Might you have more than one pause? If so, you might want to set up a related table of "anesthesia intervals" with start and end times. If there are no pauses, you have one record. With one pause, you have two records. With two pauses, three and so forth. A calculation field can then compute elapsed time for each interval of anesthesia and a summary field or sum function calculation field can compute the total anesthesia time.
But if there is never more than one pause:
If ( Is ( Empty ( PauseTime ) ; End Time - Start Time ; End time - Resume time + Pause Time - Start Time )