats per minute SEGMENT TACHYCARDIA The time length between two specific points on an ECG Increased heart rate that are supposed to be at the baseline amplitude. BRADICARDIA INTERVAL Decreased heart rate The time length between two specific ECG events. SINUS RHYTHM Usually includes a segment and one or more waveforms. Normal cardiac rhythm where depolarization begins at the SA node ARRHYTHMIA Abnormal heart rhythm How ECG works? https://www.youtube.com/watch?v=xIZQRjkwV9Q&t=426s  How ECG works? A positive and a negative charge separated by a small distance is a DIPOLE. A dipole can generate local current flow and small electrical field Cardiac muscle cells, when Depolarizing or Repolarizing, show different charges along their membranes and act like a dipole. The electrical field generated by a single dipole in the body is too small to be measured, BUT the sum of multiple dipoles can be detected How ECG works? When cardiac impulses pass through the heart, the electrical current also spreads into the adjacent tissues A small portion of the current spreads to the surface of the body Electrodes usually placed on the limbs can record the APs THE SUM OF ALL THE CURRENTS ORIGINATED FROM THE ELECTRICAL ACTIVITY OF THE HEART IS A VECTOR (ARROW)  How ECG works? Dr. Einthoven proposed the placement of the electrodes for ECG recording – THE EINTHOVEN’S TRIANGLE Electrodes are placed in the 2 arms (RA, LA) and 1 leg (LL) Compare electrical activity between any two points The position of the 3 electrodes looks at the heart from 3 different points of view A triangle can be drawn around the area of the heart STANDARD BIPOLAR LEADS WHEN THE IMPULSE TRAVELS PARALLEL TO THE (these are the most used) RECORDING ELECTRODES, LEAD I – compare LA (+) and RA (-) THE ELECTRODES “SEE” THE MAXIMUM AMOUNT OF LEAD II – compare LL (+) and RA (-) ELECTRICITY LEAD III – compare LL (+) and LA (-) (RECORD TALLER WAVES)  How ECG works? HOW WILL ECG RECORD THE DEPOL AND REPOL WAVES (VECTORS)? 1. A current of depolarization traveling toward the + electrode is recorded as a positive deflection 2. A current of depolarization traveling away from the + electrode is recorded as a negative deflection 3. A current of repolarization traveling away from the + electrode is seen as a positive deflection sister  How ECG works? OTHER LEADS Augmented unipolar leads provide three additional frontal plane views aVR – compares RA with the sum of the 2 other limbs aVL – compares LA with the sum of the 2 other limbs aVF – compares LL with the sum of the 2 other limbs Unipolar precordial chest leads Used more often in human medicine than in veterinary medicine Are transverse plane leads (dorsal and ventral planes) V1 – V6 in humans  ECG paper Allows the determination of duration and amplitude of the waveforms and intervals/segments. 604808 Standard calibration paper speed = 25 mm/sec in abut Hbert humans By convention, in small animals, paper speed is 50 how my mm/sec Can be 25 if the HR is 120 beats/min or lower The horizontal axis represents time in milliseconds If slower variations of electrical potentials occur, complexes become wider More TIME to depolarize/repolarize (ex: fibrosis) The vertical axis represents amplitude in millivolts More tissue depolarizing results increase the amplitude (height) of the complexes More muscle  higher waves  ENMsdae.dk ECG paper What is the heart rate? 9 small boxes at 25mm/s  9 x 0.04 = 0.36s 1 beat --- 0.36s ? beats --- 60s (just do the rule of three) HR = 60/0.36 = 166 BPM  Waveform and intervals – P wave The first ECG deflection is the P WAVE Represents depolarization of atrial muscle Discharge of the SA node is assumed to have occurred just before the P wave No deflection for SA node discharge – a small number of cells tie No deflection for atrial repolarization is haggen atte sue forgly y  Waveform and intervals – P-R interval The P-R INTERVAL represents the time it takes for the electrical impulse to conduct through the atria, AV node, and Bundle of HIS Mostly through AV node (impulse delayed) Increased P-R interval associated with A-V blocks From the start of the P wave to the first deflection Wi of the QRS (should be named P-Q, right? ¯\_(ツ)_/¯) Sympathetic stimulation decreases P-R interval Increases conduction velocity Parasympathetic stimulation increases P-R interval Decreases conduction velocity thebundle  of Waveform and intervals – QRS complex As the impulse activates the6B g HIS-Purkinje system and ventricular muscle, a QRS COMPLEX is generated These 3 waves together represent ventricular depolarization Total duration of the QRS complex is similar to that of the P wave Faster conduction velocity