Arrhythmias Flashcards
What is normal sinus rhythm ?
Normal heart rhythm with electrical activation beginning in the SA node
What is the PR interval ?
The time taken for the wave of depolarisation to travel from the SA node to the ventricular muscle (usually no longer than 220ms (6 boxes))
What is meant by first degree heart block ?
When there is a delay in the conduction pathway from the SA node to the ventricles
There is therefore a characteristic prolonged PR interval
What are the causes of first degree heart block ?
May be a sign of coronary artery disease, acute rheumatic carditis, digoxin toxicity or electrolyte disturbances
What is meant by second degree heart block ?
When on intermittent occasion, excitation completely fails to pass through the AV node or the bundle of His
What are the three different types of second degree heart block ?
Mobitz type 1
Mobitz type 2
2:1 / 3:1 / 4:1 conduction
What is mobitz type 1 heart block ?
Progressive lengthening of the PR interval and then failure of conduction of an atrial beat, followed by a conducted beat with a shorter PR interval and then a repetition of this cycle
What is mobitz type 2 heart block ?
Most beats are conducted with a constant PR interval, but occasionally there is atrial depolarisation without a subsequent ventricular depolarization
What is meant by 2:1 / 3:1 / 4:1 conduction ?
There may be alternate conducted and nonconducted atrial beats (or one conducted atrial beat and then two or three non-conducted beats), giving twice (or three or four times) as many P waves as QRS complexes. This is called ‘2:1’ (‘two to one’), ‘3:1’ (‘three to one’) or ‘4:1’ (‘four to one’) conduction
What is meant by third degree heart block ?
Complete heart block (third degree block) is said to occur when atrial contraction is normal but no beats are conducted to the ventricles. When this occurs the ventricles are excited by a slow ‘escape mechanism’ from a depolarizing focus within the ventricular muscle
No relationship between P waves and QRS complexes
Abnormally shaped QRS complexes, because of abnormal spread of depolarization from a ventricular focus
What causes third degree heart block ?
Complete heart block may occur as an acute phenomenon in patients with myocardial infarction (when it is usually transient) or it may be chronic, usually due to fibrosis around the bundle of His
It may also be caused by the block of both bundle branches
When is heart rhythm said to be arrhythmia ?
When the depolarisation sequence begins somewhere outwith the SA node
What is meant by fibrillation ?
When individual muscle fibres contract independently
