Ans. a. 2.5 mm (Ref: Harrison 19/e p1453, 18/e p1832; Marc Gertsch's The ECG; A Two-Step Approach to Diagnosis';Ch-4)The right atrium chronic overload is indicated by a P wave of more than 2.5 mm."Normal atrial depolarization vector is oriented downward and towards the subject's left, reflecting the spread of depolarization from the sinus node to the right and then the left atrial myocardium. This vector points toward the positive pole of lead II and toward the negative pole of lead aVR. Normal P wave is less than 2.5 mm in lead II. If it is more than 2.5 mm, this indicate right atrial overload."- Harrison 19/e p1453, 18/e p1832Right Atrial Enlargement (P Pulmonale): Peaked, tall and narrow P waves in leads II, III and aVF in lung disease are called 'P pulmonale'. The presence of P pulmonale is taken as P wave of more than 2.5 mm in leads, II, III and aVF. Though many factors such as verticalization of the heart, increased residual volume and impaired lung function have been reported to be correlated with these changes in the P wave, right atrial overload (right atrial enlargement or increased right atrial pressure or both) is still thought to be responsible for P pulmonale in lung disease."- Maeda S, Katsura H. Chida K. Imai T. Kuboki K. Watanabe C, Kida K. Ohkawa S, Matsushita S, Ueda K and Kuramoto K. Lack of correlation between P pulmonale and right atrial overload in chronic obstructive airways disease. Br Heart J. 1991; 65:132-6."Right Atrial Enlargement (P Pulmonale): As the activation of the right atrium begins earlier than that of the left atrium, an increase of the right atrial (RA) vector does not prolong the P duration. P pulmonale is characterized by tall and sometimes pointed P waves in leads II, aVF and III. The amplitude of the P waves is 2.5 mm in at least one of these leads. In the opposite 'lead aVL, the P wave is completely negative. The P wave may be higher than normal and pointed in leads V1/V2 rarely in the other precordial leads."- Marc Gertsch's 'The ECG: A Two-Step Approach to Diagnosis';- Ch-4