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Electrocardiogram

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  1. Electrocardiogram Wendy Blount, DVM Nacogdoches TX

  2. ECG – What it Detects Heart chamber enlargement • Eccentric hypertrophy • Dilation and growth • Due to volume overload • Concentric hypertrophy • Wall thickening • Due to pressure overload Conduction Disturbances

  3. ECG – When to Do • Pulse deficits detected on exam • Chaotic heart sounds (arrhythmia) detected on exam • Tachycardia • Bradycardia • Episodes of weakness or collapse Intermittent arrhythmias may not be detected in a single ECG

  4. ECG – When to Do Event Recorders • Owner/witness starts recording during an event Holter Monitors • Continuously record ECG for 24 hours • Can rent for Dr. Kate Meurs at Washington State Vet School http://www.vetmed.wsu.edu/deptsVCGL/holter/requestform.aspx

  5. ECG – Helpful Hints • Always in right lateral recumbency • Patient on a towel or rubber mat • Metal tables are more problematic • Limbs perpendicular to body • Place leads at the elbow and knee • No one moves while the ECG is being recorded • Enhance lead contact with gel or alcohol Alcohol is FLAMMABLE!!

  6. ECG – Helpful Hints Which lead goes where • “Snow and Grass are on the ground” • White and green leads are on the bottom (R) • “Christmas comes at the end of the year” • Red and green are on the back legs • “Read the newspaper with your hands” • White and black are on front legs White – RF Green – RR (ground) Black – LF Red – LR

  7. ECG – The Cardiac Cycle P wave • SA node fires • Atrial depolarization (contraction) • HS4 • Iternodal tracts (shortcut to AV node) PR interval (no deflection - baseline) • Beginning of P wave to beginning of QRS • AV node (*most of the PR interval is here*) • Bundle of HIS • bundle branches (R and L) • Purkinje fiber network

  8. ECG – The Cardiac Cycle QRS complex • ventricular depolarization (systole) • Q wave 1st negative deflection • R wave 1st positive deflection • S wave 2nd negative deflection • HS1 at beginning of QRS • HS2 at end of QRS • Pulse is generated

  9. ECG – The Cardiac Cycle T wave • Ventricular repolarization (diastole) • HS3 if myocardium is stiff QT interval (no deflection - baseline) • beginning of QRS to end of T wave • ventricular depolarization & repolarization • QRS Contains HS1 and HS2 • HS3 during T wave • Pulse generated

  10. ECG – 6 Leads Bipolar leads • I – LF+ RF- • II – LR+ RF- • III – RR+ LF- Unipolar leads • aVR – RF+ (summation lead III)- • aVL – LF+ (summation lead II)- • aVF - LR+ (summation lead I)-

  11. ECG – Systematic Interpretation • Heart Rate and Rhythm • Measurements of Deflections • P wave - width and height • PR interval - length • QRS - width and height • QT interval – length • ST segment – relative to PR interval • T wave - width and height • Mean Electrical Axis Form

  12. ECG – Measurements Take 3-5 measurements and average All measurements done in lead II Use calipers Measure from the center of the line

  13. ECG – Heart Rate At 25 mm/sec, 150mm = 6 sec • “Bic Pen Times Ten” • Accurate within 10 beats per minute At 50 mm/sec, 300mm = 6 sec • 2 Bic Pens times Ten • Accurate within 20 beats per minute Normals • Giant dogs 60-140 Med-Lg dogs 70-160 • Toy dogs 80-180 Puppies 70-220 • Cats 100-240

  14. ECG – Rhythm Normal Sinus rhythm • Regular heart rate • P, QRS and T waves in each complex Respiratory Sinus Arrhythmia • heart rate regularly irregular • P, QRS and T waves in each complex • Variable P wave – wandering pacemaker • Heart rate less than 200 Arrhythmia

  15. ECG – P Wave Measurements Atrial depolarization (contraction) Normal Dog: <0.4 mV x <0.04 sec Normal Cat: <0.2 mV x <0.04 sec • Wide P wave, Notched P wave • LA enlargement • Tall P wave (spiked) • RA enlargement • Variable P wave • “wandering pacemaker” – increased vagal tone • Lack of P wave • Atrial standstill

  16. ECG – PR Interval Conduction from atria to ventricles (AV node) Establishes the ECG baseline Normal Dog: 0.06-0.13 sec Normal Cat: 0.05-0.09 sec • Short PR Interval (tachycardia) • AV node is bypassed • “Accessory pathway” (Wolff-Parkinson-White) • Congenital or acquired • Treated in people by radioablation of the pathway • Can try calcium channel blockers

