Module title = Tutorial: Rhythm Diagnostic Criteria

Lesson title = Sinus Bradycardia

• * Rate: < 60
• Rhythm: regular
• P waves: normal
• PR interval: normal (3-5 little squares)
• QRS: narrow (< 3 little squares)

The definition of bradycardia is a HR less than 60 beats per minute (bpm).
 
Sinus bradycardia originates from the sinus node (which is why it is called "sinus"). It is caused by the SA node firing at a rate of less than 60 bpm. All other electrical conduction after the SA node is normal.

Electrophysiology:

The action potential of the sinus node looks like this:


The dotted line indicates the action potential threshold. The voltage inside the SA node cells increases during phase 4. If the slope of phase 4 is very shallow, then it will take a long time for the voltage to reach the action potential threshold. This will cause the frequency of "firing" to be reduced. If the slope is shallow enough, the frequency of firing will fall below 60 per minute and this will create sinus bradycardia.

Clinical Significance:

Rates less than 60 bpm are commonly found in healthy adults.  Healthy fit adults (especially athletes) often have a sinus bradycardia at rest or during sleeping. These do not need to be treated.
 
Rates that are inappropriately low can cause low cardiac output and hypotension and need to be treated. Treatment of sinus bradycardia can include:

• atropine intravenous bolus
• dopamine intravenous infusion
• epinephrine intravenous infusion
• artificial electronic pacemaker

• healthy cardiovascular fitness level
• negative chronotropes (drugs that slow down the sinus node such as beta blockers and calcium channel blockers)
• toxins that increase parasympathetic tone: some insecticides (also called organophosphates)
• myocardial ischemia

Examples:



This example has some variation in the rhythm. Notice that the 5th, 6th, and 7th QRS's are a little closer together? This is called sinus arrhythmia and is normal and is caused by breathing. Breathing is normal, right?


This example has really grossly shaped inverted T waves. Yuck.