Inverted P waves are upside down. This can be normal (i.e. lead aVR) or abnormal (i.e. leads II, III, aVF). If they are abnormal, the rhythm may be not originating from the SA node (e.g. atrial or junctional rhythms) or the electrodes may have been swapped around. Atrial rhythms will have a normal PR interval (i.e. 3-5 small squares) while junctional rhythms will have a short PR interval (less than 3 small squares at standard settings).
See also: P wave
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