Each wave represents a specific place of the ventricle being depolarized  Waveform and intervals – Q-T interval The Q-T INTERVAL reflects the approximate duration of ventricular systole AND the ventricular refractory period Is measured from the beginning of the Q wave to the end of the T wave The S-T SEGMENT correlates with the plateau of the ventricular AP Ending of depol and beginning of repol Gheteer go Isoelectric because all ventricular muscle is depolarized 2 waves  8 Waveform and intervals – T wave The T WAVE represents ventricular repolarization Has a longer duration than the QRS because repolarization does not occur as a synchronized propagated wave High degree of variability in dogs and cats Can be positive, negative, biphasic, or very low amplitude (but should be consistent) Waveform and intervals – R-R interval The R-R INTERVAL R-R interval = time between one R wave and the next = cycle length Used to evaluate the regularity of the heartbeats (rhythm) Used to calculate HR when the rhythm is regular to ditate Most efficient oes abnormalities ECG interpretation ANSWER THESE QUESTIONS: 1. What is the Heart Rate? 2. What is the Rhythm? Is it Regular or Irregular? 3. Is there a P Wave for Every QRS Complex? 4. Is there a QRS Complex For Every P wave? 5. What is the Relationship Between the P Waves and QRS Complexes? 6. Is the P/QRS/T waves morphology normal? 7. Are the intervals normal? https://bvna.org.uk/blog/ecg-interpretation-by-charlotte-pace/  FYI Normal values  ECG in large animals In large animals, there is considerable variability in the polarity and size of the ECG waves ECG waves vary more among horses and cattle than dogs and cats A lot of variation between individuals Cardiac depolarization pathways are not consistent ECG is only valid for the detection of arrhythmias Less useful for diagnosing structural abnormalities Ex: Ventricular hypertrophy  1. What is the Heart Rate? FYI Clinical correlation 2. 3. What is the Rhythm? Is it Regular or Irregular? Is there a P Wave for Every QRS Complex? 4. Is there a QRS Complex For Every P wave? 5. What is the Relationship Between the P Waves and QRS Complexes? 6. Is the P/QRS/T waves morphology normal? 7. Are the intervals normal? lead aVF – 50 mm/s  1. What is the Heart Rate? FYI Clinical correlation 2. 3. What is the Rhythm? Is it Regular or Irregular? Is there a P Wave for Every QRS Complex? 4. Is there a QRS Complex For Every P wave? 5. What is the Relationship Between the P Waves and QRS Complexes? 6. Is the P/QRS/T waves morphology normal? 7. Are the intervals normal? lead II – 25 mm/s  1. What is the Heart Rate? FYI Clinical correlation 2. 3. What is the Rhythm? Is it Regular or Irregular? Is there a P Wave for Every QRS Complex? 4. Is there a QRS Complex For Every P wave? 5. What is the Relationship Between the P Waves and QRS Complexes? 6. Is the P/QRS/T waves morphology normal? 7. Are the intervals normal? lead II – 25 mm/s  1. What is the Heart Rate? FYI Clinical correlation 2. 3. What is the Rhythm? Is it Regular or Irregular? Is there a P Wave for Every QRS Complex? 4. Is there a QRS Complex For Every P wave? 5. What is the Relationship Between the P Waves and QRS Complexes? 6. Is the P/QRS/T waves morphology normal? 7. Are the intervals normal? lead II – 50 mm/s  1. What is the Heart Rate? FYI Clinical correlation 2. 3. What is the Rhythm? Is it Regular or Irregular? Is there a P Wave for Every QRS Complex? 4. Is there a QRS Complex For Every P wave? 5. What is the Relationship Between the P Waves and QRS Complexes? 6. Is the P/QRS/T waves morphology normal? 7. Are the intervals normal? lead II – 50 mm/s  1. What is the Heart Rate? FYI Clinical correlation 2. 3. What is the Rhythm? Is it Regular or Irregular? Is there a P Wave for Every QRS Complex? 4. Is there a QRS Complex For Every P wave? 5. What is the Relationship Between the P Waves and QRS Complexes? 6. Is the P/QRS/T waves morphology normal? 7. Are the intervals normal? lead II – 25 mm/s  FYI Clinical correlation  Further reading Introduction to electrocardiography https://www.vin.com/apputil/content/defaultadv1.aspx?pId=11268&catId=32744&id=3866623 ECG interpretation by Charlotte Pace https://bvna.org.uk/blog/ecg-interpretation-by-charlotte-pace/ Electrocardiography https://sites.tufts.edu/cardiorush/syllabus-electrocardiography/ Reading ECGs in veterinary patients: an introduction https://www.dvm360.com/view/reading-ecgs-in-veterinary-patients-an-introduction Electrocardiography in Ruminants https://veteriankey.com/in-ruminants/ Electrocardiogram and Its Interpretation of Cardiac Diseases in Cattle https://www.intechopen.com/chapters/82061 Questions?