What is a key characteristic of the ECG in a patient with atrial fibrillation?

In atrial fibrillation, a key characteristic observed on the ECG is the absence of distinct P waves. This absence occurs because the atria are not contracting in a coordinated manner; instead, they quiver, leading to chaotic electrical activity that prevents the formation of identifiable P waves. The ventricular response can be irregularly timed, resulting in an irregularly irregular rhythm, but the defining feature that distinguishes atrial fibrillation from other arrhythmias is the lack of organized atrial activity, represented by P waves.

The other ECG findings associated with different conditions do not apply here: ST elevation is typically indicative of ischemia or myocardial infarction; QRS widening suggests a delay in ventricular conduction, which is not specific to atrial fibrillation; and regular R-R intervals would indicate a regular rhythm, which contradicts the irregular nature of atrial fibrillation. Hence, the absence of P waves is the hallmark finding in an ECG of a patient with atrial fibrillation.