Keep in mind that ADHD has two subtypes: Hyperactive and Inattentive. Where Hyperactive individuals express themselves physically, Inattentive individuals get lost in their thoughts.
If so, then your inattentiveness could be your mind trying to escape that feeling.
Could it be that you're dreading something that is "implied" by the lecture in a way? Maybe the practise that you feel obliged to do? Maybe the impact the lecture could have on your work or schedule? Maybe just the end of the lecture and back to whatever comes after?
And, I realise I'm leaning myself out of the window here, I think the inattentive type doesn't have the "dopaminergic focus problems" that strong. He zones out because he is repulsed by given tasks for different reasons. That's the insula and emotions of disgust at play there.
Let me get back to you on a source, ICD10 somewhere.