A 9-year-old boy is brought to the pediatric clinic for a routine health check-up. An ECG is performed as part of screening, and the cardiologist notes that the heart rate varies between 75 and 95 bpm during the recording, with the rate increasing during inspiration and decreasing during expiration. P waves are normal, PR interval is constant, and QRS complexes are narrow. The structure marked **D** in the diagram indicates that this finding is a benign variant in children and young adults. What is the most appropriate clinical action?