Most Likely Diagnosis: Supraventricular Tachycardia (SVT)
The most likely diagnosis is supraventricular tachycardia (SVT), specifically atrioventricular nodal reentrant tachycardia (AVNRT), given the narrow-complex regular tachycardia at 150 bpm with palpitations and lightheadedness in a middle-aged patient with hypertension.
Diagnostic Reasoning
ECG Characteristics Point to SVT
The clinical presentation of a regular narrow-complex tachycardia at approximately 150 beats per minute is the hallmark of SVT 1. The ACC/AHA/HRS guidelines emphasize that when evaluating tachycardia, you must first classify whether the QRS duration is narrow (<120 ms) or wide, and whether the rhythm is regular or irregular 1. This patient has:
- Narrow QRS complex (ruling out ventricular tachycardia or SVT with aberrant conduction)
- Regular rhythm (ruling out atrial fibrillation, multifocal atrial tachycardia, or atrial flutter with variable block)
- Heart rate of 150 bpm (classic range for SVT is 150-250 bpm) 2
AVNRT is the Most Common Form
AVNRT accounts for the majority of paroxysmal SVT cases 2. In AVNRT, the reentry circuit is confined to the AV node and perinodal tissue, with two conducting pathways (fast and slow) that have different conduction velocities and refractory periods. The key ECG feature is that atria and ventricles depolarize simultaneously, hiding the P waves within or immediately after the QRS complex 1, 2.
Clinical Context Supports SVT
The patient's symptoms align perfectly with SVT:
- Palpitations and lightheadedness are the most common presenting symptoms 1, 3
- 2-hour duration indicates sustained tachycardia requiring intervention
- Hemodynamically stable (BP 130/85 mmHg) despite tachycardia—the guidelines note that BP drops initially but partially recovers within 30-60 seconds during SVT 1
- No chest pain or dyspnea makes acute coronary syndrome or heart failure less likely
Hypertension as a Risk Factor
While hypertension is present, it serves as a predisposing factor for supraventricular arrhythmias rather than the primary diagnosis 4. Hypertensive heart disease can manifest as cardiac arrhythmias, but the acute presentation with narrow-complex tachycardia at 150 bpm is characteristic of SVT, not hypertensive emergency.
Differential Considerations
Other SVT Subtypes
- Atrioventricular reciprocating tachycardia (AVRT) using an accessory pathway would show a P wave following the QRS with RP interval >70 ms 2
- Focal atrial tachycardia typically shows distinct P waves before each QRS 5
- Atrial flutter would show flutter waves and often has atrial rate exceeding ventricular rate 1
Critical Pitfall to Avoid
Do not misdiagnose ventricular tachycardia as SVT—the guidelines emphasize this can be life-threatening 1. However, this patient's narrow QRS complex definitively excludes VT. The guideline states that "failure to correctly identify VT can be critical," but the narrow complex here makes VT extremely unlikely 1.
Fascicular VT Consideration
While fascicular VT can occasionally present with a "not classically broad" QRS complex 6, this is rare in a 55-year-old with hypertension and would typically show right bundle branch block morphology with axis deviation—not a truly narrow complex at 150 bpm.
Clinical Implications
The patient requires:
- Immediate vagal maneuvers (Valsalva is safer and more efficacious than carotid massage, especially in older patients) 2
- Adenosine as first-line pharmacologic therapy if vagal maneuvers fail 2
- 12-lead ECG during tachycardia to confirm narrow-complex and identify P wave relationship to QRS 1
- ECG in sinus rhythm after conversion to look for pre-excitation (delta waves suggesting WPW syndrome) 1
The stable hemodynamics (BP 130/85) mean electrical cardioversion is not immediately required, but should be available if the patient becomes unstable 2.