A 13-year-old's irregular apical pulse that is phasic with respiration; this finding most likely represents which normal phenomenon?

• A. Atrial fibrillation
• B. Sinus arrhythmia
• C. Sinus tachycardia
• D. Premature atrial contractions

Answer: (C)

Respiratory sinus arrhythmia is a normal phenomenon in children, reflecting how the heart rate naturally fluctuates with the breathing cycle. During inspiration, vagal tone decreases and the sinus node fires a bit faster, so the pulse quickens. During expiration, vagal tone increases and the rate slows slightly. This causes the apical pulse to be irregular and phasic with respiration, a common finding in healthy adolescents. It isn’t a sign of a pathologic rhythm. In contrast, atrial fibrillation would be irregular without a consistent breathing pattern, sinus tachycardia would show a consistently fast rate rather than beat-to-beat variation with breathing, and premature atrial contractions would produce early, isolated beats rather than a regular breath-linked pattern.