Final answer:
Atrial flutter and atrial fibrillation are both ECG abnormalities that involve missing or abnormal P waves, with atrial flutter featuring repetitive "flutter" waves, and atrial fibrillation showcasing an irregular baseline without discernible P waves.
Step-by-step explanation:
On an ECG (electrocardiogram), the waveforms that are missing or abnormal in atrial flutter and atrial fibrillation pertain to the P wave. In a normal ECG, the P wave represents the depolarization of the atria, which precedes the contraction of the atria. However, in atrial flutter, the P wave is replaced by repetitive, regular "flutter" waves that are more saw-toothed in appearance. In contrast, atrial fibrillation is characterized by a completely irregular baseline without discernible P waves, due to disorganized atrial activity that does not result in effective atrial contraction.
In both conditions, the disorderly atrial activity leads to an irregular heart rhythm which can be seen on an ECG. For atrial fibrillation, this manifests as an irregularly irregular rhythm, with a variable frequency between QRS complexes. For atrial flutter, the rhythm can be regular or irregular depending on how the ventricles respond to the rapid flutter waves coming from the atria.