For me, it's not just during a panic attack, but also occurs normally throughout the day. If I'm not fighting off fear, the "what-ifs", and hypochondria symptoms, I'm fighting off the "impending doom"...and sometimes all of them at once. The doom feeling is most commonly associated with the panic attack (because it's then that it is the strongest and most severe), but it is also a common daily symptom of Generalized Anxiety Disorder (GAD) (which is not the same as Panic Disorder (PD), though many people suffer from both--like me.
For the longest time, I didn't connect the "impending doom" with my GAD, after months in therapy and lots of research on GAD and PD, I've finally connected the two and am not as bothered anymore by those feelings. However, that's not to say I have gotten rid of them...I'm just not as troubled by them anymore because I understand they are part of the bigger picture of GAD.