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Sinus rhythm is present when the sinus node generates a P wave that conducts through the AV node to generate a ventricular (QRS) complex. P waves originating from the sinus node are upright in lead II, inverted in lead aVR, and biphasic in lead V1. The following differential for abnormal P wave axis is small, but important:
Limb lead misplacement (most common cause)
Dextrocardia
Ectopic atrial rhythm
Ventricular to atrial (V-A) conduction - abnormal P wave axis that occurs after the QRS interval
KEY PEARL:
An important additional finding associated with both limb lead misplacement and dextrocardia (but not ectopic atrial rhythm) is extreme right axis deviation (positive QRS interval in aVR & predominantly negative QRS in lead I)
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