What defines functional tachycardia?

Defining Functional Tachycardia

Functional tachycardia is defined as a heart rate greater than 100 beats per minute that is an appropriate physiological response to physical, emotional, pathological, or pharmacologic stress. 1

Characteristics of Functional Tachycardia

Functional (or physiological) sinus tachycardia has several key characteristics:

• Heart rate exceeding 100 beats per minute
• Appropriate response to underlying physiological demands
• Normal P wave morphology (positive in leads I, II, aVF; negative in aVR)
• P wave axis in frontal plane between 0° and 90°
• Expected to resolve with correction of the underlying cause 1

Common Causes

Functional tachycardia occurs in response to:

• Physical activity or exercise
• Emotional stress or anxiety
• Pathological conditions:
  • Fever/infection
  • Hypovolemia/dehydration
  • Anemia
  • Heart failure
  • Hyperthyroidism
• Exogenous substances:
  • Caffeine, alcohol, nicotine
  • Prescribed medications (salbutamol, aminophylline, atropine, catecholamines)
  • Recreational/illicit drugs (amphetamines, cocaine, cannabis) 1

Differentiating from Other Tachycardias

It's important to distinguish functional sinus tachycardia from other tachyarrhythmias:

1. Inappropriate Sinus Tachycardia (IST):

   • Unexplained by physiological demands
   • Resting heart rates >100 bpm
   • Average 24-hour rates >90 bpm
   • Associated with symptoms like fatigue, lightheadedness, and palpitations 1

2. Postural Orthostatic Tachycardia Syndrome (POTS):

   • Heart rate increment ≥30 bpm within 10 minutes of standing
   • Often reaches ≥120 bpm
   • Occurs without orthostatic hypotension
   • Associated with symptoms of cerebral hypoperfusion 2

3. Arrhythmic Tachycardias:

   • Supraventricular tachycardias (SVT) - typically have heart rates ≥150 bpm
   • Atrial fibrillation/flutter
   • Ventricular tachycardias 1

Clinical Significance

The American Heart Association guidelines note that:

• When heart rate is <150 beats per minute, symptoms of instability are unlikely to be caused primarily by the tachycardia unless there is impaired ventricular function
• The rate of tachycardia takes on clinical significance at greater extremes (typically ≥150 bpm)
• When encountering patients with tachycardia, determine whether it is the primary cause of symptoms or secondary to an underlying condition 1

Diagnostic Approach

When evaluating tachycardia:

1. Assess if the heart rate is appropriate for the clinical condition
2. Look for underlying causes (fever, dehydration, anemia, etc.)
3. Evaluate for signs of increased work of breathing and oxygen saturation
4. Determine if the tachycardia is the cause of symptoms or a response to another condition 1

Management Considerations

For functional tachycardia:

• Treatment should be directed at the underlying cause rather than at the tachycardia itself
• No specific drug treatment for the tachycardia is required
• "Normalizing" the heart rate can be detrimental when cardiac function is poor and cardiac output depends on a rapid heart rate 1

Clinical Pitfalls

1. Misdiagnosis: Failing to distinguish between functional tachycardia and pathological tachyarrhythmias can lead to inappropriate treatment.

2. Overlooking the underlying cause: Focusing on treating the tachycardia rather than identifying and addressing the root cause.

3. Inappropriate rate control: In compensatory tachycardias where cardiac output depends on heart rate, aggressive rate control can worsen hemodynamics.

4. Failure to recognize when tachycardia is inappropriate: Not identifying when a tachycardia exceeds what would be expected for the physiological stress.