Causes of Bradycardia
1. Insufficient impulse formation... such as sinus node dysfunction
2. Insufficient impulse conduction... such as AV block
Sinus Node Dysfunction (SND)
Inappropriately slow sinus rate
ECG and clinical judgement are required for this diagnosis
Sick Sinus Syndrome (SSS)
Symptoms due to sinus node dysfunction (SND).
For example, an asymptomatic 70 year old with resting sinus bradycardia of 50 bpm likely has sinus node dysfunction, BUT, doesn't have SSS until symptoms develop.
A gap of more than 3 seconds between QRS complexes.
Commonly seen in normal people when sleeping, but is abnormal when awake.
Gold Standard for Diagnosing SSS
Record inappropriately low sinus rate during symptoms.
Can be used to continuously monitor a patient for 24-48 hours.
Heart rate does not respond appropriately to exercise.
Failure to achieve 80% of age predicted heartrate.
Age predicted heartrate = 220 - age
Symptoms of Sinus Node Dysfunction
The most common is syncope.
Can also present as falls in the elderly, effort intolerance, or chronic fatigue.
Is a variant of Sick Sinus Syndrome (SSS) in which slow arrhythmias and fast arrhythmias alternate.
50% of patients with SSS will develop this.
Cause of SND
Most common cause is aging and fibrosis of the sinus node.
Treatment of SND
Reverse the correctable causes.
If this cannot be done, consider using a permanent pacemaker.
First Degree AV Block
PR interval is longer than 200ms
Every p-wave is conducted
Blockage is in either the AV node or the His-Purkinje fibers
Third Degree AV Block
Atria and ventricles are not communicating.
Hopefully an escape rhythm in the ventricles.
Blockage is either in the AV node or the His-Purkinje fibers.
Can see cannon A waves because when the atrium contracts and the valve to the ventricles does not open, all the blood shoots back into the venous system.
Indications for a pacemaker.
Second Degree AV Block - Type I
Usually a block in the AV node.
The PR interval gradually lengthens before a P-wave block, and the cycle begins again.
Second Degree AV Block - Type II
The PR interval remains constant before a P-wave blocks, and the cycle begins again.
Usually a conduction block in the His-Purkinje fibers.
This is worse than Type I. Indications for a pacemaker.