Conduction defects and heart block Flashcards
Why are the atria and ventricles electrically isolated
Connected only by the AV node. Here, the electrical signal is delayed to allow for sequential atria and ventricular contraction.
What is the reason for the existence of the plateau phase in the cardiac AP
The plateau phase PROLONGS the cardiac myocyte REFRACTORY period - ensuring sufficient time for the entire myocardium to contract prior to repolarization
Classify the causes of conduction defects
Anatomical
- IHD (Infarction/Ischaemia)
- CMO
- Post cardiac surgery
- Valvular heart disease
Physiological
- Electrolyte
- Endocrine
- Hypertension
Pharmacological
- AV blockers
- Alpha agonists
Define 1st degree heart block
PR interval - >0.2 s
(0.2 s is represented by five small squares at a paper rate of 25 mm/s)
All P waves are conducted to the ventricles
What is the clinical significance of 1st degree heart block?
First degree block is normally benign and may be seen in otherwise healthy patients. First degree heart block usually presents no problems during the conduct of anaesthesia.
What are the types of 2nd degree heart block
Mobitz type 1 (Wenkebach)
Mobitz type 2
2:1 type
What is Mobitz type 1 block
A progressive lengthening of the PR interval is seen, followed by failure of conduction of an atrial beat. The cycle then repeats itself.
What is the clinical significance of Mobitz type 1 block
This block may be associated with right coronary artery occlusion/inferior myocardial infarction. Treatment is not usually required in a perioperative setting although a 2:1 block may develop secondary to haemodynamic instability.
What is Mobitz type 2 block
This involves intermittent failure of electrical conduction through the AV node and is characterized by a P wave without a subsequent ventricular complex. This may occur in a regular pattern (e.g. 2:1) or at random. The PR interval in surrounding beats is unaffected.
What is the clinical significance of Mobitz type 2 block
Mobitz type 2 often progresses to complete heart block. It is associated with anterior myocardial infarction. Cardiology assessment and pacing is recommended in the preoperative period.
What is 2:1 block
In 2:1 block a ventricular beat follows every second atrial beat. This may also present in a 3:1 ratio.
What is the clinical significance of 2:1 block
2:1 block may develop into complete heart block. The progressive nature of conduction defect is seen in this ECG with broadening of the QRS complex. A temporary transvenous pacing wire should be considered preoperatively.
What is 3rd degree heart block
Third degree block, or complete heart block, represents complete disruption of conduction between atria and ventricles. Ventricular conduction is initiated by pacemaker cells below the AV node, either within the bundle of His or from within the ventricular myocardium. The rhythm is known as a ventricular escape rhythm.
What are the ECG features in 3rd degree heart block
ECG features: There is no association between the P wave and the QRS complex. The morphology of the QRS complex depends upon the site of origin of the ventricular escape rhythm. It may be narrow complex (<0.12 s) if the origin is more proximal, located in the bundle of His. If the origin is more distal the QRS is broad complex (>0.12 s).
What is the clinical significant of 3rd degree or complete heart block
Clinical significance: Complete heart block occurring with anterior myocardial infarction demonstrates significant myocardial damage. Temporary, followed by permanent, pacing may be required. It may also occur transiently with intense vagal stimulation. Stopping the stimulus and administration of intravenous atropine may result in restoration of a normal rhythm.
What are the ECG features of RBBB
V1 - A characteristic RSR (or M-shaped) pattern
V6 - typical W pattern with initial small downward deflection
ST segment depression may also be seen in V1-3
QRS axis is usually within the normal range
Explain the reason for the ECG changes in LBBB
Left bundle branch block (LBBB) results in a complete reversal of direction of depolarisation within the septum. This alters the direction of the initial deflection of the QRS complex in every lead.
Describe the ECG findings in LBBB
Complexes are broad and
In V1, there is a deep negative wave which may have a small upwards inflection (W pattern)
V6 has no initial negative wave and a characteristic M pattern may also be evident
What is bifascicular block?
RBBB + LAFB = right anterior hemiblock
ECG: rSR in V1 plus left axis deviation
RBBB + LPFB = left anterior hemiblock
ECG: rSR in V1 plus excessive right axis deviation
What is trifascicular block
Combination of first degree block with a bifascicular block
List the causes of RBBB
Normal variant (if normal axis)
Right ventricular hypertrophy / cor pulmonale
Pulmonary embolus
Ischaemic heart disease
Rheumatic heart disease
Myocarditis or cardiomyopathy
Degenerative disease of the conduction system
Congenital heart disease (e.g. atrial septal defect)
List the causes of LBBB
Aortic stenosis Ischaemic heart disease Hypertension Dilated cardiomyopathy Anterior MI Primary degenerative disease (fibrosis) of the conducting system (Lenegre disease) Hyperkalaemia Digoxin toxicity
When is temporary pacing required in patients with bifascicular, trifascicular or bundle branch block and why
These blocks rarely progress to complete heart block
Temporary pacing is only required if patient reports episodes of syncope
Describe management of intraoperative 3rd degree heart block with profound hypotension
- Call for help
- FiO2 100%
- Inform surgeon - discontinue
- Atropine 0.5 mg and reassess
If inadequate response - Isoprenaline (1 - 10 ug/min) or adrenalin (1 - 10 ug/min)
If inadequate response - Temporary cardiac pacing
- Transcutaneous (most practical)
- Transoesophageal (probe position guided by capture)
- Transvenous (Swan-Ganz tip into RV)
