It would be very difficult (actually impossible) to be able to say what you have been experiencing. Toprol (metoprol XL) is a beta blocker which is prescribed in a number of different situations (heart failure, arrhtyhmias, high blood pressure, etc). Without knowing your medical history and being able to perform a physical examination, there is no way of knowing for sure what is going on. However, Toprol is a long-acting beta blocker, so even missing a dose for 5 hours, should theoretically not bring the acute onset of any symptoms. If you are experiencing significant symptoms such as shortness of breath, chest pain, light-headedness, dizziness, or syncope (fainting) -- then I would be concerned enough that you should seek further evaluation. Otherwise, I think it is safe to follow-up with your regular cardiologist when he/she is available.