Heart block (1st, 2nd and 3rd degree) and LBBB/RBBB Flashcards
Give 5 causes of sinus bradycardia.
Sinus bradycardia can be
- normal in athletes
caused by
- hypothermia,
- hypothyroidism,
- vagal stimulation,
- drugs (e.g. beta blockers),
- raised intracranial pressure
- myocardial infarction.
Sinus bradycardia can also be a feature of certain infections including Legionnaire’s disease, typhoid fever and Lyme disease.
What is the normal PR interval and what does it show?
Time taken for spread of depolarisation from the SA node to the ventricular muscle
Normally <200ms - 5 small squares.
What is the heart block poem?
If the R is far from P, then you have 1st degree
Longer, longer, longer… drop, then you have a Weckenbach
If some Ps don’t get through then you have Mobitz II
If Ps and Qs don’t agree then you have a 3rd degree
Describe the cause of first degree heart block and give examples.
Delay of wave of depolarisation from SA node to ventricles, causing a prolonged PR interval - 1st degree HB
May be caused by:
- CAD
- acute rheumatic carditis
- digoxin toxicity
- electrolyte disurbances
Describe the cause of second degree heart block. What are the three typs?
Sometimes excitation completely fails to pass through AV node or bundle of His. There are three variations:
- Wenckebach/Mobitz I - progressive lengthening of PR interval then failure of conduction of atrial beat, and shorter PR interval. Then cycle repeats.
- Mobitz II - most beats are conducted with constant PR interval but ocassionally there is atrial depolarisation without ventricular depolarisation.
- 2:1, 3:1, 4:1 conduction - alternate conducted and non-conducted atrial beats e.g. one conducted atrial beat then a non conducted beat giving twice as many P waves as QRS complexes therefore 2:1 block .
What are the underlying causes of second degree HB?
Same as those of first degree HB:
- CAD
- acute rheumatic carditis
- digoxin toxicity
- electrolyte disturbances
What is the prognosis with second degree HB?
Weckenbach is usually benign
But Mobitz type 2 and 2:1, 3:1, 4:1 block may herald “complete/third degree” heart block
What type of heart block is shown?
2nd degree HB, Mobitz type 2
- PR interval of the conducted beats is constant
- One P wave is not followed by a QRS complex
What type of heart block is shown?
Second degree HB, Mobitz type 1/Weckenbach
- Progressive lengthening of the PR interval
- One nonconducted P wave
- Next conducted beat has a shorter PR interval than the preceding conducted beat
- As with any other rhythm, a P wave may only show itself as a distortion of a T wav
What type of heart block is shown?
Second degree HB, 2:1 ype
- Two P waves per QRs complex
- Normal and constant PR interval in the conducted beats
NB: P wave may only show itself as a distrotion of the T wave sometimes, as below. P wave in the T wave can be identified because of its regularity
What is the cause of third degree heart block? What are some underlying causes?
Occurs when atrial contraction is normal but no beats are conduced to the ventricles. When this occurs the ventricles are excited by a slow “escape mechanism” from a depolarising focus within the ventricular muscle.
3rd degree HB may occur in :
- acute MI (usually transient)
- fibrosis around the bundle of His (chronic)
- block of both bundle branches
What type of heart block is shown?
Third degree HB
- P wave rate is 90bpm
- No relationship between P waves and QRS complexes
- QRS complex rate 36bpm
- Abnormally shaped QRS complex, because of abnormal spread of depolarisation from a ventricular focus
What is shown on this ECG?
Complete heart block
- Sinus rhythm but no P waves conducted
- Right axis deviation
- Broad QRS complexes (160ms)
Probably from fibrosis of bundle of His
List some general causes of AV block/heart block.
- Fibrosis/calcification of the conduction system, can be age related.
- Usually male aged 50-60yrs
- CAD
- AV-node blocking drugs e.g. beta-blockers, CCBs, digitalis, adenosine
- Anti-arrhythmics e.g. Na-channel blockers, class III agents like sotalol and amiodarone
- High vagal tone
- Cardiomyopathy
- Calcification from valvular calcification
- Post-catheter ablation of arrhythmias
- Post-surgical
- Blunt cardiac injury
- Indigenous medicines
- Metabolic -electrolyte disturbance, acidosis, hypoxaemia
- Neuromuscular disorders - myotonic dystrophy, Erb dystrophy,
- Myocarditis, IE, Lyme disease
- Congenital
What are the signs/symptoms of heart block?
- Syncope
- Fatigue
- Dyspnoea
- Chest pain
- Palpitations
- N&V
- Slow HR (if <40 admit for pacemaker implantation)
- High BP with wide pulse pressure, low BP in emergency
- Cannon A waves in irreversible/complete heart block
What investigations would you do for heart block?
ECG
Serum troponin
Serum electrolytes - calcium, potassium
Serum pH
Toxicology screen - serum digitalis levels may be high and cause reversible AV block.
24hr ambulatory monitoring - measured if AV block is suspected
CXR - may show hilar lymphadenopathy (sarcoid), cardiomegaly, coronary or valvular calcification, signs of pulmonary congestion
Echo - ventricular dysfunction or hypertrophy, valvular disease, wall-motion abnormalities
EP study - may show significant infranodal condustion disease which needs pacemaker
Serological test for Lyme disease
Tilt-table testing, implantable loop recording, cardiac stress testing, coronary angiography, serum CK-MB
What is significant infranodal conduction disease?
Defined by a His-bundle-to-ventricle (HV) interval >100 ms.
How do you manage heart block?
1st degree or Mobitz I
- If asymptomatic - monitoring as low risk of progression
- If symptomatic - discontinuation of AV nodal blocking medications, infrequent PPM or cardiac resynchronisation therapy +/- ICD placement
- PPM = permanent pacemaker implantation if symptoms are severe. Usually a dual-chamber pacemaker.
2nd degree or 3rd degree
- Asymptomatic/mid-mod symptoms -
- Treat underlying cause/discontinue drugs
- Digitalis toxicity - digoxin immune Fab
- Beta blcoker toxicity - glucagon
- CCB toxicity - calcium chloride
- PPM or cardiac resynchronisation +/- ICD
- Severely symptomatic -
- Condition specific management, discontinue drugs, temporary pacing
- PPM or cardiac resynchronisation therapy +/- ICD placement
What is the prognosis with heart block?
1st degree has x2 increase risk AF and increase in all cause mortality
2nd degree, if irreversible has a high risk of progression to 3rd degree or ventricular asystole.
Symptomatic AV block treated with PPM has an excellent prognosis with low rate of complications.
What is a bifascicular block?
e.g. RBBB + LAD*
*left anterior or posterior hemiblock
What is a trifascicular block?
RBBB + LAD + 1st degree HB (long PR)
What is shown below?
Trifascicular block - RBBB + left anterior hemiblock + 1st-degree heart block
What is shown below?
Bifascicular block
- Sinus rhythm, rate 70 bpm
- Left axis deviation (S wave greater than R wave in leads II and III)
- Right bundle branch block – wide QRS complexes (135 ms); RSR 1 pattern in lead V 1 ; and a wide slurred S wave in lead V 6
