Final answer:
When suspecting atrial fibrillation, an ECG would show a rapid and irregular rhythm without distinct P waves, and the atrial heart rate may exceed 350 beats per minute. The ventricular rate can vary due to the AV node's filtering. In third-degree block and ventricular fibrillation, heart rate can be dramatically slowed or absent, which is life-threatening.
Step-by-step explanation:
If you suspect atrial fibrillation (AFib), on an electrocardiogram (ECG), you would likely see a rapid, irregular rhythm with an absence of distinct P waves before the QRS complexes. Additionally, the atrial heart rate during AFib can be quite high, typically over 350 beats per minute but can range up to 600 beats per minute due to the disorganized electrical activity in the atria. However, because of the AV node's filtering effect, not all these impulses are transmitted to the ventricles, so the ventricular rate (pulse) can vary significantly. In cases where there is an abnormal electrical pattern and increased frequency between QRS complexes, you can expect an irregular and potentially rapid heart rate for the pulse. Conversely, in the case of total lack of normal electrical activity (as observed in ventricular fibrillation), the heart rate would be ineffective or absent, leading to a medical emergency. In a third-degree heart block situation, where the P waves are not followed by QRS complexes consistently, bradycardia could occur because ventricular rate is then determined by the intrinsic pacing of the bundle branches or Purkinje fibers, which is much slower (20-40 impulses per minute) than the normal sinus rhythm directed by the SA node (60-100 beats per minute).