Heart Rhythms Part 2: Sinus Rhythms

This is part two of my cardiology lecture. See part 1, Basic Heart Anatomy, here.

A sinus rhythm is any rhythm that is generated in the sinus node. This is characterized on the rhythm strip by a P wave (unless otherwise stated all strips are considered to be in lead II).

The P wave is the electrical impulse passing through the atrium on it's way to the AV node. If the heart is working correctly, this will cause the atrium to depolarize and contract forcing more blood into the already full ventricles. This provides what is called atrial kick because when a muscle is stretched, it then contracts. So the atria force more blood into the ventricles which then stretch to accommodate the extra blood. Then, just as the muscle starts to contract, the AV node lets the electrical impulse pass through the ventricles. This provides for a very forceful contraction.

The QRS complex represents the electrical impulse passing through the ventricles, depolarizing them and causing a contraction.

The T wave is the re-polarization of the ventricles. Basically them recharging their batteries and getting ready to do it all over again....and again....and again....you hope.

The P-R interval and ST segment are important and can tell you a lot about what is going on with the conduction system of the heart but I'm not going to cover those here.

So as long as your heart rhythm looks like the one pictures above you have a sinus rhythm. Now, we must distinguish the rate.

<60 beats per minute- Sinus Bradycardia.
60-100 beats per minute- Normal Sinus Rhythm.
100-160 beats per minute- Sinus Tachycardia

Anything between 160 beats per minute and 250 beats per minute is considered to be SVT which I will cover in a later post.

Here are the requirements for a sinus rhythm.

1. 1. A heart rate between 60-100 beats per minute.
2. 2. The SA node pacing the heart.
3. 3. Regularity- Regular
4. 4. A "P" wave must be present for every "QRS" complex in a ratio of 1:1.
5. 5. PR interval is between .12 second and .20 second.
6. 6. QRS complex width should be less than .12 second.

Heart Rhythms Part 1: Basic Anatomy
Heart Rhythms Part 2: Sinus Rhythms
Heart Rhythms Part 3: Junctional Rhythms
Heart Rhythms Part 4: Ventricular Rhythms
Heart Rhythms Part 5: Premature Beats
Heart Rhythms Part 6: Heart Blocks