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P Wave

P wave

Limb Leads and the P Wave

Limb Leads and the P Wave

• Limb leads (I, II, III, aVF, aVR, aVL) view the heart in the frontal plane
• Summation (P1 + P2) atrial vector points from the SA node to the AV node, directed
• From lead aVR
• To lead II
• The more direct the vector to the lead
  • the more positive the wave
• Summation atrial vector
• directs straight to lead II (hence the P wave is best seen in lead II)
• directs away from lead aVR (hence the P wave is negative in lead aVF)

Precordial Leads and the P Wave

• The heart is rotated leftward in the horizontal plane
• The right side of the heart is oriented forward
• The left side of the heart is oriented backward
• The anatomical axis of the heart does not correspond to the electrical axis on the ECG
• The size of the vector on the ECG is directly proportional to the muscle mass in which the vector originates
• The thin atrial myocardium creates a small P wave
• The massive ventricular myocardium creates a large QRS complex

Precordial Leads and the P Wave

• Precordial leads (V1-V6) view the heart in the horizontal plane
• Precordial lead V1 views the atria from the front. The atria depolarize sequentially:
• First, the right atrium depolarizes (P1) from the SA node
• The vector of the right atrium points towards lead V1
• Then, the left atrium depolarizes (P2) from the SA node through the Bachmann's bundle
• The vector of the left atrium points away from lead V1
  
  

ECG and the P Wave

• The P wave is evaluated in leads II and V1
  • because they provide the best view of the atria
• Morphology
• Monophasic in lead II (positive)
• Biphasic in lead V1 (positive-negative)
• Frontal axis (0 - 75°)
• P wave is positive in I and II
• P wave is negative in aVR
• Duration
• < 110ms (max. 3 small squares)
• Atria depolarize within 110ms
• Amplitude
• < 2.5mm in limb leads
• < 1.5mm in chest leads

Normal P Wave and Sinus Rhythm

• Sinus rhythm with frequency of 75/min.
• P wave in lead II
• Is positive and monophasic
• Width < 110ms ( < 3 small squares)
• Amplitude < 2.5mm ( < 2.5 small squares)
• P wave in lead V1
• Biphasic (may merge with the isoelectric line)
• Amplitude < 1.5mm ( < 1.5 small squares)
• Negative wave area < 1 small square

P Pulmonale (Right Atrium Hypertrophy)

• Amplitude of P wave > 2.5mm (in lead II)
• The patient has COPD and pulmonary hypertension
• pulmonary hypertension is the most common cause of right ventricular hypertrophy and right atrial hypertrophy
• Therefore, right atrial hypertrophy on ECG is referred to as P Pulmonale

Retrograde (Negative) P Wave

• Ectopic atrial rhythm with a frequency of 60/min.
• In ectopic atrial rhythm
• Impulses originate in an ectopic focus in the atria
• Not in the AV node as in junctional rhythm
• The ectopic focus is located near the AV node in this case
• Because P wave is retrograde in lead II
• This is a similar case to above
• However, in this case the PQ interval is > 120ms, and the P wave is less deep
• Thus, the atrial vector does not exactly point from lead II

Different P Waves

• On the ECG, there is a multifocal atrial tachycardia, with a frequency > 100/min.
• The principle is exactly as described on the previous ECG
• except that it is a tachycardia (frequency > 100/min)
• On the ECG, there are 4 (or possibly 5) P waves of different shapes
• each ectopic focus has its own P wave