Final answer:
Atrial fibrillation is diagnosed based on an ECG showing an abnormal electrical pattern before the QRS complexes and an increased frequency between them, leading to an irregular heart rate.
Step-by-step explanation:
The diagnosis of an arrhythmia based on an ECG requires observation of specific patterns related to heart's electrical activity. Atrial fibrillation is characterized by an abnormal electrical pattern prior to the QRS complexes, accompanied by an increased frequency of these complexes. This irregular rhythm indicates a disorder in the heart's conduction system, where the signal, which should initiate at the sinoatrial node, leads to an unpredictable contraction pattern. Therefore, one would expect an increased and irregular heart rate (pulse) in the case of atrial fibrillation.