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Electrocardiogram Wendy Blount, DVM Nacogdoches TX 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 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 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 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!! 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 Black – LF Green – RR (ground) Red – LR 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 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 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 ECG – 6 Leads Bipolar leads • I – LF+ RF• II – LR+ RF• III – RR+ LFUnipolar leads • aVR – RF+ (summation lead III)• aVL – LF+ (summation lead II)• aVF - LR+ (summation lead I)- ECG – Systematic Interpretation 1. Heart Rate and Rhythm 2. 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 3. Mean Electrical Axis Form ECG – Measurements Take 3-5 measurements and average All measurements done in lead II Use calipers Measure from the center of the line 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 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 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 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 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 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 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 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 ECG – ST Segment Other Causes of ST segment depression or elevation • hypothermia • hypokalemia • Digitalis toxicity • Bundle branch block 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 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 ECG – Mean Electrical Axis (MEA) +1.5 - 5 = -3.5 +10 -1.5 = +8.5 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) 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 3.5 (MEA) ECG – Mean Electrical Axis 3.5 17.5 3 J 2.5 5.1 Right Bundle Branch Block (RBBB) -15.5 2.5Rate 2.8 Heart – 160 per minute (normal 80-180) 5.2 point Rhythm – Jnormal sinus with 2 complex types Complex 1 Complex 2 P wave 0.05 sec x 0.35 mV PR interval 0.104 sec QRS wave 0.056 sec x 1.75 mV(R) ST segment -0.1 mV depressed MEA – 85o 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 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 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 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 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