Category:Altered Ventricular Impulse Formations

Small Animals: Not as common as atrial abnormalities

Large Animals: Uncommon

1. Ventricular Tachycardia

• A rapid, regular heart beat caused by continuous ventricular premature complexes (VPCs).

• Serious condition which has the capability of causing ventricular fibrillation and ultimately death

Causes

Significant myocardial damage to the heart or disease in other systems, Drug reactions, Hypoxia, Anaemia, Electrolyte and acid-base disturbances, etc.

ECG Appearance: Wide and irregular QRS complexes, P waves without QRS complexes following

Treatment

Treat underlying conditions

Small Animals

Dog: lidocaine (1st line of treatment), quinidine or procainamide (2nd line of treatment)

Cat: propranolol (1st line of treatment), small dose of lidocaine (2nd line of treatment)

Large Animals

• Digitalis

• Quinidine Sulfate

• Magnesium Sulfate supplementation (used in horses)

2. Ventricular Premature Complexes (VPCs)

• An ectopic beat beginning in the ventricles below the AV node and occurring prematurely in relation the the existing cardiac rhythm.

• Often VPCs are following by a compensatory pause.

Causes

Significant myocardial damage to the heart or disease in other systems, Drug reactions, Hypoxia, Anaemia, Electrolyte and acid-base disturbances, etc.

ECG Appearance: P waves absent from the QRS complexes associated with the VPCs, wide and abnormal QRS complexes

Treatment

Treatment similar to ventricular tachycardia.

3. Ventricular Fibrillation

• Total lack of organized ventricular depolarization causing cardiopulmonary arrest and death.

Causes

Cardiac or systemic pathology, Electrolyte disturbances, Electric Shock, Untreated ventricular tachycardia, Hypoxia etc.

ECG Appearance: Wavy baseline without any sign of an organized P, QRS, or T wave

Treatment

Small Animals

• Cardiopulmonary resuscitation: electrical defibrillation, open-chest cardiac massage, intratracheal lidocaine

Large Animals

• Rarely detected in large animals before death

• There is no treatment for large animals with this condition

4. Ventricular Asystole

• The complete absence of electrical cardiac activity

====Causes====Cardiac or systemic pathology, Electrolyte disturbances, Electric Shock, Untreated ventricular tachycardia, Hypoxia etc.

ECG Appearance: Flat base line

Treatment

Small Animals

• Cardiopulmonary resuscitation

• Epinephrine

5. Escape Rhythms

• These rhythms occur in an effort to maintain heart function when the SA or AV nodes are not functioning properly. Escape rhythms occur in pacemaker tissue found in the atria, around the AV node, or in the ventricles. These ectopic rhythms occur at a regular, but slower depolarization rate compared to the dominant SA and AV nodal pacemakers.

ECG Appearance: Depending on where the escape rhythms originate, they can appear either as ectopic beats with a normal or totally abnormal shape.

====Treatment==== Do not use antiarrhythmic drugs in the presence of escape rhythms or else you risk death.

Impulse Conduction Abnormalities

1. SA Block

• The delay (sinus block) or failure (sinus arrest) of the sinoatrial node to conduct impulses.

• SA Block=when an impulse does not leave the SA node

• SA Arrest=momentary failure in the SA node to start an impulse causes a pause in the normal cardiac cycle

Causes

Sick sinus syndrome, electrolyte imbalances, atrial diseases, hypothyroidism etc.

ECG Appearance: Absence of P-QRS-T complexes for a brief period of time

-Sinus block=no P-QRS-T complexes for twice the R-R interval

-Sinus arrest=no P-QRS-T complexes for greater than two R-R intervals

Treatment

Treat the underlying causes; atropine etc., surgical implantation of a pacemaker

2. AV Block

• The delay or failure of the atrioventricular node to conduct impulses.

1st Degree (Partial AV Block)

• Delayed conduction through the AV node

Causes Electrolyte imbalance, drug reactions, degenerative changes in the AV node

ECG Appearance: Longer P-R interval

Treatment Treat the underlying cause

2nd Degree (Partial AV Block)

• Intermittent failure of conduction through the AV node

Mobitz Type I.= prolonged P-R interval

Mobitz Type II.= no prolongation of the P-R interval

Type A: normal QRS length; block is above the bundle of His division (Most commonly seen with Mobitz Type I.)

Type B: longer QRS length; block is below the bundle of His division (Most commonly seen with Mobitz Type II.)

Causes Electrolyte imbalance, drug reactions, degenerative changes in the AV node

ECG Appearance: Some P waves are not followed by QRS complexes

Treatment

Mobitz Type I.: Treat underlying cause if necessary

Mobitz Type II. Treat with atropine etc. or surgical implantation of a pacemaker

3rd Degree (Total AV Block)

• Total failure of AV conduction causing uncoordinated and independent atrial and ventricular depolarisations

Causes Electrolyte disturbances, drug toxicities (e.g. digitalis), acquired nodal diseases, cardiomyopathies, hypothyroidism, endocarditis, lymes disease etc.

ECG Appearance: Independent regular and fast P waves; independent regular and slow QRS waves

Treatment Treat underlying disease, atropine etc., implantation of a cardiac pacemaker

Atrial Standstill

• Temporary or permanent lack of atrial activity resulting from a failure of atrial depolarisation. Ventricles still function normally.

Causes

Electrolyte abnormalities, cardiomyopathies, muscular dystrophy, hypoadrenocorticism

ECG Appearance: Absence of P waves

Treatment

Treat the underlying cause

Bundle Branch Block

• Abnormal ventricular excitation as the result of blockage to the right bundle branch, the left bundle branch, or left anterior fascicular block.

Right Bundle Branch Block

Characterised by the failure or delay of conduction through the right bundle branch. Left ventricle is unaffected.

Causes

Pulmonary embolism, heart worm, electrolyte disturbances, congenital diseases etc.

ECG Appearance: Wide QRS complex, Wide S wave

Treatment

Treat the underlying cause

Left Bundle Branch Block

Characterised by the failure of conduction through the left bundle branch. Right ventricle is unaffected.

Causes

Congenital abnormalities, myocardial diseases etc.

Treatment

Treat the underlying cause

Left Anterior Fascicular Block=characterised by the failure of conduction through the anterior fascicle of the left bundle branch.

Causes: Electrolyte abnormalities, hypertrophic cardiomyopathy (cat), etc.

ECG Appearance: Normal QRS, tall R waves, deep S waves

Treatment: Treat the underlying cause

Wolff-Parkinson-White Syndrome (Ventricular Preexcitation Syndrome

• This syndrome occurs when the conduction travels from the SA node and then completely bypasses the AV

node and travels to the bundle of His or the ventricles.

Causes

Hypertrophic cardiomyopathy (cats); congenital heart disorders

ECG Appearance: Short PR interval; Prolonged QRS complex; Notch in the R wave

Treatment

Treatment for tachycardia if necessary: digoxin, quinidine etc.

Sick Sinus Syndrome

• This is a blanket term used to describe multiple abnormalities of the SA node involving impulse formation and conduction disturbances.

Causes

Myocardial diseases, idiopathic, breed predispositions (e.g. miniature schnauzer)

ECG: Variable depending on the specific SA abnormality

Treatment

Surgical implantation of a pacemaker; +/- antiarrhythmic treatments