Possible error in judgement?

Yes. The one performed in the wheelchair had a lot of artifacts that made it difficult to read, so I redid the EKG while the pt was laying down. There were no ST elevations or other notable abnormalities beyond the heart rhythm, which the EKG read-out interpreted as A-fib. Manual pulse check did not confirm the BPM recorded on the EKG.

No, they did not appear to be

But now I'm concerned that maybe my interpretation was wrong? Worst case scenario, if the pt was having paroxysmal a-fibb will the physician likely be upset that I didn't notify them (given that this is a known and treated condition)?

Yes, I am not super comfortable interpreting EKG's, but I can do basic interpretations. I had my charge nurse review the EKG as well, and he saw no outstanding abnormalities. No significant findings from pt assessment. O2 sat above 95% on RA, no c/o of fatigue, dizziness, SOB, chest pain, or palpitations. LOC x4. No abnormal breathing pattern or other signs of distress noted. Pt conversational throughout exam.

I did the EKG in the wheelchair to minimize the pt's discomfort (she was obese and had extreme difficulty changing positions); however, now I understand that the seated position is likely to give faulty readouts.

It's generally best to alert the MD to any abnormal EKGs. The machine will spit out an analysis, and that may be wrong, but it should still be reported. If there was artifact on the EKG, was it not possible to repeat and get the patient to lie still? Was it due to tremoring? A span of 100 bpm between your pulse measurement and the EKG does not seem like a good test. What did you feel as a manual pulse? Automated readings should never be considered the gold standard because machines are more fallible than human assessment.

The first EKG w/ the pt in the seated position had a significant number of artifacts due to tremoring and positioning, so I repeated the EKG laying and the reading did not have any significant artifacts. The manual pulse was 56. The automatic vitals sign machine also recorded a pulse of 56.

The day shift nurse got back to me and said they would have preferred to have been notified, but did not seem overly upset. She was also told to trust the EKG over a manual pulse when the pt has a dx of A-fibb. Moving forward, I will always alert the physician to an abnormal EKG, regardless of pt diagnosis/hx.

Pulse and Heart Rate by EKG are two different things, typically the two match up but particularly with tachyarrhythmias they may be completely different.

'Pulse' is the number of detectable perfusing beats, the EKG HR is number of electrical impulses to the ventricles. This can often be seen in variations between the EKG HR and a pleth pulse or A-line pulse.

With a markedly elevated HR the ventricles may not have time to fill sufficiently to empty enough with a contraction to produce a perfusing beat, so some beats may not perfuse if the ventricle remains underfilled.