P-Wave Abnormalities: Measurement

P Wave

P wave is the first wave in ECG. It represents Atrial depolarization. It is the first positive wave, i.e., above the isoelectric baseline. P wave has 2 halves.

• The first half is seen due to Right atrial depolarization.
• The second half is due to Left atrial depolarization.

Relationship between Six Chest Leads and Heart

• Electrical activity is towards the electrode: Positive complex.
• Electrical activity is away from the electrode: Negative complex.
• Lead V1
  • Placed in right side parasternal area.
  • Picks electrical activity of the right atrium directly.
  • The electrical activity of the right atrium is towards lead V1.
    • Positive complex 
  • Depolarization of left atrial activity, slightly away from lead V1
    • Negative complex
  • Biphasic
• Progression from V1→V6 is Monophasic.
• The P wave is Negative in Lead aVR
  • aVR is the Augmentation of the Right arm.
• Electrical activity moves away from aVR - Negative P wave.

Atrial Repolarization 

• Wave of Atrial Repolarization has a small amplitude. 
• Small amplitude is Masked by the QRS complex. 
• In Precordial leads, i.e., V1-V6, Amplitude is <1.5 mm.
• In chest leads, P wave amplitude is < 1.5 mm.
• Duration is <120 msec.

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Abnormalities of P wave

1. Tricuspid Stenosis

ECG findings in Tricuspid Stenosis

• Lead II is considered a rhythm strip.
• P wave is first seen in Lead II.
• P wave amplitude is increased (>2.5 mm) and appears like a peak
• This P wave is called p pulmonale.
• P pulmonale is seen in patients with tricuspid stenosis
• There is right atrial hypertrophy, so depolarization activity is greater hence, the amplitude of the P wave of the first half is greater.

2. Right atrial abnormality

• In right atrial enlargement, the first half of the p wave is affected.
• But the duration remains the same <120 msec). 
• Lead V1 has a biphasic P wave. 

ECG of Right Atrial Abnormality in lead V1

• In right atrial abnormality, first half of P wave in V1 lead is changed.
• The amplitude of first half of P wave is > 1.5 mm.

Criteria for right atrial hypertrophy/abnormality 

• P pulmonale
  • Amplitude > 2.5 mm in limb leads. 
  • Voltage > 0.25 mV
• Initial part of P wave in lead v1
  • > 1.5 mm  
• Rightward shift of P wave
  • Axis - + 75° 
• The area under the initial positive portion of the P wave in lead V1 is >0.06 mm/sec.

Conditions that cause right atrial hypertrophy                                        

• Cor pulmonale 
• PAH (pulmonary arterial hypertension) 
• TOF and pulmonic stenosis in right to left-sided shunts.
• Tricuspid stenosis

1. Left atrial abnormalities

• Inou balloon is used in the treatment of left atrial hypertrophy.

• In the case of P mitrale
  • Duration - >120 msec 
  • Amplitude is same 
• P wave to be called P mitrale or in case of left atrial hypertrophy, the duration between 2 peaks of M - > 40 msec. 

Morphology of P wave in left atrial abnormality in Lead V1

• Prolonged P wave duration: >120 msec in lead ll.
• Distance between two peaks: > 40 msec.
• 2^nd half of the P wave represents left atrial abnormality. 
• Duration of negative component in lead l: > 40 msecs and Depth of negative component in lead l: >1-1.5 mm 
• The leftward shift of p wave: -30 to -45
• The negative component of the P wave is deep as well as wide, called the P terminal force.
• These criteria are considered for left atrial hypertrophy.

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Measurement of P wave: Morris index and Macruz index

• Morris index:
  • The negative component of the P wave in lead V1
    • Duration: > 40 m sec
    • Depth: > 2 mm
    • Suggests LT atrial hypertrophy.
• Macruz index:
  • Measurement of the P/PR segment

Atrial flutter

• Narrow complex tachycardia (normal QRS complex).

ECG features of Atrial flutter

• P Waves - saw-toothed pattern (flutter waves) in lead V1.
• Normal QRS complex
• Atrial rate ~300 BPM
• Loss of isoelectric baseline.

