Final answer:
The nurse should further assess the client for irregular heartbeats.
Step-by-step explanation:
The nurse should further assess the client for irregular heartbeats when an older adult with a pacemaker and a sedentary lifestyle reports being unable to perform activities that require physical exertion. This is because irregular heartbeats, also known as arrhythmias, can affect the heart's ability to contract and pump blood effectively. A pacemaker helps regulate the heart rate, so any irregularities should be monitored closely.