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____________ may increase voltage or prolong the P-wave.
Atrial dilatation or disease
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What effects might ventricular hypertrophy (concentric or eccentric) have on the ECG? (3)
increase voltages, wider QRS complex, change in the frontal axis
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Describe the difference between primary and secondary T wave abnormalities.
primary T wave abnormalities occur with a normal QRS, from hyperkalemia hypotension/ischemia, etc; secondary T wave abnormalities occur due to an abnormality in the QRS, which is what we really care about diagnostically
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Increased height/ voltage/ taller P waves are consistent with __________.
right atrial dilation (P pulmonale)
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Wider than normal P waves are consistent with ___________.
left atrial dilation (P mitrale)
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What would eccentric LV hypertrophy look like on ECG?
increased voltages of QRS with normal frontal axis (morphology)
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What are potential causes of increased QRS voltages and normal shape? (4)
DCM, LV volume overload (such as PDA, MR, AR)
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Which 2 leads normally have the largest (most positive) QRS complexes?
Leads II and aVF
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What causes increased QRS duration/ wider QRS complexes?
left bundle branch block (b/c myocardial spread from cell to cell is ay slower than if it goes through the BB), LV hypertrophy, LV myocardial disease
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What is left axis deviation?
instead of leads II and aVF being the most positive, leads I and aVL dominate (frontal axis has shifted to the left cranial quadrant)
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What are potential causes of left axis deviation? (3)
normal variation, concentric LV hypertrophy, left anterior fascicular block
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What are potential causes of right axis deviation? (4)
RV hypertrophy due to pulmonic stenosis, atrial septal defect, pulmonary hypertension, or RV conduction delay
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What is right axis deviation?
instead of leads II and aVF being the most positive, leads III and aVR dominate
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What are the key ECG features of RV hypertrophy? (3)
right axis deviation, prominent S wave in lead I, often an S wave in leads I, II and III (so-called S1,2,3 pattern)
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What does an S wave in lead I indicate?
that means the terminal activation is going to the right, which is abnormal
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What are primary changes in repolarization (ST interval)? (4)
ischemia, hypoxia, pericarditis, hyperkalemia
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What ECG changes are associated with hyperkalemia? (2)
absent P waves (sodium channels don't open b/c have passed necessary voltage, atrial muscle inexcitable), tented T waves
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"Rescue" complex from subsidiary pacemaker cells.
escape beat
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Impulse arising outside of the SA node.
ectopia
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Delay or interruption of current flow.
Block
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Rapid, disorganized electrical activity affecting atria or ventricles; disorganized electrical activation and ineffective myocardial contraction.
Fibrillation
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Absence of electrical and mechanical activity.
Asystole
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What are potential hemodynamic consequences of arrhythmias? (5)
reduced cardiac function, decreased BP, reduced tissue perfusion, limited exercise capacity, syncope
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What are potential electrical consequences of arrhythmias? (3)
myocardial fibrillation, asystole, sudden cardiac death
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What are the general causes of arrhythmias? (5)
cardiac diseases (structural diseases, heart failure, ischemia, primary electrical disorders), metabolic/ endocrine disorders, autonomic nervous system, drugs/toxins, usual suspects (GDV, splenic disease, etc)
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Describe normal sinus rhythm.
normal HR, every P is followed by a QRS, every QRS is preceeded by a P, and the rhythm is very regular
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Describe sinus arrhythmia.
normal rate and irregular rhythm, varying sinus node discharge rate causes cyclical mild variation in HR; occurs due to varying amounts of vagal tone to SA node (often associated with respiration)
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Describe wandering pacemaker.
varying vagal influence to the SA node, causing the node to fire from different foci, which may alter P wave morphology
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What are the types of supraventricular arrhythmias? (4)
PACs, atrial tachy, atrial flutter, atrial fib
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What is respiratory sinus arrhythmia?
SA node speed during inspiration and slows during expiration due to changes in vagal tone
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Wandering pacemaker is completely normal in the setting of _____________.
sinus arrhythmia
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How is sinus arrest defined?
no atrial activity for a period exceeding two normal P to P intervals
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What are PACs?
beats that occur earlier than the current R-R interval, but whose QRS complex appears nearly identical to the sinus QRS complexes
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What is considered atrial tachycardia?
a string of more than 3 PACs in a row
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What is considered atrial flutter?
sawtooth looking F waves in the baseline; contrast from afib because they are uniform and consistent (afib is disorganized and haphazard)
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Describe afib.
highly chaotic and irregular, the AV node slows conduction, only allowing a random number of beats to be conducted and therefore, causing the irregular ventricular rhythm (sounds like tennis shoes in the dry on auscultation)
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Which patients benefit from rhythm control? (3)
those with relentless tachycardia, atrial flutter, or afib
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How do you manage a dog with heart failure and an atrial arrhythmia?
digoxin (the only positive inotrope of the 3 rate control drugs) + diltiazem
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Which type of rate control drug should NOT be used in a heart failure patient and why?
beta blockers because they are negative inotropes
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How should you manage sinus tachycardia?
identify underlying cause of increased sympathetic activity, manage hypotension or heart failure
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How do you manage sinus bradycardia?
identify underlying reasons for high vagal tone, atropine or catecholamine if needed
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How do you manage sinus arrest- sick sinus syndrome?
pacemaker
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How do you pharmacologically control how fast the ventricle beats?
regulate the AV node because the atria control how fast the ventricles go
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In sinus arrest, the blame is with the _________; with atrial standstill, the blame is with the _________ because it is _________.
sinus node; atrial muscle; inexcitable (usually due to hyperkalemia)
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_____________ determine ventricular rate.
AV nodal conduction
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____________ determines which fibrillation waves get through to cause a QRS complex.
