Hello from JustAnswer.
If there is no identifiable cause of the low heart rate, such as the use of a medicine that can lower the heart rate, the usual next step in this situation would be a consultation with a Cardiologist for consideration of placement of a pacemaker.
It is appropriate to consider pacemaker placement in someone having a heart rate below 40, even if only happening intermittently. The consideration for a pacemaker would be greater if there are symptoms that could be related to the low heart rate, such as dizziness or loss of sight.
When someone is having intermittent low heart rate, it is common that an ECG done by your doctor or hospital will miss the low heart rhythm, and we will frequently used a prolonged monitoring method, such as a Holter monitor. But since the low heart rate was documented when you were scheduled for the procedure, the Cardiologist would be able to make a decision based on this documentation.
Since the ECG ordered by your doctor did not show the rhythm, you may need to ask the hospital to send a copy of the documented low heart rate to your doctor, and then your doctor can consider the referral to a Cardiologist.
If I can provide any further information, please let me know.