Most Likely Diagnosis: Sinus Rhythm with Premature Atrial Contractions (PACs)
The most likely diagnosis is sinus rhythm with premature atrial contractions (PACs), given the patient's young age, absence of hemodynamic compromise, irregular rhythm on exam, and benign clinical presentation with palpitations as the sole symptom. 1
Clinical Reasoning
Key Distinguishing Features
Irregular palpitations in a healthy patient without syncope, presyncope, lightheadedness, or chest pain strongly suggest premature depolarizations rather than conduction block. 1 The ACC/AHA guidelines explicitly state that irregular palpitations may be due to premature depolarizations, atrial fibrillation, or multifocal atrial tachycardia 1.
Why PACs Are Most Likely
- PACs occur in nearly all individuals and commonly cause palpitations without other symptoms 2
- The irregular rhythm with normal S1 and S2 is characteristic of intermittent premature beats disrupting an otherwise regular sinus rhythm 1
- PACs are described as "pauses or nonconducted beats followed by a sensation of a strong heart beat, or irregularities in heart rhythm" 1, which matches this patient's presentation
- The absence of hemodynamic symptoms (no lightheadedness, dizziness, syncope, or chest pain) argues strongly against significant conduction block 1
Why the Other Diagnoses Are Unlikely
Third-degree heart block is definitively excluded because:
- Complete heart block typically presents with profound symptoms including syncope, presyncope, or severe fatigue due to inadequate cardiac output 1
- Patients with third-degree block would have bradycardia and hemodynamic compromise, not isolated palpitations 1
- The normal vital signs exclude this diagnosis 1
Second-degree AV block Mobitz type I is unlikely because:
- Mobitz type I is often asymptomatic in healthy, active patients, but when symptomatic, it causes exertional intolerance or dizziness, not isolated palpitations 1
- The clinical presentation lacks the characteristic features of progressive conduction delay 1
- Mobitz type I typically occurs at the AV node level and would not present with weeks of palpitations as the sole symptom 3
Wandering atrial pacemaker is less likely because:
- This rhythm is typically seen in patients with underlying pulmonary disease or increased vagal tone 1
- The patient is described as healthy with no mention of underlying conditions
- Wandering atrial pacemaker would show varying P-wave morphology on ECG, which is not the primary clinical concern here
Diagnostic Approach
The next step should be obtaining a 12-lead ECG during symptoms to confirm the diagnosis. 1 The ACC/AHA guidelines emphasize that "patients with a history of sustained arrhythmia should always be encouraged to have at least one 12-lead ECG taken during the arrhythmia" 1.
What to Look For on ECG
- Premature P waves with different morphology from sinus P waves 1
- Irregular R-R intervals with occasional early beats 2
- Normal QRS complexes following the premature P waves (conducted PACs) or pauses (nonconducted PACs) 4, 5
Management Considerations
After excluding reversible causes (electrolyte abnormalities, hyperthyroidism, stimulant use), symptomatic PACs can be treated with beta blockers if needed. 2 However, the ACC/AHA guidelines note that frequent PACs are increasingly recognized as associated with developing atrial fibrillation, warranting appropriate follow-up 2.