Final answer:
The nurse should first notify the healthcare provider immediately due to signs that suggest a worsening pericardial effusion, possibly leading to cardiac tamponade, which is a medical emergency.
Step-by-step explanation:
The nurse assessing a client with pericardial effusion observes a change in the client's vital signs from an apical pulse of 72 and a blood pressure (BP) of 138/94 at 1600 to an apical pulse of 70 and a BP of 106/94 at 1800, along with neck vein distention. Neck vein distention, alongside a drop in systolic blood pressure but unchanged diastolic pressure, indicates an increased likelihood of cardiac tamponade, a potentially life-threatening condition. The first action the nurse should take is b. Notify the health-care provider immediately. This is crucial to provide timely intervention for the condition. While awaiting the healthcare provider's instructions or intervention, the nurse should then consider other actions such as staying with the client and reassurance, positioning, or administration of medications as ordered.