CRAT Pacemaker Rhythm Analysis 2 — Questions and Answers

Question 1: A pacemaker spike followed by a wide, abnormal QRS complex indicates what type of pacing?

Ventricular pacing with successful capture (Correct answer)
Atrial pacing with successful capture
Failure to capture
Oversensing

Correct answer: Ventricular pacing with successful capture

A pacemaker spike immediately followed by a wide QRS complex confirms ventricular pacing with successful capture — the pacemaker stimulus has successfully depolarized the ventricular myocardium.

Ventricular pacing produces wide QRS complexes (> 120 ms) resembling left or right bundle branch block because the activation spreads from the pacing electrode through myocardium rather than the normal His-Purkinje system. A spike without a following QRS indicates failure to capture. The spike itself has no hemodynamic effect — only the subsequent myocardial depolarization produces contraction.

Question 2: Pacemaker failure to capture is identified by which ECG finding?

Pacemaker spikes visible without subsequent QRS complexes (Correct answer)
Pacemaker spikes followed by wide QRS complexes
Absence of pacemaker spikes despite slow heart rate
Pacemaker spikes occurring at twice the programmed rate

Correct answer: Pacemaker spikes visible without subsequent QRS complexes

Failure to capture occurs when the pacemaker fires (spike visible) but the electrical stimulus fails to depolarize the myocardium, so no QRS complex follows the spike.

Failure to capture can result from lead dislodgement, lead fracture, elevated pacing threshold (due to scar tissue, drugs, or electrolyte disturbances), insufficient pacing output, or battery depletion. Increasing the pacing output, checking lead integrity, and treating reversible causes are management steps. If symptomatic bradycardia persists, temporary transcutaneous pacing may be required.

Question 3: Pacemaker oversensing causes which problem?

Inappropriate inhibition of pacing pulses due to sensing signals that are not cardiac in origin (Correct answer)
Failure to sense intrinsic cardiac activity
Pacing at a rate faster than programmed
Pacing spikes that do not produce QRS complexes

Correct answer: Inappropriate inhibition of pacing pulses due to sensing signals that are not cardiac in origin

Oversensing occurs when the pacemaker incorrectly interprets non-cardiac signals (muscle potentials, electromagnetic interference) as intrinsic cardiac activity, inhibiting appropriate pacing and causing bradycardia.

Oversensing causes the pacemaker to be inappropriately inhibited when it detects non-cardiac signals such as skeletal muscle potentials (pectoral muscle near the lead), electromagnetic interference from appliances, or T wave sensing. The result is absent pacing pulses when they should be delivered. In pacemaker-dependent patients, oversensing can cause severe symptomatic bradycardia or asystole.

Question 4: The NBG (NASPE/BPEG) pacemaker code uses letters to describe pacemaker function. In a VVI pacemaker, what does the second 'V' indicate?

The ventricle is the sensed chamber (Correct answer)
The ventricle is the paced chamber
The mode is inhibited
The device is rate-responsive

Correct answer: The ventricle is the sensed chamber

In the NBG code, the first letter indicates the paced chamber, the second letter indicates the sensed chamber. In VVI, the second V means the ventricle is the sensed chamber.

The NBG pacemaker code: Position 1 (chamber paced): V=ventricle, A=atrium, D=dual. Position 2 (chamber sensed): V=ventricle, A=atrium, D=dual, O=none. Position 3 (response to sensing): I=inhibited, T=triggered, D=dual, O=none. Position 4: R=rate responsive. VVI = ventricle paced, ventricle sensed, inhibited by sensed ventricular activity — a common single-chamber pacemaker mode.

Question 5: In a DDD pacemaker, what does the first 'D' represent?

Both atrium and ventricle are paced (dual chamber pacing) (Correct answer)
Only the atrium is paced
Only the ventricle is paced
The device is in triggered mode

Correct answer: Both atrium and ventricle are paced (dual chamber pacing)

In DDD mode, the first D indicates that both the atrium and ventricle can be paced (dual chamber pacing), making it the most physiologic pacemaker mode.

DDD (Dual-Dual-Dual) is the most physiologic pacing mode: both chambers can be paced, both can be sensed, and it has a dual response (inhibited by sensed atrial activity AND triggered to pace the ventricle after sensed atrial activity). It maintains AV synchrony, which is important for optimizing cardiac output. DDD is preferred for patients with AV block who have intact sinus node function.

Question 6: Pacemaker undersensing is identified by which ECG pattern?

Pacemaker spikes firing at inappropriate times during or after intrinsic cardiac beats (Correct answer)
Complete absence of pacemaker spikes
Pacemaker spikes only during tachycardia
Pacemaker spikes exactly timed to the R wave

Correct answer: Pacemaker spikes firing at inappropriate times during or after intrinsic cardiac beats

Undersensing means the pacemaker fails to detect intrinsic cardiac activity, causing it to fire at its programmed interval even when the heart has already depolarized spontaneously.

In undersensing, the pacemaker does not detect the patient's own QRS complexes or P waves, so it fires at its set rate regardless of spontaneous cardiac activity. This produces pacemaker spikes appearing on top of or immediately after intrinsic beats. If a spike falls on the T wave (vulnerable period), it could theoretically trigger VF (R-on-T phenomenon), though this is rare with modern pacemakers.

CRAT Pacemaker Rhythm Analysis 2

A pacemaker spike followed by a wide, abnormal QRS complex indicates what type of pacing?