Sinus Tachycardia - Defining the Normal

We’ve talked a lot about sinus tachycardia. But what is? And how do we even know if a rhythm is sinus rhythm or not? 

A sinus rhythm is any rhythm that originates from the Sino atrial node. It’s that simple. But on an ECG, how do we identify if a rhythm originates from the SA node or from somewhere else?

In order to understand this, we need to understand one basic principle of ECG electrophysiology and how the waves on the ECG are actually formed. The concept is simple. The electrodes needed to measure the electrical impulse are placed on the chest and on the limbs. When the electrical impulse is generated and moves towards the lead, then the wave that is recorded is a positive wave. When the impulse moves away from the electrode, the wave that is recorded is a negative wave. When the impulse moves perpendicular to the electrode then the recorded wave is biphasic. 

How electrical currents are translated into waves on the ECG.

Reproduced from Goldberger’s Clinical Electrocardiography, 9th Edition

Now imagine an impulse that arises from the SA node and moves towards the AV node and then anteriorly and inferiorly into the ventricles. In the inferior leads (II,III and aVF) the recorded wave is positive because the impulse travels inferiorly towards the inferior leads. In the superior lead (especially in aVR) the recorded wave is negative because the impulse is traveling away from the right shoulder. Finally in lead V1, the recorded wave is biphasic because the impulse is travelling perpendicular to where the lead is. 

So these are the three criteria we apply to identify a sinus rhythm.

1. Negative P wave is II, III. aVF

2. Positive P wave is aVR

3. Biphasic P wave is V1

The location of the SA node and the various deflections a sinus rhythm produces on an ECG in various leads.

Reproduced from Harrisons Principles of Internal Medicine, 20th Edition

Once we know that the rhythm is sinus, we have to figure out if it is pathological or physiological tachycardia.

Physiological sinus tachycardia occurs in states of high stress and physical exertion. 

Pathological sinus tachycardia occurs rarely and usually due to some underlying condition. 

Idiopathic sinus tachycardia is a condition in which there is tachycardia at rest or is out of proportion to the stressing event. It is commonly seen in women aged 40-50 years. Generally, it settles on its own. 

Postural orthostatic tachycardia syndrome also known as POTS is an increase in heart rate by 30 beats per minute or over 120/min within ten minutes of standing WITHOUT any associated hypotension. It usually occurs due to an underlying viral illness. 

Pathological sinus tachycardias are rare and not very clinically significant. The major purpose of this article was to understand how to identify a sinus rhythm because in the following articles we will see what happens if you don’t get a sinus rhythm. 

Author: Narendran Sairam (Facebook)

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