Category:Impulse Conduction Abnormalities

Sino-Atrial Block

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