The P wave on the ECG represents atrial tissue activation. During normal sinus rhythm, atrial depolarization is conducted from right atrium to left atrium through Bachmann’s bundle, and a normal P wave axis which is measured on the frontal plane is between 0º and + 75º [3]. Therefore, the atrial depolarization vector projection on the lead II and aVR results in positive polarity of lead II and negative polarity of lead aVR. Besides, P wave morphology shows positive deflection in lead I. When the limb and precordial leads are placed in their standard location, the aVR and V5 or V6 leads are in the same alignment but have deflections in the opposite polarity [4].

In our case, P wave morphology shows biphasic deflection (initially positive followed by negative deflections) in lead II and positive polarity in lead aVR, indicating that it isn’t likely the result of normal sinus rhythm. P wave morphology shows negative deflection in lead I, indicating that it is likely the result of conduction from the left atrium to the right atrium. The 5 most likely causes of depolarization sequence are the mirror-image dextrocardia, acquired dextrocardia, left atrial rhythm, the right arm-left leg lead reversal and the right arm-left arm lead reversal [5, 6]. The characteristics of QRS complex in leads V1 to V6 are helpful to preliminarily differential diagnosis. Figure 1 shows a RBBB morphology in chest leads. There is a progressive increase in R-wave amplitude from leads V1 to V6, which isn’t favor of mirror-image dextrocardia. Transthoracic echocardiography also didn’t show mirror-image dextrocardia. The causes of acquired dextrocardia (dextroposition) include lung, pleural, and diaphragm lesions, such as right pneumonectomy, left pneumothorax, and left lung hypoplasia, which can induce negative P wave in lead I. Recently, Tsai et al. [6] reported one patient with inverted P and T waves in lead I and aVL, and dominant R wave in lead V1 in acquired dextrocardia. Our patient denied any above medical history and intra-abdominal or thoracic symptoms. The echocardiography showed left atrial enlargement, which can cause left atrial rhythm. If this was an example of a left atrial rhythm, QRS complex morphology is consistent with conduction to the ventricles through the normal His-Purkinje atrioventricular conduction system (normal QRS complex or BBB). The aVR and V5 leads displayed identical polarity deflections, excluding left atrial rhythm [4]. At this time, we should suspect the presence of abnormal placement of limb leads. Besides, in a 2021 study, Littmann [7] proposed that lead V1 can mimic aVR. The V1 and aVR discordance also should raise the suspicion of the presence of electrode reversal. Right arm-left leg reversal leads is uncommon. Right arm-left leg reversal produces highly abnormal-looking limb leads, with leads I, II, III, and aVF being negative and aVR being upright, mimicking inferior myocardial infarction. The aVL is unchangeable. In our case, P waves in leads III and aVF are positive and the QRS complex morphology in lead aVL is inconsistency with lead V5, therefore it is highly unlikely to have a right arm-left leg reversal. Right arm-left arm reversal lead is common. In a patient with a right arm-left arm lead reversal, the P-QRS-T in lead I should be flipped, the pattern of lead aVR resembles a normal aVL, and lead II resembles a normal lead III [8]. The initial ECG (Fig. 1) corrected according to the aforementioned features was consistent with typical RBBB. The repeat ECG in Fig. 2 after correcting right arm-left arm lead connection shows sinus rhythm with RBBB, confirming right arm-left arm lead reversal on initial ECG.

The ECG is a simple and inexpensive tool which can be recorded by many different types of health personnel, including physician doctors, ECG technicians, nurses from a variety of hospital units. The value of the ECG depends upon the accuracy of how it is obtained. When right arm-left arm lead reversal is not complicated with arrhythmia, ECG features are typical and easy to diagnose. If combined with other abnormal ECG changes, such as the RBBB in this patient, electrocardiographic manifestations are diverse and atypical. Recognizing right arm-left arm lead reversal in patients with RBBB is an important and intellectually rewarding skill. Close evaluation of the lead characteristics of P-QRS-T changes such as lead I and chest leads may provide clues in diagnosing right arm-left arm lead reversal and its differential diagnosis. Besides, it is important to identify anatomic landmarks and to clear up any other misconceptions. Finally, a quality improvement program should monitor the incidence of common errors such as right arm-left arm lead wire reversal and provide retraining when indicated.