Final answer:
Muffled heart sounds following ICD insertion is a critical finding that may signal cardiac tamponade, requiring swift medical intervention.
Step-by-step explanation:
The complication that requires immediate intervention by the nurse after the insertion of an implantable cardioverter-defibrillator (ICD) is muffled heart sounds. This symptom may indicate cardiac tamponade, a potentially life-threatening condition where fluid accumulates in the pericardial sac around the heart, restricting the heart's movement. It's important to promptly assess and manage this condition to prevent cardiac arrest or other serious complications.