How is good capture determined in a paced rhythm?

Determining Good Capture in a Paced Rhythm

Good capture in a paced rhythm is confirmed when each pacing stimulus produces a corresponding mechanical systole, verified by observing a QRS complex following each pacing spike on ECG and confirming peripheral pulse generation through arterial pressure monitoring, pulse oximetry plethysmogram, or manual pulse palpation. 1

Primary Methods for Assessing Capture

ECG-Based Assessment

• Verify that each pacing artifact is followed by a QRS complex (for ventricular pacing) or P wave (for atrial pacing) with appropriate morphology 1
• Measure the paced QRS duration - successful ventricular capture typically produces a wide QRS complex, though left bundle branch pacing may produce narrower complexes (QRS <120 ms) 2
• Optimize ECG lead selection to minimize pacemaker artifact and maximize QRS visualization, as large pacing artifacts can obscure the QRS complex 1

Critical Pitfall with Transcutaneous Pacing

Because transcutaneous pacing produces large artifacts that may obscure or mimic the QRS complex, making ECG assessment unreliable, you must use concomitant non-ECG monitoring methods (arterial pressure waveform, pulse oximetry plethysmogram, or manual pulse palpation) to confirm mechanical capture 1

Mechanical Confirmation Methods

When ECG assessment is difficult or unreliable:

• Monitor peripheral pulse through arterial line waveform, pulse oximeter plethysmogram, or manual palpation to confirm each pacing stimulus produces a mechanical systole 1
• Verify hemodynamic response - adequate blood pressure and perfusion confirm effective mechanical capture 1

Device-Specific Verification

For Permanent Pacemakers

• Interrogate the device to assess capture thresholds, typically measured in volts at 0.4-0.5 ms pulse width 3, 4
• Evaluate sensing parameters - appropriate sensing prevents oversensing that could inhibit pacing output 1
• Check diagnostic counters for loss of capture events or backup pulse delivery, which indicate intermittent capture failure 5

For Biventricular/CRT Devices

• Confirm biventricular capture by analyzing the paced QRS morphology - look for positive R-wave in V1, as absence may indicate LV lead displacement or loss of LV capture 1
• Verify 100% biventricular pacing through device diagnostics, as even small percentages of non-captured beats reduce CRT efficacy 1
• Compare ECG with post-implant template - significant morphology changes suggest loss of capture or lead displacement 1

Threshold Testing for Capture Verification

• Perform threshold testing by gradually decreasing pacing output until loss of capture occurs, then program output with adequate safety margin 1, 3
• Maintain at least 100% voltage safety margin above threshold (e.g., if threshold is 1.0V, program output to ≥2.0V) to prevent intermittent capture loss between follow-ups 5
• Use incremental approach from below threshold during automatic measurements, as beat-to-beat threshold variations can cause underestimation if only 1-2 beats are tested 4

Common Causes of Capture Failure

When capture is lost or intermittent:

• Lead dislodgement - most common cause, occurring in 1-2% of pacemaker cases and up to 5.7% of CRT cases, with 50% occurring within first 24 hours 1
• Sudden threshold increase - may indicate lead fracture, insulation breaks, or cardiac perforation 3
• Oversensing - large P or T waves, muscle artifact, or electrical interference can inhibit pacing output 1
• Battery depletion or loose connections between lead and generator 1

Monitoring Recommendations

• Continuous ECG monitoring for 12-24 hours post-implantation to detect early lead dislodgement 3
• All patients with temporary transvenous pacing require continuous monitoring until device removal or replacement with permanent system 1
• Check thresholds twice in first 6 months, then annually for single-chamber or every 6 months for dual-chamber devices 3