What do PMT (Pacemaker-Mediated Tachycardia) events on a Boston Scientific pacemaker indicate?

PMT Events on Boston Scientific Pacemakers: Clinical Significance and Management

PMT (Pacemaker-Mediated Tachycardia) events on a Boston Scientific pacemaker indicate episodes where the pacemaker has detected and attempted to terminate a form of reentrant tachycardia involving the pacemaker's sensing and pacing functions.

What is Pacemaker-Mediated Tachycardia (PMT)?

PMT is a form of reentrant tachycardia that occurs in patients with dual-chamber pacemakers who have intact ventriculoatrial (VA) conduction. The basic mechanism involves:

1. A ventricular paced beat conducts retrograde to the atrium
2. The pacemaker senses this retrograde P wave
3. This triggers another ventricular paced beat
4. The cycle repeats, creating a "endless loop" tachycardia

Common Triggers for PMT

• Premature ventricular contractions (PVCs) - 16% of cases 1
• Premature atrial contractions (PACs) - 9% of cases 1
• Prolongation of AV delay, particularly with RYTHMIQ™ algorithm - 69% of cases in patients with this feature 1
• Atrial flutter 2
• Loss of atrial sensing
• Electromagnetic interference

Clinical Significance

PMT events recorded by Boston Scientific devices have important clinical implications:

• True PMT episodes (80% in non-AV dependent patients) require intervention to prevent:

  • Symptoms (palpitations, dizziness, shortness of breath)
  • Tachycardia-induced cardiomyopathy with prolonged episodes 3
  • Hemodynamic compromise

• False PMT detection (common during exercise):

  • Occurs in 66-74% of AV block and CRT patients 1
  • Can cause inappropriate algorithm intervention leading to:
    • Non-conducted P waves
    • Loss of cardiac resynchronization therapy
    • Potential pro-arrhythmic events

Diagnostic Approach

When PMT events are detected on device interrogation:

1. Review intracardiac electrograms (EGMs) - More accurate than standard 12-lead ECG 2
2. Analyze pattern: Look for:
   • Ventricular pacing with short RP interval
   • Superiorly directed P waves (retrograde)
   • Changes in ventricular pacing rate causing corresponding changes in atrial rate

Management Algorithm

1. Immediate termination of ongoing PMT:

   • Apply magnet over device (causes temporary asynchronous pacing) 2
   • Use device programmer for overdrive pacing 2

2. Preventive reprogramming:

   • Extend post-ventricular atrial refractory period (PVARP)
   • Adjust AV delay (first-line approach to preserve exercise capacity) 4
   • Enable PMT intervention algorithms
   • In patients with RYTHMIQ™ algorithm, consider deactivation if PMT is recurrent 1

3. For false PMT detections during exercise:

   • Adjust maximum tracking rate
   • Modify PMT detection algorithm sensitivity

Important Caveats

• PMT may be underrecognized in patients with AV synchronous leadless pacemakers due to lower rates 3
• Boston Scientific devices have a high rate of incorrect PMT diagnosis during exercise (66-74% in AV block and CRT patients) 1
• PMT algorithm intervention during exercise can cause loss of CRT and potentially induce ventricular arrhythmias 1
• Patients with single-pass VDD leads may experience a unique form of PMT unrelated to retrograde atrial activation but due to abnormal sensing of terminal QRS forces or T waves 5

Follow-up Recommendations

• Regular device checks to monitor for PMT episodes
• Review of stored EGMs to confirm true vs. false PMT detection
• Reassessment after any reprogramming to ensure resolution
• Patient education about symptoms that might indicate recurrent PMT

PMT events on Boston Scientific pacemakers represent an important diagnostic finding that requires careful evaluation and management to prevent adverse clinical outcomes.