AV nodal conduction
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What drugs are used for heart rate control in animals with afib? (3)
Digoxin, Diltiazem, Beta-blockers (-lol)
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How are persistent PACs managed sometimes?
drugs that suppress automaticity or modify conduction (rhythm control)- sotalol, beta-blockers, amiodarone, flecainide
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What drug is used in horses to convert afib to normal sinus rhythm? What is the initial effect of this drug on the HR prior to conversion?
quinidine (horses don't usually have associated structural heart disease, unlike dogs, which almost always have structural disease as the cause); initially causes sinus tachycardia because of increased AV nodal conduction
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Describe electrical cardioversion.
electrical shock delivered to heart right at the R wave to convert the animal from afib back to normal sinus rhythm (horses and dogs)
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What is Tosades de pointes?
ventricular arrhythmia in which the QRS complexesgo from positive to negative, like a ballerina turning on a point
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What are two life-threatening and lethal ventricular arrhythmias?
ventricular fibrillation and ventricular standstill/asystole
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When is a QRS complex considered premature (VPCs)?
it's very very close to the prior T wave and causes secondarily irregular following T wave [the QRS is wide and bizarre and P waves do not occur at a regular interval]
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Why is the QRS of VPCs "wide and bizarre"?
the depolarization spreads out from the ectopic focus from cell to cell, which takes more time than through the his-purkinje system
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PCVs originating from the LV are usually __________ in lead II,while PCVs originating from the RV are usually _________ in lead II.
negative; positive
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What is sick sinus syndrome?
disease of the SA node as well as the distal conducting system that results in periods of SA arrest without appropriate escape activity
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How do we TRY to assess PVCs? (6)
frequency, timing (R on T), morphology, runs of Vtach, rate of Vtach, complexity
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What are oral drugs for the chronic therapy of ventricular and atrial arrhythmias?
SPAAM: sotalol, procainamide, atenolol, amiodarone, mexiletine
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What is the usual in-hospital therapy for ventricular tachyarrhythmia?
- First line: lidocaine
- Second line: amiodarone, procainamide
- Third line: esmolol
- Supportive: potassium and magnesium salts
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Describe first degree AV block.
prolonged PR interval but all P waves are conducted
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Describe second degree AV block.
incomplete atrial-ventricular conduction- some P waves are blocked
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Describe third degree AV block.
complete AV block- patient survives on an escape rhythm below the level of block
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Hyperkalemia causes __________.
atrial standstill
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Bundle branch block occurs when...
only a portion of the normal conduction system is blocked; the normal conduction system splits from the bundle of His into the left and right bundles, supplying the left and right ventricles respectively; if one of the bundles cannot conduct, then half of the heart is depolarized normally, while the other half is depolarized from cell to cell, causing the QRS to widen (takes longer)
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If the right bundle branch is blocked, the QRS will be...
wider than normal and will be predominantly negative in lead II.
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If the left bundle branch is blocked, the QRS will be...
wider than normal and positive in lead II.
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What is the clinical significance of an ECG with a normal frontal axis and very large R waves in lead II from a young dog with a continuous heart murmur?
PDA causing LV dilatation to increase SV to feed the shunt and maintain normal CO
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What is the clinical significance of an ECG with a normal frontal axis, absent Q waves in lead II, and a very long QRS duration in a dog with DCM?
LV eccentric hypertrophy; can lead to BBB
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What are causes of ST segment elevation and depression?
- ST elevation: normal variant "early repolarization", ischemic heart disease, acute pericarditis, LV hypertrophy, LBBB, advanced hyperK+, hypothermia
- ST depression: usually ischemia, normal variant/artifact, hyperventilation, ventricular hypertrophy, Digoxin, hypoK+, MV prolapse, CNS disease, secondary to arrhythmia
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J point depression occurs with ____________.
sinus tachycardia
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What is sick sinus syndrome? How is it managed?
disease of the SA node and distal conducting system that results in periods of SA arrest without appropriate escape activity; the only effective management is a pacemaker.
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Describe the clinical features of premature atrial complexes on ECG. (3)
irregular rhythm, premature QRS initiated by a premature ectopic P wave, P waves have weird morphology
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Describe the clinical features of focal atrial tachycardia on ECG.
ectopic atrial rhythm from outside the SA node
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Contrast defibrilliation from electrical cardioversion.
defibrillation is when you have no QRS complexes and are going to die, so you shock/depolarize all of the cells of the heart at once to get them conducting again; cardioversion is when you give a shock right on the R wave to depolarize and allow the sinus node to resume normal function/rhythm
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Using afib as an example, explain the circumstances when you rate versus rhythm control.
- We cannot use rhythm control in animal with structural heart disease (ie. almost ALL dogs with afib); conversely, horses do not usually have structural heart disease with a fib, so we manage them with quinidine first.
- For SA with afib, we use rate control, such as Digoxin, Diltiazem, or beta-blockers.
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Which 3 drugs are commonly used to obtain rate control in SAs with atrial tachyarrhythmias? What is the mechanism of each drug?
- Digoxin: increases vagal tone through sensitization of the baroreceptor reflex
- Diltiazem: blocks Ca2+ entry into AV node
- Beta-blockers: reduced Ca2+ entry across L-Ca2+ channels by decreasing sympathetic tone
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What is ventricular bigeminy?
ectopic beats/VPCs occur after every sinus beat, causing alternating long and short heart beats
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What is ventricular tachycardia?
a string of VPCs linked together
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Contrast monomorphic and polymorphic vtach.
- monomorphic: accelerated idioventricular rhythm- "slow" vtach
- polymorphic: very fast vtach that is varying in appearance; more serious
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What are the indications for pacemakers in dogs?
high grade second degree AV block, complete AV block (third degree), sinus arrest/sick sinus syndrome
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