Module title = Tutorial: Rhythm Diagnostic Criteria

Lesson title = Sinus Tachycardia

• * Rate: > 100
• Rhythm: regular
• P waves: normal
• PR interval: normal (3-5 little squares)
• QRS: narrow (less than 3 little squares)

 
The definition of tachycardia is a heart rate greater than 100 beats per minute (bpm).
 
Sinus tachycardia originates from the sinus node (which is why it is called "sinus"). It is caused by the SA node firing at a rate of greater than 100 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 steep, then it will take a short time for the voltage to reach the action potential threshold. This will cause the frequency of "firing" to be increased. If the slope is steep enough, the frequency of firing will exceed 100 per minute and this will create sinus tachycardia.

Clinical Significance:

Rates greater than 100 bpm are commonly found in children and adults. In fact, if you got off your lazy butt right now and ran up and down a couple flights of stairs, you will most likely experience sinus tachycardia.  

Sinus tachycardia is a good thing if:

• it is physiological: you are exercising and you need increased cardiac output to your very attractive ripped muscles that you are checking out in the mirror after your workout
• it is compensatory: you are bleeding and hypovolemic and need a compensatory increased rate to maintain a bare minimum cardiac output to stay alive.
• it is compensatory: you are anemic and need increased cardiac output to deliver an appropriate amount of oxygen to your tissues

• it is driven by a pathological process such as an over active thyroid gland (hyperthyroidism)
• you have coronary artery disease and will experience myocardial ischemia at high(er) heart rates

• treating the underlying cause of the tachycardia (treat hyperthyroidism or anemia for example)
• give a negative chronotrope: beta blocker or calcium channel blocker

Examples:

At high heart rates like this one, the P wave and T wave can be hard to differentiate.


This one has a rate of 105, so "just barely" tachycardia.