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Electrocardiogram Wendy Blount, DVM Nacogdoches TX Wendy Blount, DVM Nacogdoches TX Dr. Callan Video

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ECG What it Detects Heart chamber enlargement Eccentric hypertrophy Dilation and growth of heart chambers Due to volume overload Concentric hypertrophy Wall thickening of heart chambers Due to pressure overload Conduction Disturbances

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ECG What Doesnt Detect Type of Heart chamber enlargement Eccentric vs. Concentric hypertrophy Congestive Heart Failure A Short ECG wont detect many arrhythmias Arrhythmias can be intermittent 10 minutes is

Wide P wave (Sometimes Notched) 50 mm/sec > 2.5 boxes wide 25 mm/sec > 1.25 box wide LA enlargement Tall P wave (often spiked) Dog > 4 boxes tall, cat > 2 boxes tall RA enlargement Variable P wave normal variation wandering pacemaker increased vagal tone Lack of P wave Atrial standstill

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ECG P Wave Measurements Wandering pacemaker

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ECG PR Interval Conduction from atria to ventricles (AV node) Establishes the ECG baseline Normal Dog: 0.06-0.13 sec 50mm/sec 3-6.5 boxes 25mm/sec 1.5-3.25 boxes Normal Cat: 0.05-0.09 sec 50mm/sec 2.5-4.5 boxes 25mm/sec 1.25-2.25 boxes

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ECG PR Interval Short PR Interval (tachycardia) AV node is bypassed Accessory pathway (Wolff-Parkinson-White) Congenital or acquired Treated in people by radioablation of the pathway Sudden onset of tachycardia in a dog Can try calcium channel blockers Diltiazem SR (Plumb dose) If you dont treat right away, the myocardium will poop out & rapidly progressive CHF will ensue

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ECG PR Interval Normal Dog: 0.06-0.13 sec (3-6.5 boxes) Normal Cat: 0.05-0.09 sec (2.5-4.5 boxes) Long PR Interval Slow conduction through abnormal AV node AV Blocks

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ECG PR Interval Normal Dog: 0.06-0.13 sec 3-6.5 boxes Normal Cat: 0.05-0.09 sec 2.5-4.5 boxes 1 st degree AV Block Every P wave is followed by a QRS Due to increased vagal tone Non-pathogenic 50 mm/sec

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ECG PR Interval

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2 nd degree AV Block Some P waves not followed by a QRS Mobitz type I PR progressively longer until QRS dropped (Wenkebach Phenomenon)

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ECG PR Interval 2 nd degree AV Block Some P waves not followed by a QRS Mobitz type 2 no pattern PR interval does not change P-P interval is consistent, so SA node is working fine PR interval may be prolonged and may be normal Occasionally, a P wave is not followed by a QRS Not necessarily pathogenic

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Physiology - Cardiac Pacemakers Automatic cells in the heart Depolarize on their own during phase 4 of the cardiac cycle (escape beat) 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 ) Ventricular myocytes (20-40 beats/min)

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Physiology - Cardiac Pacemakers Automatic cells in the heart The fastest functioning pacemaker in the heart takes over, by default The closer to the AV node, the more the escape beat will resemble normal QRS The closer to the ventricle, the more wide and bizarre the QRS will appear Escape rhythm pacemaker other than SA node takes over, because SA node fails to fire

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ECG PR Interval 3 rd degree AV Block (complete AV block) No relationship between P waves and QRS P waves have their own rate (faster), determined by the normal SA node QRS has its own rate (slower), determined by the automaticity of the fastest remaining functioning pacemaker Treatment Pacemaker, if escape rhythm rate doesnt support normal activity Prognosis Cats without anesthesia, potentially very good, as they usually escape from the AV node Dogs eventual asystole is likely, if no pacemaker implanted, as they escape from Purkinje fibers, HIS or ventricles

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ECG PR Interval 3 rd degree AV Block (complete AV block) Pacemaker above bifurcation of bundle of His Pacemaker left ventricle

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ECG QRS Complex Measurements Normal Dog: