Approach to Arrhythmia part 1: Bradycardia

Approach to Arrhythmia part 1: Bradycardia

Bradycardia is defined as heart rate <60/min. To understand the cause of bradycardia we have to understand the structures involved in the production and conduction of cardiac impulse.

The normal cardiac structures involved in electrical activity of the heart are
1. SA node- It is the pacemaker of the heart, because it fires at the highest rate hence predominates over other pacemakers of the heart.
2.AV node – In normal hearts its function is to conduct impulses generated in the SA node to the ventricles through the Bundle of His and bundle branches.It can act as a slow pacemeker when the SA node is diseased
3. Bundle of His and bundle branches- Normally their function is conduction of cardiac impulses. They can act as a slow pacemeker when the proximal structures (SA node & AV node) are diseased.
4. Purkinje fibres
5. Ventricular myocardium- In complete heart block the ventricular myocardium produces escape rhythm at a slow rate of 20-40/min

Disease in any of the structures can lead to bradycardia.

The diagnosis is made from ECG in most of the cases.
Now we will discuss how to systematically analyze an ECG for diagnosing a bradycardia.

Step 1: Calculate the rate first. Bradycardia by definition heart rate <60/min
Step 2: Analysis of rhythm begins with search for P-wave. Normally P-waves are produced by SA node, so absence of P-waves indicate disease of SA node. Which is called as sick sinus syndrome
Step 3: Absent P-waves can be due to
1. Sick sinus syndrome with escape rhythm. (R-R intervals fixed)
2. Atrial fibrillation with slow ventricular rate. (R-R intervals variable)

Sick sinus syndrome: no visible P waves, fixed R-R intervals.

Sick sinus syndrome: no visible P waves, fixed R-R intervals.

 

Step 4: P-waves present
If P-waves are present look for P-P interval, PR-interval and relation between P and R waves.

Step 5: P-P interval variable
Nonconducted APCs
Step 6: P-P interval fixed
The next stepis evaluation of PR-interval and relation between P and R waves

Step 7: PR interval normal and fixed: Sinus bradycardia

Sinus bradycardia

Sinus bradycardia

PR interval prolonged but fixed and each QRS complex is preceeded by P wave: First degree AV block

1st degree AV block: Prolonged fixed PR interval

1st degree AV block: Prolonged fixed PR interval

(ECG courtesy of www.lifeinthefastlane.com)
PR interval lengthens then dropped beat and return with short PR interrval: Mobitz type 1 second degree AV block

Wenckebach block

Wenckebach block

(ECG courtesy of www.lifeinthefastlane.com)
PR interval fixed then dropped beat : Mobitz type 2 second degree AV block
No PR relationship : Third degee AV block

Complete heart block

Complete heart block

(ECG courtesy of www.lifeinthefastlane.com)

 

The flow chart below summarizes the whole approach

ECG  approach to bradycardia

ECG approach to bradycardia

 

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