Interpreting Electrocardiograms

Evaluate cardiac arrythmias
Detect cardiac chamber enlargement
Detect pericardial disease
Identify electrolyte abnormalities
Help to identify myocardial ischemia and/or necrosis
Evaluate changes in sympathetic and parasympathetic tone
Detect the presence and effects of drug-mediated changes on the heart

RMP: -90 mV (same as phase 4)
Phase 0: Depolarization. Fast due to Na influx when cells are stimulated to threshold
Phase 1: Rapid repolarization (brief)
Phase 2: Continued depolarization due to delayed onset of K permeability and continued flow of Na and Ca via slow L-type Ca channels.
Phase 3: Repolarization actually occurs when flow through L-type Ca channels ceases and K permeability increases, return to RMP marks the end.
Phase 4: Transmembrane potential is constant

RMP: -60 mV (unstable due to leaky Na "funny" channels)
Phase 0: Depolarization. Slow due to Ca and Na influx through SLOW Ca channels.
Phase 1,2,3: No phase 1 rapid repolarization.
No clear phase 2 "delayed depolarization"/plateau. Leaky Na channels make repolarization more gradual
Phase 3 repolarization occurs when flow through slow Ca channels ceases and K permeability increases
Phase 4: RMP gradually increases until threshold is reached, initiating the next depolarization.

SA node
AV node
Bundle of HIs
Purkinje fibers
These cells display automaticity due to instability in phase 4 of their action potentials due to Na leakage into the cells --> gradual decrease in their electronegativity.

The pacemaker cells with the fastest rate of spontaneous depolarization will be the primary pacemaker, so in a normal heart, the SA node is the primary pacemaker.

SA paper speed: 50 mm/sec 1 big box = 0.1 seconds
LA paper speed: 25 mm/sec, 1 big box = 0.2 seconds
Standard sensitivity: 10 mm = 1 mV = 2 big boxes
Double sensitivity: 20 mm = 1 mV = 4 big boxes
Half sensitivity: 5 mm = 1 mV = 1 big box

Near the junction of the cranial vena cava and the right atrium.
70-140 bpm in dogs.

Atrial internodal tracts, they run throughout the atrial myocardium.

AV node is located between the atria and ventricles.
40-60 bpm in dogs.

The Bundle of His runs between the interatrial septum and the ventricular septum, though the atrioventricular ring

They branch from the Bundle of His to each ventricle.

Purkinje fibers are the terminal branches for electrical impulse throughout the ventricles in the subendocardial layer.
His-Purkinje fibers have a rate of 15-40 bpm in dogs.

False. You can get information about chamber enlargement in small animals, but not large animals.

Lead I: Left arm (+), Right arm (-)
Lead II: Left leg (+), Right arm (-)
Lead III: Left leg (+), Left arm (-)
Lead aVR: Right arm (+), left arm and left leg (-)
Lead aVL: Left arm (+), right arm and left leg (-)
Lead aVF: left leg (+), left arm and right arm (-)

Depolarization of the atria.
Will be shown as positive on leads I-III
Repolarization of the atria is usually obscured by the QRS complex.

The time it takes for the sinus node impulse to get through the conduction system to the Bundle of His and Purkinje system, marked by the beginning of the Q wave.

Depolarization of the ventricles.
Q waves are the depolarization of the intraventricular septum.
The R and S waves represent the direction of the spreading depolarization of the contracting ventricles.
Time is marked by the beginning of the Q wave to the beginning of the ST segment.

Ventricular repolarization

Use this if the HR is regular
If paper speed is 50 mm/sec, divide mm between QRS complexes into 3000.
If paper speed is 25 mm/sec divide into 1500

Use this if the HR is irregular.
Count the number of beats in 3-12 seconds, then multiply appropriately.

The average of all of the instantaneous vectors recorded of the QRS complex.

Normal for dogs: +40 to +103
MEA > +103 suggests RV enlargement.

1) Take difference between the height of all positive QRS deflections and all negative QRS deflections in the two chosen leads. Plot units on lead axes, draw perpendicular lines, then draw vector from origin to intersection of these lines. (Not widely used)
2) MEA = perpendicular to isoelectric lead.

Increased P wave amplitude: R atrial enlargement
Increased P wave duration: L atrial enlargement

The time from SA node impulse to traverse AV node and His-Purkinje system.
Measured from beginning of P to beginning of Q.
Prolonged = delayed AV nodal conduction

Amplitudes of R and S measured from baseline.
Duration from beginning of first wave of the QRS complex to the start of the ST segment
Wide QRS + no signs of R heart enlargement suggests L heart enlargement.

ST elevation or depression suggests myocardial ischemia, acute myocardial infarction, electrolyte disturbances, digitalis toxicity, and cardiac trauma.

QT interval: total time for ventricle to depolarize and repolarize, from beginning of the QRS to the end of T.

Prolonged QT interval suggests hypoCa, hypoK, quinidine toxicity, ethylene glycol toxicity, strenuous exercise, hypothermia, and CNS disorders

Shortened QT interval suggests hyperCa, digitalis toxicity, hyperK

Diagnosis when two or more of these criteria exist:
-Increased R amplitude in II, III, aVF (w/o signs of RVH)
-Prolonged, widened QRS
-T waves directed opposite to the main QRS deflection in that lead
-Slurring of ST segment (doesn't flatten)
-Tall T waves

Diagnoses when 2 or more criteria are present:
-Frontal plane MEA > 100+ in dogs, >+180 in cats
-Terminal negative deflections (S waves) in leads with positive poles to the left
-Positive T wave in lead V10
-R/S < 0.87 in CV6LU

Individual criteria for RVE and LVE
Suggested that deep Q waves in I, II, III, aVF + LVE criteria = BVE, but not objectively established.