Pacemaker Spike Without QRS: Failure to Capture
A pacer spike on telemetry without a resultant QRS complex in a VVI pacemaker indicates failure to capture (Answer D). This represents the pacemaker's inability to depolarize the myocardium despite delivering an electrical impulse, which is a critical malfunction requiring immediate intervention 1.
Understanding the Mechanism
Failure to capture occurs when the pacing stimulus is delivered but fails to produce ventricular depolarization. In VVI mode (ventricular pacing, ventricular sensing, inhibited response), you should see a pacing spike followed by a wide QRS complex when the pacemaker fires 1. When the spike appears without the QRS, the electrical energy is insufficient to stimulate myocardial contraction 1.
Why This is Particularly Concerning with Transvenous Pacemakers
Temporary transvenous pacemakers carry a higher risk of loss of capture compared to permanent pacemakers due to several mechanical vulnerabilities 1:
- The lead wires are stiffer to facilitate rapid insertion, making them more prone to displacement
- They lack active and passive fixation mechanisms present in permanent leads
- Lead perforation through the right ventricular free wall or interventricular septum is more likely
- Lead dislodgement is a well-documented early complication 1
Distinguishing from Other Pacemaker Malfunctions
Undersensing (Answer A) would show pacing spikes occurring inappropriately when intrinsic QRS complexes are present, but the spikes would still capture and produce QRS complexes 1.
Oversensing (Answer C) would result in inappropriate inhibition of pacing—you would see fewer pacing spikes than expected because the pacemaker incorrectly interprets noise as cardiac activity 1.
Failure to pace (Answer B) means no pacing spike is generated at all when one is needed—the complete absence of electrical output 1.
Immediate Management Priorities
When you identify failure to capture, verify both electrical AND mechanical capture—never rely on ECG alone 2:
- Confirm pulse and blood pressure immediately
- Large pacing artifacts can obscure QRS complexes, creating diagnostic confusion 2
- Use arterial pressure monitoring or pulse oximetry to confirm mechanical capture when ECG is unreliable 2
Troubleshooting steps include 1, 3:
- Increase the pacing output (mA) to overcome increased capture threshold
- Reposition the patient (left lateral decubitus may improve lead contact)
- Consider lead dislodgement requiring repositioning
- Evaluate for lead perforation or sudden threshold increase
Critical Clinical Context
This patient is pacemaker-dependent given the indication of heart block, making loss of capture a potentially life-threatening emergency 1. Temporary pacing equipment and defibrillation capability must be immediately available 1. The goal is rapid transition to stable pacing—either by correcting the transvenous lead position or proceeding to permanent pacemaker placement, as transvenous pacing is a bridge therapy, not a destination 2.