Normal atrial activation

ECG: Sinus rhythm with a rate of 56 bpm; normal P-wave: positive in leads I, II, V5, V6 and negative in aVR; normal P-wave axis (54°); normal P-wave duration (80 ms); no increase in P-wave voltages; absence of P-wave morphology abnormalities (notched, biphasic, etc.); the remaining ECG also within normal limits;

Comments: This tracing shows an example of a strictly normal P-wave. Atrial activation is triggered in response to the depolarization of the sinus node and spreads radially and rapidly through the adjacent right atrial myocardium. Atrial activation totally differs from ventricular activation due to the thinness of the walls and the absence of specific conductive tissue or Purkinje network in the atria. The subendocardial and subepicardial layers are activated in parallel.

There is a gap between the beginning and end of right atrial activation and that of the left atrium:

• right atrial activation begins upon sinus discharge and lasts on average 70 ms in adults
• left atrial activation begins 30 ms after activation of the right atrium and has a broadly equivalent duration (approximately 60 ms)
• overall atrial activation therefore lasts 90 ms on average in adults

The P-wave, the electrocardiographic reflection of atrial activation, typically has a rounded shape and can be divided into three phases:

1. the initial slope of the P-wave corresponds to the onset of right atrial activation; the progression of the activation originates at the sinoatrial node and travels downward and forward;
2. its peak corresponds to the end of right atrial activation, activation of the atrial septum and activation of the upper and posterior aspects of the left atrium (biatrial phase);
3.  its terminal slope corresponds to the end of left atrial activation (activation of the inferior aspects of the left atrium and left atrial appendage); the direction of activation of the left atrium is therefore to the left and slightly backward;

Right atrial activity is essential directed downward such that this activity is better recorded in the frontal plane (limb leads). Left atrial activity is mainly directed toward the left and thus better projected in the horizontal plane (precordial leads) than in the frontal plane. The overall activation of the two atria resulting from both right and left vectors is directed downwards and to the left slightly forward. The morphology and axis of the P-wave reflect the various phases of this activation. The direction of the P-wave in the frontal plane does not vary substantially with age and is between 0 and 80° (P-wave axis).  Right atrial activation propagates in the direction of leads II and V1; left atrial activation also propagates in the direction of lead II but in the opposite direction of V1. This explains the biphasic P-wave pattern commonly observed in V1: the initial component is positive (right atrial activity) and the second component negative (left atrial activity). The P-wave is always positive from V3 to V6. The atria, given their anatomical position, are in proximity of leads V1 and V2; P-waves are frequently more prominent in these leads than in leads V3-V6 which are usually of low amplitude, under 0.2 mV.

The normal P-wave is therefore:

• always positive in leads I and II
• always negative in aVR
• variable in lead III, either positive, negative, biphasic or isoelectric
• most often positive in aVL, but may be negative
• almost always positive in aVF
• often biphasic in V1 (positive component followed by a negative component)
• always positive from V3 to V6

In lean subjects, the electrical axis of P is directed downward and more vertically such that the P-wave is taller in leads II, III and aVF and is smaller in lead I. In contrast, in short stocky patients, the mean electrical axis of P is near 0°: the amplitude of the P-wave tends to be greater than usual in lead I whereas in lead III, it can be biphasic or inverted.

If the “P-wave” appears negative in lead I, the highest probability is a reversal of the right arm/left arm electrodes; other possibilities include a dextrocardia or an ectopic atrial rhythm originating from the left atrium.

The normal duration of the P-wave varies between 60 and 110 ms (tendency toward a moderate prolongation of the P-wave with age), with a mean value of 80 ms. Its pattern is typically rounded with no possibility of differentiating between right atrial activation from left atrial activation. Its amplitude is generally maximal at lead II (90% of cases) and does not exceed 2.5 mm in limb leads.

While the P-wave reflects atrial depolarization, atrial repolarization is invisible on normal tracings being masked by the QRS-complex and is of very low amplitude. It only becomes visible if its size is increased or in the presence of an atrioventricular dissociation, and corresponds to an offset relative to the flatline, in the opposite direction of the P-wave. A marked repolarization may lead to a false diagnosis of ST segment depression when the rhythm is rapid (sinus tachycardia).

When the atrial myocardium is diseased or scarred, the morphology of the P-wave may be changed with evidence of notching or slurring, depending on the leads, highlighting the altered activation. The duration of the P-wave can also be increased by the presence of conduction blocks delaying and prolonging atrial activation.

The electrocardiographic expression of atrial enlargement stems from the mode of activation of the atria. As seen above, the P-wave is comprised of an exclusively right atrial initial component, a middle biatrial component and an exclusively left atrial terminal component. Right atrial enlargement tends to increase the initial voltage of the P-wave without lengthening the duration, while left atrial enlargement tends to prolong the duration of the P-wave and to dissociate the left and right atrial action potential by increasing the voltage of the terminal portion of the P-wave.

Take-home message: The normal P-wave is positive in leads I, II, V4 to V6, negative in aVR and often biphasic in V1 (occasionally V2); the duration of the P-wave does not exceed 110 ms (standard between 80 and 110 ms); the amplitude of the P-wave is less than 2.5 mm in limb leads, the positive deflection is less than 1.5 mm in V1 while the negative deflection is less 1 mm in V1.