Physiologic Basis of
the Abnormal ECG:
Disorders of Rate
Created by Diane R. Karius, Ph.D.
Just to make sure everyone is on the same page, let's start with some definitions:
- Sinus rhythm: The SA node is driving the heart. This is the normal situation that was animated above.
- Tachycardia: the heart rate is greater than >100 beats/min at rest.
- Bradycardia: the heart rate is slower than normal (< ~ 45 b/min - this is subject to debate and the condition of you patient - a well-trained athlete's heart rate may be 45 b/min but the same heart rate in an elderly, sedentary individual would likely be cause for concern)
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:
- The ECG appears basically normal, just at an increased rate (assuming there is no additional problem in the heart)
- There is one P wave before every QRS complex unless the atrial rate gets very fast (not likely to happen in a simple tachycardia).
- The PR interval may be shorter than usual
- The QT interval is shorter than usual (this can disguise a longer-than-normal QT interval and make it look 'normal').
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:
- Normal appearance of cardiac cycle, longer periods in diastole.
- rate < ~ 45 b/min
- Waves are normal appearing as long as there no additional conduction problem in the heart.
- The heart rate may get so slow that another pacemaker (most likely the AV node or the Bundle of His) takes over - if that happens, you see more significant changes in the ECG).
Once again,this may be a normal thing (lots of parasympathetic tone in a well-trained athlete) or abnormal.