8.1 Paced rhythms & pacemaker malfunctions

Why patients are paced

An implanted pacemaker is used when the heart's own conduction system is too slow or unreliable — symptomatic sinus bradycardia, sinus arrest, high-grade or complete AV block, and chronotropic incompetence are the classic indications. For the rhythm technician the clinical reason matters less than one core skill: reading a paced strip correctly and deciding whether the device is working. Devices are described with the NBG code, in which the first letter is the chamber paced, the second the chamber sensed, and the third the response to sensing (I = inhibited, T = triggered, D = dual). A VVI device paces and senses the ventricle and is inhibited by intrinsic beats; a DDD device paces and senses both chambers. You do not program these devices, but knowing that "DDD" means dual-chamber helps you predict how many spikes to expect on the strip.

The pacer spike and capture

Every time the generator fires it prints a pacer spike — a narrow, near-vertical line, often of high amplitude, that is unrelated to the normal P-QRS-T waveform. A spike proves only that the device discharged; it does not prove the heart responded. Capture is the term for a spike that successfully depolarizes its target chamber.

• Atrial capture: a spike is immediately followed by a P wave.
• Ventricular capture: a spike is immediately followed by a wide QRS (0.12 s or wider), because the impulse spreads muscle-to-muscle rather than through the His-Purkinje system. The paced ventricular beat typically resembles a left bundle branch block, with a T wave pointing opposite the QRS.

Judging capture is simply asking, spike by spike, "did a complex follow?"

Identifying the type of paced rhythm

Atrial pacing places the spike just before each P wave; the ventricle is usually depolarized by the patient's own conduction, so that QRS stays narrow. Ventricular pacing places the spike just before each wide QRS. AV-sequential (dual-chamber) pacing produces two spikes per cycle — an atrial spike, then after a programmed AV delay a ventricular spike — reproducing normal AV timing. Biventricular pacing (CRT, cardiac resynchronization therapy) stimulates the right and left ventricle together to re-coordinate contraction in heart-failure patients who have a wide QRS; success is signaled by a paced QRS that is narrower than the patient's underlying bundle-branch-block beat.

Fusion and pseudofusion beats

Two normal findings are commonly mistaken for malfunction, and the exam likes to test the difference. A fusion beat occurs when a paced ventricular stimulus and the patient's own intrinsic beat depolarize the ventricle at nearly the same instant; the resulting QRS is a hybrid shape — part paced, part native — and it is a benign timing coincidence, not a failure. A pseudofusion beat occurs when a pacer spike lands on an intrinsic QRS that was already underway; the spike distorts the complex but contributes no depolarization, again a benign timing artifact rather than undersensing. Recognizing these keeps a technician from over-calling failure to sense: the giveaway is that the underlying rate is appropriate and no beat is actually lost.

The four pacemaker malfunctions

The exam consistently tests four failure modes. The fastest way to sort them is to ask two questions: Is a spike present? and Did it capture?

• Failure to capture: the spike is present but is not followed by a P or QRS. Spikes march across the strip with no complex behind some or all of them. Causes include lead dislodgement, a rise in stimulation threshold (fibrosis, ischemia, hyperkalemia, antiarrhythmic drugs), or output programmed too low.
• Failure to sense (undersensing): the device does not recognize the patient's own beats, so it fires spikes too early, sometimes landing on or just after native complexes. A spike that falls on a T wave is dangerous because it can provoke a ventricular arrhythmia.
• Failure to pace / failure to output: no spike appears where one is due, so the paced rate falls below the programmed lower limit. In a pacemaker-dependent patient this exposes the underlying escape rhythm or asystole. Causes are battery depletion, lead fracture, or a loose connection.
• Oversensing: the device senses signals that are not true beats — tall T waves, muscle potentials, or electrical noise — and is inappropriately inhibited, so expected spikes go missing and the rate drops. It looks like failure to output on the strip, but the mechanism is a sensing error, not a dead generator.

Turning the malfunction into an action

The discrimination the CRAT exam rewards is "spike but no beat" versus "no spike at all." A spike with no following complex is a capture problem; a missing spike is an output or oversensing problem; a spike that lands too early on native rhythm is undersensing. Because a pacemaker-dependent patient in failure to capture or failure to output can become profoundly bradycardic or asystolic, the technician should flag any spike without a complex and any pause that drops the heart rate below the programmed floor, then escalate the tracing for evaluation.