Cardiology (Arrhythmia) Flashcards
describe the pathophysiology of heart block
- Heart block describes impaired electrical conduction in the heart.
- The sinoatrial node (SAN) sends impulses to the atrioventricular node (AVN) which then sends impulses down the Bundle of his and via the bundle branches towards the apex of the heart which split into fascicles which supply the myocardium.
types of heart block
1) Sinoatrial node blocks - blocks within the SAN
2) Atrioventricular blocks- blocks within AVN
- 1st degree
- 2nd degree : Mobitz 1 and Mobitz 2
- 3rd degree: complete heart block
3) Hisian block: blocks within bundle of His
4) Bundle branch blocks- within the left and right bundle branches
5) Fasicular/hemiblocks- within the fasicles of the left bundle branch
first degree heart block
abnormally slow conduction through the AV node
- despite slow conduction, every impulse that origiates from the SAN is passed to the ventricles
- leads to PR interval prolongation
On an ECG, a first-degree AV block is defined as a PR interval >0.20 seconds (>5 small squares)
causes of first degree heart block
1) Common in younger, athletic patients due to increased vagal tone
2) Ischaemic heart disease:
* Coronary heart disease
* Myocardial infarction
3) Electrolyte imbalances:
* Hypokalaemia
* Hypomagnesaemia
4) Infection:
* Rheumatic fever
* Infective endocarditis
5) Drugs:
* Digoxin
6) Autoimmune and inflammatory conditions:
* Sarcoidosis
* Systemic lupus erythematosus
* Rheumatoid arthritis
* Systemic sclerosis
second degree heart block
Second-degree atrioventricular (AV) block describes impaired conduction between the atria and ventricles leading to PR interval progression. More specifically, second-degree heart block describes when one or more (but not all) atrial impulses fail to pass to the ventricles.
Second-degree AV block can be divided into two subtypes:
- Mobitz type I (Wenckebach)
- Mobitz type II
Mobitz type I (Wenckebach)
there is a progressive prolongation of the PR interval until a P wave is completely blocked (known as a dropped beat)
- each impulse leads to the prolongation of the refractory period of the AV node
- when an impulse from the atria arrives at the AV node during the rleative refractory periodm it is conducted more slowly
- as the refractory period gets longer with each impulse, an atrial impulse arrives at the AV node during its absolute refractory period and is not conducted
- After this the AV node resets and the cycle repeats
On an ECG, this appears as progressive PR prolongation until a P-wave is dropped, resulting in a P-wave with no QRS complex following it.
causes of mobitz type 1
- Normal variant in people with high vagal tone and no structural heart disease (e.g. athletes)
- Inferior myocardial ischaemia
- Hyperkalaemia
- Some drugs (e.g. beta-blockers, calcium channel blockers, digoxin)
Mobitz type 2
- In a Mobitz type II heart block, the PR interval is constant, however, each P wave is associated with a QRS complex until one P wave arises that does not have a QRS complex.
- There is usually a fixed number of P-waves for every successful QRS complex.
- For example, if there are three P waves for every QRS complex, this is a 3:1 Mobitz II block.
There is a high risk of asystole with Mobitz type 2 heart blocks. This is because dropped beats can happen suddenly and unexpectedly, and progress to third-degree (complete) heart block or asystole, resulting in sudden cardiac death
why are the PR intervals consistency different in Mobitz type 1 vs 2
This occurs because the block is lower down than Mobitz type I, below the AV node.
Mobitz type 2
Mobitz type II blocks are most commonly due to structural heart damage.
It is rarely seen in patients without structural heart disease. Its causes include:
- Anterior myocardial infarction
- Myocardial fibrosis/sclerosis
- Amyloidosis
- Haemochromatosis
- Rheumatic fever
- Autoimmune disorders (e.g. systemic lupus erythematosus, systemic sclerosis)
- Hyperkalaemia
- Some drugs (e.g. beta-blockers, calcium channel blockers, digoxin, adenosine, amiodarone)
2:1 block
A 2:1 block describes the presence of two P waves for each QRS complex. Because Mobitz type I blocks occur in regular cycles, there is always a fixed number of P waves to QRS complexes. In general, in the P:QRS ratio, Mobitz type I blocks have one more P wave than QRS complex (e.g. 5 P waves and 4 QRS complexes per cycle are a 5:4 Mobitz Type I block). Mobitz type II blocks generally have a fixed ratio of P:QRS complexes and are generally X:1 (e.g. 3:1, 4:1, 5:1). Ratios of 3:1 and above are known as ‘high-grade AV blocks’. With 2:1 blocks, it is difficult to tell whether it is due to a Mobitz type I or type II block.
third degree heart block
COMPLETE HEART BLOCK
- no impulses from the SAN are conducted to the ventricles
- SAN continues sneding impulses and ventricles activate via escape rhythms
- leads to bradycardia (45-50bpm) and haemodynamic instability
On an ECG, there is no association between the P waves and QRS complexes. There may be a regular P-P interval and regular R-R interval, but the P-R interval may vary.
There is a high risk of asystole with third-degree heart blocks.
causes of a third-degree heart block
are the same as Mobitz I and II, such as:
- Inferior myocardial infarction
- Drugs (beta-blockers, calcium channel blockers, amiodarone, adenosine, digoxin)
bundle branch block
The left and right bundle branches emerge from the bundle of His in the heart. They transmit impulses from the bundle of His to the Purkinje fibres. The bundle branches are found along the interventricular septum and each bundle branch depolarises their respective ventricles. The interventricular septum itself is depolarised by the left bundle branch and is depolarised from left to right. The ventricles contract simultaneously.
left bundle branch block pathophysiology
Left bundle branch block (LBBB) describes slowed or absent conduction through the left bundle branch. This leads to delayed depolarisation of the left ventricle. This results in the left ventricle being depolarised via the right bundle branch, whose impulses travel through the right ventricle, and then to the left ventricle via the septum.
causes of new LBBB
ALWAYS PATHOLOGICAL
- MI
- Aortic stenosis
- HTN
- Cardiomyopathy
findings of LBBB on ECG
- Prolonged, positive R waves in the left ventricular leads (I, V5-6)
* These are usually negative, but since depolarisation is happening in the opposite direction, - Secondary R waves in the left ventricular leads (I, V5-6) giving “M-shaped” R waves
* This is due to delayed activation between the right ventricle and left ventricle - QRS prolongation
* Due to delayed conduction as the left ventricle now needs to be depolarised via the right ventricle through a slower and less efficient pathway
Right bundle branch block (LBBB)
describes slowed or absent conduction through the right bundle branch. This leads to delayed depolarisation of the right ventricle. This results in the right ventricle being depolarised via the left bundle branch, whose impulses travel through the left ventricle first, then to the right ventricle via a slower and less efficient pathway.
findings of RBBB
- Secondary R waves in the right ventricular leads (V1 and V2) giving “M-shaped” R waves
* This is due to delayed activation between the left ventricle and right ventricle - QRS prolongation
* Due to delayed conduction as the right ventricle now needs to be depolarised via the left ventricle through a slower and less efficient pathway - Wide, slurred S waves in leads I and V6
causes of RBBB
- RBBB can be a normal variant
- Ischaemic heart disease
- Pulmonary disorders (e.g. COPD)
- Right ventricular hypertrophy
- Pulmonary embolism
what does bifascicular block mean?
RBBB and blockage of one of the fascicles of the left bundle branch
Trifascicular block:
Bifascicular block and third-degree heart block
tachycardia defined as
≥100 bpm
how can tachycardias be classified
- QRS morphology
- Rhthm regularity
