ACLS Advanced cardiac life support (Latest 2020 AHA)

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Heart rhythms

First degree heart block

What is first degree heart block

When there is a delay of transmission from SA node to ventricles, there will be a prolongation of PR interval on ECG (longer than 200 milli seconds) which is called as first degree heart block.

Is first degree heart block dangerous?

Most of the times, this is harmless as a heart block.

What causes first degree heart block?

Mostly due to degeneration of heart’s conducting system. Temporary causes can include imbalance in electrolytes.

ECG Intervals
Image credits-wikipedia

The rhythm will look identical to sinus rhythm with the only exception of prolonged PR interval.
On ECG paper, each smallest box is equivalent to 40 milli seconds.
Each big box has 5 small boxes inside, meaning the big box takes 200 milli seconds.
Measure the distance between start of P wave to start of QRS complex to see if PR is prolonged.
Normal PR interval is 120-200milli seconds (that is 3 to 5 small boxes).
If you check the number of small boxes in the following ECG strip, you can see that there are 6 small boxes for the middle beat.

First degree heart block showing more than 5 small (1 big) boxes from start of P to start of QRS.
Normal ECG strip
Normal ECG strip to compare –
The image also shows a way to calculate heart rate by looking at number of big boxes between R waves. If there is 1 box, rate is 300, if there are 2 boxes between adjacent R waves, the rate is 150 and so on.