ECG features and diagnostic challenges
The term "supraventricular paroxysmal tachycardia" is used in clinical practice as an umbrella term. Due to the extremely high heart rate, the P wave is often not visualized on the tracing, making it impossible to precisely differentiate between atrial and atrioventricular tachycardia.
Key ECG features of paroxysm:
- Rhythm: sudden onset and equally sudden termination.
- QRS complex: in the vast majority of cases (about 90%), it remains narrow, with normal morphology and width.
- P wave: indicates retrograde conduction (from the ventricles to the atria), thus becoming negative (inverted) in leads II, III, and aVF. In practice, it is most often hidden behind the QRS complex.
The formal ECG criterion for establishing the diagnosis is the registration of a "run" consisting of at least three consecutive supraventricular premature beats.
Mechanisms of occurrence
The development of paroxysmal supraventricular conditions is based on several pathophysiological mechanisms:
- Excitation wave circulation (re-entry): a pathological conduction loop is formed within the atrioventricular junction.
- Wolff-Parkinson-White (WPW) syndrome: the presence of accessory pathways causes pre-excitation of the ventricles. In apparently healthy individuals, it may be an incidental ECG finding, but its main clinical manifestation is tachycardia paroxysms debuting in childhood or adolescence via a re-entry mechanism.
Epidemiologically, rhythm disorders often manifest in children (even in the neonatal period) with a peak during puberty, during which hormonal and autonomic reorganization of the body occurs.
Termination of the attack: first stage
Prehospital tactics strictly depend on hemodynamic stability (level of consciousness, blood pressure, skin color). In stable hemodynamics and a narrow QRS, therapy begins with vagal maneuvers for reflex termination by stimulating the vagus nerve:
- Carotid sinus massage: performed sequentially on each side for 15–20 seconds with mandatory pulse monitoring.
- Reflex maneuvers: Valsalva maneuver (straining during inspiration), inducing the gag reflex, Aschner reflex (pressure on the eyeballs), applying ice to the face.
In young children, carotid sinus massage and eye pressure are used with extreme caution due to the risk of injury.
Pharmacotherapy
If vagal maneuvers fail, pharmacotherapy is initiated (strictly in a hospital setting or under ECG monitoring).
Algorithm for a narrow QRS complex:
- Triphasadenine (ATP/adenosine): first-line drug. Administered as a rapid intravenous bolus (10–20 mg). It acts within seconds, but causes unpleasant sensations (feeling of warmth, chest discomfort, brief asystole), which the patient must be warned about in advance.
- If ineffective: intravenous administration of verapamil or beta-blockers.
- If previous steps fail: drugs for antidromic tachycardia (e.g., propafenone).
Many potent antiarrhythmic drugs (amiodarone, lidocaine) are used in young patients with great caution due to the risk of hypotension and conduction depression.
Electrical cardioversion
Synchronized electrical cardioversion (ECV) is indicated when drug therapy and vagal maneuvers have proved ineffective, or when unstable hemodynamics (hypotension, signs of shock) are present initially. Delaying cardioversion in marked instability is life-threatening.
Subsequent decisions on prescribing prophylactic therapy are made in the hospital. For mild, rare, and short paroxysms, long-term drug therapy is generally not recommended.
The "Three U" rule for supraventricular tachycardia: Accelerated rate (150–250), Narrow QRS (in 90% of cases), Elusive P wave (hidden behind QRS or negative).
Class and exam questions
Why are atrial and AV tachycardias often combined under a single term?
Due to the extremely high heart rate, the P wave superimposes on the QRS complex or merges with it, making precise topographic diagnosis (differentiation into atrial and junctional forms) impossible on a standard ECG.
What does a patient feel when given triphasadenine (ATP)?
The drug acts very rapidly and may cause a transient feeling of warmth, chest discomfort, transient bradycardia, or even brief asystole. The patient must be warned about this in advance.
When should electrical cardioversion be used immediately during a tachycardia paroxysm?
Synchronized electrical cardioversion is immediately used in the presence of signs of unstable hemodynamics: a drop in blood pressure, loss of consciousness, or the development of signs of shock.
What else to study on this topic
- Criteria and forms of atrioventricular premature beats
- ECG picture in Wolff-Parkinson-White (WPW) syndrome
- Differences between supraventricular tachycardia and ventricular tachycardia by QRS complex width
- Sick sinus syndrome and tachycardia-bradycardia syndrome
- Mechanism of excitation wave circulation (re-entry) in the myocardium
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