Give the patient a day and time when he can expect action. Keep him abreast of the progress. Do more than promised. If you promised something for Thursday, come through on Wednesday. Thank the person for mentioning the problem.
Most people never complain.
They just leave. A complaint gives you the chance to improve your operation while keeping that patient. It's a blessing in disguise. Personally follow through to make sure the problem is solved adequately. People really get hot under the collar when you promise to fix something and it doesn't happen.