Classification of Heart Rates

When we classify heart rates, we typically consider the physiological heart rate which solely looks at the number of heart beats per minute (bpm), regardless of where in the heart it originates from. In that sense, the normal heart rate is 60 to 100 bpm. Any heart rate slower than 60 bpm is a bradycardia, and any heart rate faster than 100 bpm is a tachycardia.

For the most part, the heart can compensate for slower and fast heart rates by adjusting stroke volume in order to main an appropriate cardiac output. If a person has a tachycardia/bradycardia and are not showing signs of poor perfusion, we say they are stable.

Conversely, if they are showing signs of poor perfusion, we say they are unstable. While there are many ways we could define instability, most emergency settings utilize the HSF/AHA criteria for instability.

Unstable Signs & Symptoms
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Bradycardia

Bradycardias aren't particularly taxing to the heart and most patients can simply be monitored. If a bradycardia is not normal for the patient, then efforts are made to correct the underlying cause.

If the bradycardia is causing symptoms of instability, then we need to intervene and speed up the heart via:

  • Medications like atropine or epinephrine, that speed up the heart.
  • Electrical pacing, which generates an impulse for the heart.

Tachycardia

Tachycardias could be more taxing to the heart, but are still well tolerated by most patients if it's meant to meet some sort of physiological demand. For example, if a patient is exercising, then a tachycardia is expected and can be resolved with rest. If a patient is bacteremic and dehydrated, then a tachycaria is expected and can be resolved by fluids and treating the infection with antibiotics.

There are some tachycardias, however, that are caused by unusual processes within the heart. In these cases, we will need to intervene and slow down the heart directly via:

  • Medications which directly slows the heart rate like beta-blockers or calcium channel blockers.
  • Medications which converts the heart rhythm to a normal, slower rhythm like adenosine or other antiarrhythmics.
  • Electrical cardioversion, which overrides the heart's electrical system and attempts to reset it to a normal rhythm.

The exact method that will work depends on the arrhythmia.