In which scenario does hyperventilation become abnormal?

Hyperventilation is considered abnormal in the context of an EEG when it leads to a focal delta build-up. Focal delta waves on an EEG typically indicate a pathological process or an area of dysfunction in the brain. In a normal scenario, hyperventilation may cause a general increase in slow waves or even a variety of EEG changes; however, the appearance of localized or focal delta activity suggests that there is an underlying issue that is being exacerbated by the hyperventilation. This could indicate a sensitivity in a specific brain region to changes in metabolism or blood flow, which may uncover or aggravate a pre-existing condition, such as a seizure focus.

In contrast, hyperventilation might lead to yawning, which is a physiological response and typically not indicative of an abnormality in the context of an EEG. An increase in muscle tension can be a normal physiological response to stress or anxiety and is not an EEG-related abnormality. Similarly, a lack of response to hyperventilation may suggest that the brain is stable and functioning normally under the condition, which does not constitute an abnormal response. Therefore, the development of focal delta waves serves as a key indicator of abnormality induced by hyperventilation.