Blocked PACs

When a PAC fails to cause ventricular depolarization, it is known as a blocked or nontransmitted PAC. On some occasions, the transmission of an atrial impulse from a PAC can be blocked at the AV node. As you can imagine, blocked PACs can be very difficult to diagnose. Once again, careful evaluation of the waves of a complex preceding a pause may yield the diagnosis. (See Figures 13-29 to 13-31.)

A rhythm strip from lead 2 illustrates a blocked P wave.

Figure 13-29 Blocked P wave. In this example, the T wave and P wave have fused into one larger combination wave. Notice the pause that follows due to the lack of ventricular depolarization.

From Arrhythmia Recognition: The Art of Interpretation, courtesy of Tomas B. Garcia, MD.

Description
A rhythm strip from lead 2 shows a short and curved peak of the P wave, a sharp dip of the QRS complex, and a broad peak of the T wave. One of the P and T waves are fused forming a double hump.

Figure 13-30 Blocked P wave. This strip shows a distinct double-humped quality to the labeled T wave. Evaluation of the subsequent pause leads us to take a closer look at the T wave. Notice how that T wave is different from the others. This is a combination of T and P wave due to a blocked PAC.

From Arrhythmia Recognition: The Art of Interpretation, courtesy of Tomas B. Garcia, MD.

A rhythm strip from lead 2 shows a short and broad peak of the P wave, a sharp dip of the QRS complex, and a broad peak of the T wave. One of the P and T waves are fused forming a larger complex.

Figure 13-31 Blocked P wave. Here we clearly see a pause with a morphologically different T wave preceding it. This morphologic difference is due to an inverted P wave. The PAC is nonconducted.

From Arrhythmia Recognition: The Art of Interpretation, courtesy of Tomas B. Garcia, MD.

Blocked PACs, if frequent, may lead to hemodynamic compromise. Careful monitoring and removal and treatment of the offending cause is critical in these cases. In extreme cases, transcutaneous and transvenous pacing may be needed if the hemodynamic compromise cannot be controlled.

ARRHYTHMIA RECOGNITION

Premature Atrial Contractions

Rate: Single complexes
Regularity: Regular with an event
P wave:

Morphology:

Upright in II, III, and aVF:

 Present

Different in event

Variable in event
P:QRS ratio: 1:1
PR interval: Different in event
QRS width: Normal
Grouping: None
Dropped beats: None

DIFFERENTIAL DIAGNOSIS

Premature Atrial Contractions

  1. Idiopathic and benign
  2. Anxiety
  3. Fatigue
  4. Drugs: Nicotine, alcohol, caffeine, etc.
  5. Enlarged atria
  6. Heart disease
  7. Electrolyte disorders

This list is not inclusive, since the causes of PACs are extensive. Normally, PACs are benign and cause no hemodynamic compromise. However, hemodynamic compromise can rarely occur.