CCT Pacemaker Rhythm Interpretation 2 — Questions and Answers

Question 1: On an ECG, what is the characteristic appearance of a pacemaker spike followed by a wide QRS complex?

A thin vertical spike immediately followed by a broad, LBBB-morphology QRS greater than 0.12 seconds (Correct answer)
A narrow P wave followed by a normal-width QRS
A tall peaked T wave without a preceding spike
A broad, notched P wave before a narrow QRS

Correct answer: A thin vertical spike immediately followed by a broad, LBBB-morphology QRS greater than 0.12 seconds

Ventricular paced beats produce a sharp spike (the pacemaker stimulus artifact) immediately followed by a wide QRS complex resembling LBBB morphology, because ventricular activation spreads cell-to-cell rather than via the His-Purkinje system.

A ventricular pacemaker lead (typically placed in the right ventricular apex) delivers a stimulus that depolarizes the ventricular myocardium directly, bypassing the specialized conduction system. Because impulse propagation is slower through myocardium than the Purkinje network, the resulting QRS complex is wide (>0.12 seconds) and has a LBBB morphology in most standard leads (since activation starts in the right ventricle and spreads leftward). The preceding pacemaker spike is a narrow, near-vertical deflection that may be large or small depending on the pacing output and lead configuration.

Question 2: What is pacemaker failure to capture?

The pacemaker fires (spike visible) but does not depolarize the myocardium (no QRS follows) (Correct answer)
The pacemaker does not fire at all, producing no spikes
The pacemaker fires too slowly, below the programmed rate
The pacemaker senses intrinsic beats correctly but does not pace

Correct answer: The pacemaker fires (spike visible) but does not depolarize the myocardium (no QRS follows)

Failure to capture occurs when a pacemaker spike is visible on the ECG but is not followed by a QRS complex (or P wave for atrial pacing), meaning the electrical stimulus failed to depolarize the myocardium.

Failure to capture means the pacemaker spike reaches the myocardium but the stimulus is insufficient to trigger depolarization. Causes include: lead dislodgement, lead fracture, high pacing threshold (e.g., after MI, electrolyte imbalance, medication effects), inadequate pacing output setting, or battery depletion. On the ECG, you see pacing spikes occurring at the expected intervals, but the myocardium does not respond — there is no QRS following the atrial spike or no broad QRS following the ventricular spike. This is a potentially life-threatening malfunction in pacemaker-dependent patients.

Question 3: What does the middle letter in a three-letter pacemaker code represent? (e.g., DDD, VVI, VDD)

The chamber sensed by the pacemaker (Correct answer)
The chamber paced
The response to sensing
The programmability level

Correct answer: The chamber sensed by the pacemaker

In the NBG pacemaker code (first three letters), the first letter indicates the chamber paced, the second letter indicates the chamber sensed, and the third letter indicates the response to sensing (I=inhibited, T=triggered, D=dual).

The NBG (NASPE/BPEG Generic) pacemaker code is the international standard for describing pacemaker function. The first letter: chamber(s) paced (A=atrium, V=ventricle, D=both, O=none). The second letter: chamber(s) sensed (A, V, D, O). The third letter: response to sensing (I=inhibited — pacing inhibited when intrinsic beat sensed; T=triggered — pacing triggered by sensed event; D=dual for dual-chamber devices; O=none). Example: VVI = ventricular pacing, ventricular sensing, inhibited response. DDD = dual chamber paced, dual sensed, dual response (most physiologic mode).

Question 4: A patient with a DDD pacemaker shows pacing spikes before both P waves and QRS complexes on the ECG. What pacemaker mode is currently functioning?

AV sequential (DDD) pacing — both atrium and ventricle are being paced (Correct answer)
VVI pacing — only the ventricle is being paced
AAI pacing — only the atrium is being paced
The pacemaker is malfunctioning

Correct answer: AV sequential (DDD) pacing — both atrium and ventricle are being paced

Visible pacing spikes before both P waves and QRS complexes confirms that both the atrium and ventricle are being paced sequentially (DDD or DOO mode), which is the most physiologic form of pacing.

In DDD pacing, the device monitors intrinsic atrial and ventricular activity. When both chambers require pacing (no intrinsic activity detected), the device fires an atrial spike, waits the programmed AV interval, then fires a ventricular spike. This maintains AV synchrony (atrial kick) and is the most hemodynamically beneficial pacing mode. AV sequential pacing is identifiable on ECG by a spike before the P wave (atrial pacing) and a second spike before a wide QRS (ventricular pacing), with a fixed paced AV interval between them.

Question 5: What is undersensing in a pacemaker, and what ECG pattern does it produce?

The pacemaker fails to detect intrinsic cardiac activity and fires at inappropriate times, producing spikes that may fall on T waves (Correct answer)
The pacemaker fires too rapidly, exceeding the programmed rate
The pacemaker cannot capture the myocardium despite sensing correctly
The pacemaker fires at a rate slower than programmed

Correct answer: The pacemaker fails to detect intrinsic cardiac activity and fires at inappropriate times, producing spikes that may fall on T waves

Undersensing means the pacemaker's sensing circuit fails to detect the patient's own heartbeats, so it fires according to its escape interval as if no intrinsic beat occurred, potentially producing spikes falling on T waves (R-on-T phenomenon).

In undersensing, the pacemaker is 'blind' to intrinsic depolarizations — it fires at its programmed escape interval regardless of whether a native beat has occurred. On ECG, this appears as inappropriately timed spikes: spikes too close to or on top of intrinsic QRS complexes, or even spikes landing on T waves (R-on-T). An R-on-T spike from undersensing can theoretically trigger ventricular fibrillation in vulnerable patients. Causes include: programming issue (sensitivity set too low), lead dislodgement, lead fracture, or signal too small for the device to detect. Compare with oversensing (pacemaker sees noise as real beats and is inappropriately inhibited).

Question 6: Which pacemaker code indicates rate-responsive pacing?

A fifth letter 'R' added after the standard three-letter code, as in DDDR or VVIR (Correct answer)
A capital 'S' after the first letter
A number after the three-letter code indicating the sensor type
The letter 'P' added as a prefix before the standard code

Correct answer: A fifth letter 'R' added after the standard three-letter code, as in DDDR or VVIR

Rate-responsive pacing is indicated by the letter 'R' as the fourth position in the NBG code (e.g., DDDR, VVIR, AAIR), indicating the pacemaker can vary its rate in response to physiologic demand (activity, minute ventilation, etc.).

Rate-responsive pacemakers use sensors (accelerometer for motion, impedance-based minute ventilation sensor, or others) to increase pacing rate during physical activity or metabolic demand, mimicking the normal chronotropic response. The letter R in the fourth position of the NBG code indicates this feature. For example, VVIR = ventricular pacing, ventricular sensing, inhibited, rate-responsive. This mode is particularly important for patients with chronotropic incompetence (inability to raise heart rate with exercise) due to SA node disease or AV node disease.

CCT Pacemaker Rhythm Interpretation 2

On an ECG, what is the characteristic appearance of a pacemaker spike followed by a wide QRS complex?