Cardio - AV blocks

Question 1

what is 1st degree AV block? suggest possible causes.

Answer 1

Decrease in impulse conduction through AVN, defined as a prolonged PR interval (>0.2secs).

• highly trained athletes with supranormal cardiovagal tone
• post-MI
• congenital or acquired AVN disease/fibrosis
• inflammation, e.g. SLE, Lyme disease
• electrolyte abnormalities
• drugs, esp. flecainide, propafenone, beta-blockers, CCBs, digoxin and magnesium

Question 2

what is the usual presentation of a pt with 1st degree AV block?

Answer 2

usually asymptomatic (incidental ECG finding, no follow-up required)

Question 3

describe the ECG appearance of 1st degree AV block

Answer 3

PR interval >5 small squares (>0.2 secs)

Question 4

what is 2nd degree, mobitz type I AV block?

Answer 4

Progressive prolongation of PR interval, until an atrial impulse fails to be conducted to ventricles (‘dropped’ QRS)… AV conduction recovers its baseline optimal level and cycle repeats.

Question 5

what is 2nd degree, mobitz type II AV block?

Answer 5

Intermittent failure of atrial impulse conduction to ventricles (without progressive lengthening of PR interval). May be a regular pattern to no. of conducted/dropped atrial impulses, e.g. 2:1 or 3:1 block.

Question 6

describe the QRS complex in Mobitz type I and II

Answer 6

type I: due to impaired AVN conduction so QRS complexes tend to be normal width.

type II: due to impaired conduction in bundle of His or bundle branches so QRS complexes tend to be wider than normal.

Question 7

suggest possible causes for 2nd degree heart block

Answer 7

• drugs: digoxin, beta-blockers, CCBs and other anti-arrhythmics
• structural heart abnormalities, e.g. congenital, valvular heart defects (esp. aortic stenosis and following aortic valve replacement)
• idiopathic age-related fibrosis of cardiac matrix
• coronary heart disease and MI
• inflammatory cardiac disease, e.g. SLE, ankylosing spondylitis, RA, Lyme disease and rheumatic fever
• cardiac infiltration caused by tumour, leukaemia, amyloidosis, haemochromatosis and sarcoidosis
• endocrine and metabolic disorders, e.g. hyperkalaemia, hypermagnesaemia and Addison’s disease
• cardiomyopathies
• familial causes

Question 8

how may mobitz type II present?

Answer 8

Stokes-Adams attack (episodes of syncope caused by sig. slowing of ventricular rate)

Question 9

how should 2nd AV block (acute and chronic) be managed?

Answer 9

Acutely symptomatic pts with low ventricular rates: atropine and/or temporary pacemaker insertion.

Treatment by insertion of a permanent cardiac pacemaker may be required, esp. for mobitz type II

Question 10

what is 3rd degree AV block? how is it caused?

Answer 10

Complete failure of transmission of atrial impulses to ventricles. Block can be located at AVN or His-Purkinje system.
May occur as a progression from 2nd degree block, or acutely, esp. after MI, so same causative conditions.

Question 11

what are the ECG features of 3rd degree AV block?

Answer 11

• severe bradychardia
• constant P-P interval (usually rate around 75bpm)
• constant R-interval
• unsychronisation of P and R waves

Question 12

how should 3rd AV block (acute and chronic) be managed?

Answer 12

Acutely symptomatic pts with low ventricular rates: atropine and temporary pacemaker insertion.

Treatment by insertion of a permanent cardiac pacemaker.