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Physiologic Basis of

the Abnormal ECG:

Disorders of Rate

 

Created by Diane R. Karius, Ph.D.
Updated 11/14/2018
 

Just to make sure everyone is on the same page, let's start with some definitions:

Tachycardia and bradycardia can arise from either the SA node or from other areas of the cardiac muscle. The distinction is important. We're going to start with the assumption that you are looking at a sinus rhythm for this section. In the next section, we'll discuss ectopic rhythms.

Sinus Tachycardia: In sinus tachycardia, the SA node is driving the heart, just at a faster rate then normal (> 100 b/min). It may or may not represent a cardiac abnormality - for example, if I'm having an anxiety attack, I may have a pronounced sinus tachycardia although my heart is completely normal. Sinus tachycardia may also represent the normal response to a physical problem - for example, if my blood pressure is low due to shock, it would be ABNORMAL if I didn't show some degree of sinus tachycardia (remember that BP = CO x TPR. If I am hypotensive, the brain should be increasing my heart rate to increase cardiac output).

Electrocardiographic appearance of sinus tachycardia: This ECG illustrates a typical sinus tachycardia - because the SA node is driving the heart, we see a normal progression of the ECG (P waves followed by QRS complexes followed by T waves The PR interval may be shorter than usual if the tachycardia is the result of sympathetic stimulation as sympathetic stimulation will increase the conduction velocity through the AV node. The QT interval is also shorter than you usually see (remember we have to correct the QT interval for heart rate!).

Characteristics of sinus tachycardia:


Sinus Bradycardia

As was the case with sinus tachycardia, sinus bradycardia means that the SA node is driving the heart, but now it is slower than the normal range. This can be a result of a lot of parasympathetic tone or because of damage to the SA node as might occur with myocardial infarction on the posterior wall.

Luckily for us, sinus bradycardia is easy to 'see' on the ECG:

Sinus Bradycardia is the result of a prolonged phase 4 in the pacemaker cells of the SA node - they just take longer to get to threshold and start the next heartbeat. Since the SA node is driving the heart, we continue to see the normal sequence of events (P wave followed by QRS complex followed by T wave, but the PR interval may be prolonged and the QT interval (uncorrected) will be long.

Sinus bradycardia has the following characteristics:

Once again,this may be a normal thing (lots of parasympathetic tone in a well-trained athlete) or abnormal.