Treatment for atrial flutter

• Control the heart rate.
  • Beta-blockers
  • Non-dihydropyridine calcium channel blockers
    • Verapamil
    • Diltiazem
• Drugs to control the rhythm.
  • Class lll anti-antiarrhythmic drugs
    • Ibutilide
    • Dofetilide
• In lead ll, only one P Wave is abnormal, which appears earlier than normal.
• The morphology of the abnormal P wave is different from the morphology of another sinus P wave.
• This indicates that depolarization is not from the SA node but from atrial extrasystole.
• In atrial premature complexes
  • Abnormal P Waves have different morphology and appear earlier than normal.

Causes of Atrial Flutter

• Excessive caffeine intake
• Alcohol consumption
• Stress
• Fatigue
• Poor sleep

Atrial fibrillation

• In lead II
  • Variable RR intervals (irregularly irregular rhythm).
  • P wave is absent in some of the sinus rhythms (fibrillatory waves)

Supraventricular tachycardia

• P wave is almost merged with QRS complex or can occasionally appears after QRS complex.
• Seen in AV nodal reentrant tachycardia (AVNRT) - a type of supraventricular tachycardia.

Pseudo P pulmonale

• If amplitude is > 2.5 mm - seen in patients with tricuspid stenosis
• In the absence of tricuspid stenosis, if amplitude is >2.5 mm then it is considered as pseudo P pulmonale.
  • It is seen in case of hypokalemia. 

ECG Abnormalities in Hypokalemia

• Appearance of U wave
• T wave get flattened/inverted
• Prolonged QT interval
• Pseudo P pulmonale

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Frequently Asked Questions

Q. In which of the following leads P is biphasic?

A Lead I

B. Lead III

C. Lead aVR

D. Lead V1

Answer. D. Lead V1

• Above the isoelectric line: Right atrial depolarization.
• Below the isoelectric line: Left atrial depolarization.

Q. Normal P wave axis?

A. Approximately 90 degrees

B. Approximately 60 degrees

C. Approximately 120 degrees

D. Approximately 30 degrees

Answer. B. Approximately 60 degrees

Q. Amplitude of Atrial Repolarization?

A <50 mV

B. <100 mV

C. <1 mV

D. <1 volt

Answer. B. <100 mV

• Small amplitude is masked by the QRS complex. 

Q. Amplitude and duration of normal P wave in limb leads?

A. <1.5 mm & <120 msec

B. <2.5 mm & <120 msec

C. <2.0 mm & <210 msec

D. <3.0 mm & <110 msec

Answer. B. <2.5 mm & <120 msec

• In Precordial leads, i.e., V1-V6, Amplitude is <1.5 mm.

Q: You see a 57-year-old woman who presents with pedal edema, and on examination, there is pedal edema and tender hepatosplenomegaly. What is the diagnosis? 

1. Mitral stenosis
2. Tricuspid stenosis
3. Aortic stenosis
4. Mitral regurgitation

Answer: B. Tricuspid stenosis 

• The presence of pedal edema and tender hepatosplenomegaly are features of right-sided heart abnormalities.

Q: Following are causes of P pulmonale except

1. PAH (pulmonary arterial hypertension) 
2. TOF and pulmonic stenosis in right to left-sided shunts
3. Tricuspid stenosis
4. Aortic regurgitation

Answer: 4. Aortic regurgitation

Q: What should this patient undergo?

1. Rx with adrenaline
2. Surgery - PBMV
3. Rx with digoxin
4. Surgery - CABG

Answer: B. Surgery - PBMV 

Explanation

• M-shaped P wave - P mitrale 
• P mitrale seen in mitral stenosis
• In mitral stenosis, left atrial hypertrophy is seen

Q: What is the Macruz index for Mitral stenosis?

1. 1
2. 1.6
3. >1.6
4. <1

Answer: C. >1.6

Q: Rhythm?

1. Normal sinus rhythm
2. Ventricular rhythm
3. Irregular rhythm
4. Accelerated junctional rhythm.

Answer: D. Accelerated junctional rhythm

Q: Rhythm in ECG is best assessed in? 

1. Lead ll
2. Lead lll
3. Lead l
4. Lead aVR

Ans: A. Lead ll 

Explanation

• Lead ll is considered a rhythm strip. 
• Hence lead ll is used for assessing all the waves.

Q: Comment on the ECG findings.

1. Atrial fibrillation
2. Atrial premature complexes
3. PSVT
4. WPW

Ans: B. Atrial premature complexes

Q: Pseudo P pulmonale is seen in?

1. Hypokalemia
2. Hypocalcemia
3. Hyperkalemia
4. Hypomagnesemia

Ans: A. Hypokalemia

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