• Ectopic Focus
• 3 Foci and Multifocal Atrial Tachycardia
• Physiological P Wave
• ECG and Multifocal Atrial Tachycardia
• Multifocal Atrial Rhythm
• Heart Rate Calculation (6-Second Rule)
• Differential Diagnosis of Atrial Fibrillation

Ectopic Focus

• Most commonly occurs in a structurally altered atrium
• Never located in the SA node
• Size is up to 5mm
• Generates impulses at a frequency of 130-250/min.
• Precedes the SA node and dictates heart rhythm (overdrive suppression)
• The focus may start generating impulses through 3 mechanisms:
• Increased automaticity
• Trigger activity
• Micro-reentry
• Focal Atrial Tachycardia
• Has 1 focus in the atrium (mechanism is not reentry)
• Intra-Atrial Reentry Tachycardia
• Has 1 focus in the atrium (with a reentry mechanism)

Physiological P Wave

• The atrial vector originates in the SA node and directs
• Away from the aVR lead
• Towards the II lead
• The physiological P wave is
  • Positive in lead II
  • Negative in lead aVR
    
    
• In Multifocal Atrial Tachycardia (MAT), the P wave does not have a physiological shape
• Because the vector does not originate from the SA node

ECG and Multifocal Atrial Tachycardia

• Frequency 100-250/min.
• 3 foci generate impulses independently, the common frequency of impulses is 100-250/min.
• Narrow QRS complexes (< 0.12s)
• QRS complexes may be widened (> 0.12s) in case of aberrant conduction
• Heart rate is irregularly irregular
• As in atrial fibrillation
• 3 P waves of different shapes alternate
• Each ectopic focus creates a different P wave
• PQ interval varies
• Each ectopic focus is at a different distance from the AV node

Multifocal Atrial Tachycardia (MAT)

• Frequency: 280/min.
• 3 P waves of different shapes alternate
• PQ interval varies

Heart Rate Calculation (6-Second Rule)

• If the heart rate is irregularly irregular:
• Multifocal Atrial Tachycardia
• Atrial Fibrillation
• Atrial Flutter (with variable conduction)
• To calculate the heart rate, use the 6-second rule
• This is the average number of QRS complexes over 6 seconds (30 squares)
• Heart Rate = Number of QRS in 6s x 10

Atrial Fibrillation and Heart Rate 130/min.

• Atrial Fibrillation
• No P waves are present
• Heart rate is irregularly irregular
• The number of QRS complexes in 6s (30 squares) is 13
• 13 x 10 = 130/min.

Atrial Fibrillation

• Atrial Fibrillation
• In the atria, there are many micro-reentry circuits
• These circuits generate impulses independently with a frequency of 350-600/min.
• Heart rhythm is irregularly irregular (RR intervals vary in length)
• Fibrillatory f waves (deformed baseline, P waves are absent)
  • P waves cannot be differentiated

Multifocal Atrial Tachycardia

• There are 3 ectopic foci in the atria generating impulses independently
• 3 different P waves are seen on the ECG
• The ECG shows 3 P waves of different shapes (each focus generates its own P wave)
• The heart rhythm is irregularly irregular (similar to atrial fibrillation)
• This is because the ectopic foci generate impulses (P waves) independently of each other
• However, each P wave is followed by a QRS complex

Multifocal Atrial Tachycardia

• Frequency: 110/min.
• The heart rhythm is irregularly irregular
• 3 different P waves (best seen in the continuous II lead)
• Right heart overload
• Right Ventricular Hypertrophy (Dominant R in V1 and deep S in V6)

Atrial Fibrillation

• Frequency: 90/min.
• The heart rhythm is irregularly irregular
• P waves cannot be differentiated
• This is atrial fibrillation (which is often confused with MPT)
• Atrial fibrillation never has P waves