Definition of Sinus Tachycardia
Sinus tachycardia is defined as a sinus rhythm with a heart rate exceeding 100 beats per minute (bpm). 1
Heart Rate Parameters
- Normal resting sinus rate in healthy adults ranges between 50-90 bpm, reflecting baseline vagal tone 1
- Sinus tachycardia threshold is any sinus rate >100 bpm 1
Electrocardiographic Characteristics
The ECG features that distinguish sinus tachycardia from other tachyarrhythmias include:
- P wave morphology is upright in leads I, II, and aVF 1
- P wave in V1 is characteristically biphasic 1
- P wave amplitude increases in inferior leads as heart rate increases, because activation arises from more superior aspects of the right atrium at faster rates 1
Clinical Classification
Sinus tachycardia can be categorized into two distinct types based on clinical context:
Appropriate (Physiological) Sinus Tachycardia
- Occurs in response to physiological stimuli or identifiable exogenous factors where the elevated heart rate is expected 1
- Common causes include fever/infection, dehydration, anemia, heart failure, hyperthyroidism, pain, hypotension/shock 1
- Exogenous substances such as caffeine, beta-agonist medications (albuterol, salmeterol), and illicit stimulants (amphetamines, cocaine) 1
- Expected to resolve with correction of the underlying cause 1
Inappropriate Sinus Tachycardia (IST)
- Defined as sinus tachycardia that is unexplained by physiological demands, where the heart rate exceeds what would be expected for the level of physical activity or clinical circumstances 1
- Associated with debilitating symptoms including weakness, fatigue, lightheadedness, and uncomfortable sensations such as heart racing 1
- Resting heart rates commonly exceed 100 bpm even at rest 1
Critical Diagnostic Distinction
When heart rate is <150 bpm in the absence of ventricular dysfunction, the tachycardia is most likely secondary to an underlying condition rather than the primary cause of symptoms, which fundamentally determines the treatment approach 2