In multifocal atrial tachycardia (MAT), is every P (P wave) followed by a QRS (QRS complex) complex?

Multifocal Atrial Tachycardia: P Wave and QRS Relationship

In multifocal atrial tachycardia (MAT), not every P wave is followed by a QRS complex due to the variable P-R intervals characteristic of this arrhythmia. 1

Electrocardiographic Characteristics of MAT

MAT is defined by specific ECG criteria:

• At least 3 distinct P wave morphologies on the surface ECG
• Irregular atrial rhythm with a rate >100 bpm (or >90 bpm in some definitions)
• Distinct isoelectric period between P waves (unlike atrial fibrillation)
• Variable P-P, P-R, and R-R intervals 1

The variability of the P-R interval is a key diagnostic feature of MAT, which means that the relationship between P waves and QRS complexes is inconsistent. This variability may be related to:

• Decremental conduction through the AV node
• Different origins of the P waves
• Occasional AV block 1

Differentiating MAT from Other Arrhythmias

MAT can be difficult to distinguish from atrial fibrillation, especially on physical examination or a single ECG tracing. Key differences include:

• MAT has distinct P waves with isoelectric periods between them
• Atrial fibrillation has no distinct P waves and no isoelectric baseline 2

When analyzing the ECG:

• First classify the rhythm as regular or irregular
• If irregular, consider MAT, atrial fibrillation, or atrial flutter with variable AV conduction
• Look for ≥3 distinct P wave morphologies to confirm MAT 1

Pathophysiology and Conduction

The mechanism of MAT is not well established, but:

• It likely involves triggered activity (abnormal automaticity)
• The variability of P-R intervals suggests inconsistent conduction through the AV node
• This inconsistent conduction means some P waves may not conduct to the ventricles 1, 2

Clinical Context and Management

MAT is commonly associated with:

• Pulmonary disease and pulmonary hypertension
• Coronary and valvular heart disease
• Electrolyte abnormalities (particularly hypomagnesemia)
• Theophylline therapy 1

Management should focus on:

1. Treating underlying conditions
2. Correcting electrolyte abnormalities
3. Considering intravenous metoprolol or verapamil for acute rate control in hemodynamically stable patients 1, 3

Important Pitfalls

• Misdiagnosing MAT as atrial fibrillation can lead to inappropriate treatment strategies
• Cardioversion is not effective for MAT and should not be attempted 1
• Beta blockers should be used cautiously in patients with severe pulmonary disease or bronchospasm
• Always obtain a 12-lead ECG to confirm the diagnosis, as MAT cannot be reliably diagnosed from a rhythm strip alone 1, 4

In conclusion, the variable P-R intervals in MAT mean that not every P wave will be followed by a QRS complex, which is an important diagnostic feature that helps distinguish it from other supraventricular tachycardias.