le(a)d astray…

the case.

a 66 year old male is referred to your ED with recurrent bouts of dizziness over the past 2-3 weeks. His past medical history is significant only for pacemaker insertion (4 years earlier) for symptomatic heart block.

On examination, he appears well & his observations are within normal limits. There is no evidence of congestive cardiac failure.

This is his ECG….

[DDET Describe & interpret his ECG…]

Annotated ECG below for further explanation

• Rate.
  • ~66 /min
  • Atrial pacing at 50 /min.
• Rhythm.
  • Irregular.
    • Atrial pacing [Complexes 1, 5,  7, 9, 10 & 11]
    • Possible native atrial rhythm [Complex 3]
    • Ventricular ectopics [Complexes 2 & 6]
    • Junctional ectopics [Complexes 4 & 8]
• Axis.
  • Normal [-26*] (for normal conducted beat)
• Intervals.
  • PR ~ 120 msec
  • QRS.
    • Complexes 2 & 6 ~ 120  msec.
      • LBBB appearance
    • All remaining complexes ~ 80 msec.
      • Incomplete RBBB appearance
  • QTc ~ 365 msec
• Segments.
  • Essentially isoelectric.
• Other.
  • P-wave morphology varies between paced & native atrial conduction.
  • No evidence of ventricular pacing.
  • Concerning long pause [between beats 4 & 5]
  • ?ventricular undersensing [Beats 5 to 9]; failure to reset atrial rate following ectopic beats.
  • ?atrial failure to capture [Blue circles map out the ongoing 1200msec rate]
    • Suggestion of atrial pacing spike [middle blue circle].

Interpretation.

Initial irregular atrial pacing & subsequent ventricular undersensing is highly suggestive of pacemaker malfunction.

[/DDET]

[DDET What is happening here ?]

Pacemaker malfunction.

This can be separated into three broad categories.

(1) Failure to capture.

Either NO pacemaker spikes or pacemaker spikes WITHOUT associated myocardial depolarisation or capture. Can occur with atrial or ventricular (or both) pacing. Native electrical activity (± escape rhythms) is seen & may occur simultaneously with pacemaker spikes.

CAUSES:

• Lead displacement
  • Most common.
  • Detectable on CXR – displaced tip from RV apex.
  • Lead may have intermittent contact with endocardium resulting in intermittent failure.
• Lead fracture
  • Uncommon.
  • Failure to capture results from current leakage (from break in insulation)
• Battery depletion
  • Initially, ↓ pacing rate followed by falling voltage output below the required threshold.
  • Gradual process.
• Disconnection of lead from generator
• Exit block.
  • The failure of an adequate stimulus to depolarise the paced chamber.
  • Consider diagnosis with functioning generator & intact lead system fails to result in capture.
  • Causes:
    • Ischaemia / infarction
    • Hyperkalaemia
    • Drugs [esp. Class III antiarrhythmics]

 

(2) Inappropriate sensing.

Normally a pacemaker (in non-competitive mode) must sense intrinsic/native electrical activity.

Undersensing.

Typically seen on ECG as a premature pacemaker spike, occurring earlier than programmed. This may or may not be followed by a paced complex (depending on the timing of the atrial or ventricular refractory period). Alternatively, a pacemaker may not detect the native conduction & still attempt to pace simultaneously with native conduction (or shortly afterwards).

Results from a change in sensing parameters (most commonly from RV infarction or fibrosis). Lead displacement, fracture or poor-contact can also cause undersensing.

Oversensing.

Rarely, the pacemaker may detect electrical activity that is not of cardiac origin. The result may be intermittent, irregular pacing or an apparent complete absence of pacemaker function.

This can result from pectoralis myopotentials, T-waves (from intrinsic electrical activity) or surgical electrocautery. It is important to note, that not all abnormal electrical signals (resulting in oversensing) are seen on the actual ECG. There are reported interferences from everyday items such as microwaves & mobile phones which may cause inappropriate pacemaker inhibition.

 

(3) Inappropriate pacemaker rate.

SLOW.

Pacing rate below the programmed rate is a typical finding in generator battery depletion & occurs gradually (not abruptly).

FAST.

An extreme increase in pacing rate (aka. runaway pacemaker) is rare.

Endless-loop tachycardias can develop with dual-chamber pacing & ventriculoatrial conduction. Retrograde atrial depolarisation results in a stimulated (ie. paced) ventricular depolarisation. The ventricular rate should not exceed the programmed upper limit of the pacemaker (typically 110-120 bpm)

[/DDET]

[DDET His initial results return…]

Full blood count & electrolytes unremarkable. (K+ 4.4 mmol/L)

Troponin negative.

Here is his CXR…

[/DDET]

[DDET You decide to review his old CXR…]

[/DDET]

[DDET The case concludes…]

Formal pacemaker check confirms that the ventricular lead is not capturing; however continues to sense atrial activity.

The pacemaker is reprogrammed to an AAI mode [What does this mean ?? Check out OFF THE PACE…].

Our patient is admitted to a monitored bed for pacemaker lead replacement which takes place the following day.

[/DDET]

[DDET References.]

1. Rosenʼs Emergency Medicine. Concepts and Clinical Approach. 7th Edition
2. Tintinalli’s Emergency Medicine: A Comprehensive Study Guide. 7th Edition.
3. Chan TC, Brady WJ, Harrigan RA, Ornato JP, Rosen P. ECG in Emergency Medicine and Acute Care. Elsevier Mosby 2005.
4. Hayes, DL & Vlietstra, RE. Pacemaker malfunction. Ann Intern Med. 1993;119(8):828-835.
5. Pacemaker – ECGpedia
6. Float Nurse – EKG Rhythm Strip Quiz 157
7. KG-EKG Press – Pacemaker Mediated Tachycardia

Special thanks to Adam Lee for his assistance in this ECG interpretation.

[/DDET]