Which finding should be monitored closely while a patient is taking timolol?

Timolol is a non-selective beta-adrenergic antagonist primarily used in the management of glaucoma by reducing intraocular pressure. However, it also has systemic effects due to its action on beta receptors throughout the body, particularly in the cardiovascular system. While taking timolol, monitoring heart rate closely is essential because the medication can cause bradycardia (a slower than normal heart rate) by blocking beta-1 receptors in the heart. In patients who have pre-existing conditions like bradycardia or other cardiovascular issues, this could pose significant risks.

When considering other parameters, such as intraocular pressure, while important in assessing the effectiveness of timolol, it is the systemic effects on heart rate that require more vigilant monitoring, especially since serious cardiovascular effects can directly impact patient safety. Additionally, changes in blood glucose levels and serum potassium levels could occur for various reasons that aren't directly linked to timolol, making heart rate the most critical parameter to observe while the patient is on this medication.