  17. ECG – PR Interval • Long PR Interval • Slow conduction through abnormal AV node • AV Blocks 1st degree AV Block • Every P wave is followed by a QRS • Due to increased vagal tone • Non-pathogenic

  18. ECG – PR Interval 2nd degree AV Block Some P waves not followed by a QRS • Mobitz type I – PR progressively longer until QRS dropped • Mobitz type 2 – no pattern • Not necessarily pathogenic 3rd degree AV Block (complete AV block) No relationship between P waves and QRS • P waves have their own rate (faster) • QRS has its own rate (slower) • Treated by pacemaker

  19. ECG – QRS Complex Measurements Normal Dog: <40 lbs - <0.05sec x <3.0 mV >40 lbs - <0.06sec x <3.0 mV Normal Cat: <0.04sec x <0.9 mV R wave measured from baseline to top • Tall R wave, wide QRS • LV enlargement • Left Bundle branch block

  20. ECG – ST Segment Late ventricular depolarization “J point” should be within 0.2 mV of baseline in dogs Should be on baseline in cats • Between QRS and T waves • Elevation or depression means regional myocardial hypoxia • Myocardial infarction not as common in dogs and cats as in people • Can see in cats with HCM

  21. ECG – ST Segment Other Causes of ST segment depression or elevation • hypothermia • hypokalemia • Digitalis toxicity • Bundle branch block

  22. ECG – Mean Electrical Axis (MEA) • when a wavefront spreads toward an electrode, the largest possible deflection will occur • When a wavefront spreads perpendicular to a lead, the smallest or no deflection occurs • ECG shows the sum of all wavefronts relative to the lead being used to measure (MEA) • Isoelectric lead • lead with the smallest deflection • Perpendicular to the MEA

  23. ECG – Mean Electrical Axis (MEA) • The normal MEA is 40o to 100o (dog) • Lead II is most perpendicular to the normal MEA • Lead II shows the largest deflections in the dog with normal MEA • This is why lead II is the most often used lead in veterinary medicine • aVL is most often the isoelectric lead

  24. ECG – Mean Electrical Axis (MEA) +1.5 - 5 = -3.5 Estimating MEA • Find the isolectric lead • MEA is perpendicular to that, in the direction of net deflection Calculating MEA by graph • Graph net deflection in lead I on “x axis” • Graph net deflection in lead aVF on “y axis” • Draw the vector between the two (MEA) +10 -1.5 = +8.5

  25. ECG – Mean Electrical Axis (MEA) Right Axis Shift • Right ventricular enlargement • RV hypertrophy or dilation • Right bundle branch block Left Axis Shift • HCM in cats • hyperkalemia

  26. ECG – Mean Electrical Axis (MEA) 3.5 17.5 3.5 3 Right Bundle Branch Block (RBBB) Heart Rate – 160 per minute (normal 80-180) Rhythm – normal sinus with 2 complex types J 2.5 5.1 -15.5 2.5 2.8 5.2 J point Complex 1 P wave0.05 sec x 0.35 mV PR interval 0.104 sec QRS wave0.056 sec x 1.75 mV(R) ST segment -0.1 mV depressed MEA – 85o Complex 2 P wave 0.05 sec x 0.35 mV PR interval 0.102 sec QRS wave 0.06 sec x -1.55 mV(S) ST segment 0 deviation from PR MEA -90o to -120o

  27. ECG – Bundle Branch Blocks Bundle Branch blocks • Depolarization wave through myocardium rather than through Purkinje network on affected side • depolarization takes longer • depolarization wave “appears bigger” on ECG • Can be persistent or intermittent • Intermittent often precipitated by increased heart rate (delayed refractory period) • Bilateral BBB looks like 3rd degree AV block

  28. ECG – Bundle Branch Blocks Right Bundle Branch Block (RBBB) • Causes: • primary conduction system disease • Disruption of moderator band • RV enlargement • Congenital (beagles) • ECG • Deep S save leads I, II, III, aVF • Wide QRS • May cause a split S2

  29. ECG – Bundle Branch Blocks Left Bundle Branch Block (RBBB) • Causes: • primary conduction system disease • Widespread LV myocardial disease • Unlike RBBB, not usually benign • ECG • Tall R wave • Wide QRS • Looks like a VPC, but follows a P wave with normal PR interval

  30. Physiology - Cardiac Pacemakers Automatic cells in the heart • Depolarize on their own during phase 4 of the cardiac cycle • Rate of depolarization affected by autonomic nervous system • SA node (60-180 beats/min dog) (100-240 cat) • AV node (40-60 beats/min dog) (80-130 cat) • Purkinje fibers (20-40 beats/min) • Bundle of HIS (20-40 beats/min) • Escape rhythm – pacemaker other than SA node, because it fails to fire