Tutorial 2a: Sinus Rhythm

Normal sinus rhythm means a regular rhythm generated by the SA node. The parts of a normal EKG waveform are probably familiar to you and we review it here only briefly in the picture below.

1. Note that not all waves are easy to see in all leads, for example there are very small S-waves only in lead aVF here.
2. In any given lead, whatever negative wave comes first is called a Q-wave in that lead. Here there is a little Q-wave in lead aVL and some larger Q-waves in aVR.
3. The first positive wave in a lead is called the R-wave in that lead. Here there are R-waves in aVL and aVF.
4. A negative wave that comes after an R-wave is called an S-wave. Here there are small S-waves in lead aVF

Other simple things to note about a standard EKG tracing are:

1. Time runs horizontally across the paper - that is leads I,II,III are recorded at the same time and leads aVR,aVL,aVF are recorded together but after I,II and III. Then V1,V2,V3 and finally V4,V5,V6 are recorded together.

This means that if you saw a strange event like a premature beat in lead V1, it should appear in V2 and V3, but not in the other leads. This is clear in the tracing below.

For practice, calculate the rate for the above EKG.

1. To identify an EKG as showing sinus rhythm, we want to see a P-wave before each QRS.

We also want to see that the P-waves are actually coming from the SA node - which is located in the right atrium up toward the patient's right shoulder. So we expect the P-waves to be going away from the right shoulder (downward, and from right to left). On the EKG this means the P-waves should be positive in leads I and II as shown below.

So our second way of identifying normal sinus rythym is:

2. The P-waves should be positive in lead I and II.

And finally, we don't want any blocks in conduction, so:

3. The rhythm should be regular.

And now try to analyse the example below. Get the rate and decide whether this is normal sinus rhythm or not.