Final answer:
The nurse would implement placing the client on a cardiac monitor as the first action.
Step-by-step explanation:
The nurse would implement placing the client on a cardiac monitor as the first action. When a client has a serum potassium level of 6.9 mEq/L, it indicates hyperkalemia, which can lead to serious cardiac complications. Placing the client on a cardiac monitor allows for continuous monitoring of the heart rhythm and can detect any abnormalities or arrhythmias caused by the high potassium level.