A 28-year-old athlete presents with palpitations during a routine cardiology screening. His ECG shows progressive lengthening of the PR interval followed by a P wave that is not conducted to the ventricles (marked **B** in the diagram), after which the PR interval resets to a shorter duration. The QRS complexes are narrow. Which of the following best describes the clinical significance and expected natural history of the conduction abnormality demonstrated at **B**?