Inverted P wave

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Ayokunle Olubaniyi, M.B,B.S [2]

Overview

Ordinarily, an impulse traveling from a point high in the atrium to the ventricle is right side up on the electrocardiographic tracing, but if this pacemaker impulse originates in lower part of the atrium, the orientation of the electrical vector may cause it to appear upside down or to be an "inverted P-wave".

Classification

Inverted P waves can be classified into two based on the leads affected. They are:

Those Causing Inverted P waves in Leads I and aVL

• Dextrocardia
• Misplacement of leads a.k.a technical dextrocardia

Those Causing Inverted P waves in Leads II, III, and aVF (inferior leads)

This involves all the conditions which cause a retrograde atrial activation i.e. an upward flow of electrical currents from an ectopic focus in the lower atrium or AV node. They include:

• Ectopic foci in the lower atrium
  • Focal atrial tachycardia
  • Multifocal atrial tachycardia

• Atrioventricular Origin
  • AV nodal reentrant tachycardia (AVNRT)
  • AV reentrant tachycardia (AVRT)
  • Junctional tachycardia

• Ventricular origin
  • Premature ventricular complexes
  • Ventricular bigeminy
  • Ventricular trigeminy
  • Ventricular couplets

Pathophysiology

During the normal atrial depolarization, the electrical impulse originates from the SA node in the right atrium and flows downwards through the right and left atrial muscle to the AV node (right to left depolarization). This movement of electrical current points towards the positive pole of lead II and the negative pole of lead aVR. Therefore, the normal sinus P wave is positive (upright) in lead II and negative (inverted) in lead aVR. However, when the flow of electrical current is from an ectopic focus either in the lower part of the atrium or in the AV junction, the current moves in the upward direction from the left to the right (left to right depolarization), consequently, leading to a reversal of the normal i.e. negative P wave in lead II and positive in aVR. The abnormalities in the morphology of P waves are best evaluated in the inferior leads of the EKG - leads II, III, and aVF.

Dextrocardia

This is a condition in which the heart is situated on the right side. Therefore, the flow of electrical impulse originates from the left side and terminates on the right (a reversal of the normal). The resultant effect of EKG include:

• Negative P wave and QRS complex in lead I
• R wave inversion across the precordium; the R wave is tallest in V1 and progressively decreases in amplitude in leads V2 to V6
• Right axis deviation and loss of voltage across the precordium

Misplacement of Leads

This refers to the inadvertent misplacement of limb or precordial leads which results into an abnormal P wave, QRS, and abnormal R wave progression.

Retrograde Atrial Activation

This occurs when there is a conduction of cardiac impulse from the ventricular to atrial direction. The origin of the electrical impulses could either be from:

• Ectopic foci in the atrium - This could be a single or multiple foci in the atrium. Foci within the atrium usually originate below the crista terminalis in the right atrium for it to have a significant effect on the p wave.

• AV node - This results into the various junctional rhythms e.g. AVNRT, AVRT.

• Ventricles - This results into ventricular tachycardias or rhythm.

Causes

• Dextrocardia
• Lead switch or misplacement
• Causes of retrograde atrial activation