Introduction The ECG is one of the most frequently performed diagnostic tests in Emergency Medicine. Electrode misplacement can lead to incorrect diagnoses, initiation of incorrect or withholding appropriate treatment. Right arm – left arm (RA-LA) misplacement is the most common and most recognisable limb lead misplacement. Inverted P waves and QRS complexes in lead I are characteristic features of RA-LA misplacement. Inverted T waves in lead I and the presence of R waves in lead aVR may also be present. High quality research is lacking and therefore this unique study aims to investigate the ECG features that are statistically significant for the presence of RA-LA misplacement.

Methods Over a 10-year period in-patient and Emergency Department records were scrutinised for evidence of RA-LA misplacement. Statistical analysis of both paired normal and RA-LA misplacement ECGs was performed using MDCalc Diagnostic Test Calculator and Fischer’s exact test.

Results Of the 335 lead misplacements identified 143 (42.7%) were RA-LA misplacement. ECG machines detected only 58% of RA-LA misplacements. In lead I inverted P waves were present in 103 of 117 (88%) ECGs and inverted QRS complexes in 100% of ECGs. T waves were inverted in 103 ECGs (72%), upright in 31 (21.7%) and flat in 9 (6.3%). R waves were identified in lead aVR in 113 (79%). Atrial fibrillation was present in 26 (18.2%) ECGS and only 2 (7.7%) were correctly diagnosed by the ECG machine with lead misplacement.

Conclusion Highly statistically significant ECG features of RA-LA misplacement are inverted P waves and inverted QRS complexes in lead I and the presence of R waves in lead aVR Their absence does not exclude the diagnosis. In this study ECG machines failed to detect almost half of all RA-LA misplacements. Education and training is required to recognise ECG lead misplacements. ECG interpretative software should incorporate these findings into their diagnostic algorithms.