How do you differentiate and manage sinus tachycardia (ST) versus supraventricular tachycardia (SVT) in a patient presenting with tachycardia?

Differentiating Sinus Tachycardia from SVT

The most reliable way to distinguish sinus tachycardia from SVT is by assessing heart rate characteristics, P-wave morphology, and response to vagal maneuvers: sinus tachycardia shows gradual rate changes with identifiable physiologic causes and normal P-wave axis, while SVT demonstrates abrupt onset/termination with extreme regularity and abnormal P-wave morphology. 1

Rate and Rhythm Characteristics

Heart rate thresholds differ significantly between the two arrhythmias:

• Sinus tachycardia rarely exceeds 180 bpm in adults and is almost always <230 bpm in infants, with gradual acceleration and deceleration over several seconds 1
• SVT typically presents at >150 bpm in adults and 260-300 bpm in infants, with extreme regularity after the first 10-20 beats 1
• The R-R interval in SVT shows metronome-like extreme regularity after initial beats, whereas sinus tachycardia demonstrates variation over several seconds 1

P-Wave Analysis: The Critical Distinguishing Feature

P-wave morphology is your most important diagnostic tool:

• Sinus tachycardia maintains the same P-wave axis as normal sinus rhythm with clearly visible P waves 1
• In SVT, approximately 60% show visible P waves, but these almost always have different morphology from sinus P waves 1
• In typical AVNRT (the most common SVT), P waves are hidden within the QRS complex, creating a pseudo-S wave in inferior leads (II, III, aVF) and pseudo-R' in lead V1 2, 1
• In orthodromic AVRT, the P wave appears in the early ST segment, separated from QRS by ≥70 ms 2, 1

Clinical Context Integration

Sinus tachycardia always has an identifiable physiologic trigger:

• Look for fever, sepsis, hypovolemia, pain, anxiety, or medications as precipitating factors 1
• SVT often occurs in patients with otherwise normal hearts without obvious precipitating factors 1

Diagnostic Maneuvers

Vagal maneuvers or adenosine administration provide definitive differentiation:

• Sinus tachycardia will gradually slow during vagal maneuvers or adenosine, then return to the previous rate 1
• SVT will either abruptly terminate or transiently slow, revealing underlying atrial activity 1
• A 12-lead ECG recording is desirable during adenosine administration or carotid massage 2
• Carotid sinus massage is useful for differentiation in narrow complex regular tachycardias 3

QRS Complex Assessment

QRS width provides the initial framework:

• Narrow QRS complex (<120 ms) almost always indicates a supraventricular origin (either sinus tachycardia or SVT) 1, 4
• If ventricular action is narrow, the tachycardia is almost always supraventricular, and the differential diagnosis relates to its mechanism 2

RP Interval Classification (When P Waves Are Visible)

The relationship between P waves and QRS complexes narrows the SVT differential:

• Short RP interval (RP < PR) is typical of AVNRT or AVRT, where the P wave is closer to the prior QRS 1
• Long RP interval (RP > PR) suggests atypical AVNRT, permanent form of junctional reciprocating tachycardia (PJRT), or atrial tachycardia 1

Critical Pitfalls to Avoid

Common diagnostic errors that lead to inappropriate management:

• Never initiate class I or class III antiarrhythmic drugs without documented arrhythmia due to proarrhythmia risk 2
• Do not assume all regular narrow complex tachycardias are benign—obtain a 12-lead ECG whenever possible, though this should not delay immediate therapy if hemodynamic instability is present 2
• Compare the tachycardia ECG with previous ECGs during normal sinus rhythm to assess P-wave morphology changes 1
• If no P waves are visible on surface ECG, esophageal pill electrodes can be helpful for visualization 2

Management Approach Based on Diagnosis

Once differentiated, management diverges significantly:

• For sinus tachycardia: Treat the underlying cause (fever, hypovolemia, pain, etc.) rather than the rhythm itself 1
• For SVT in stable patients: Vagal maneuvers are first-line, followed by adenosine if ineffective 5
• For SVT in unstable patients: Synchronized cardioversion is first-line management 5
• Beta-blocking agents may be prescribed empirically for SVT, provided significant bradycardia (<50 bpm) has been excluded 2