How do you check sensitivity and capture on a DDD (Dual Chamber, Dual Mode) Osypka epicardial pacemaker?

Checking Sensitivity and Capture on DDD Osypka Epicardial Pacemaker

For a DDD epicardial pacemaker, verify capture by applying a magnet to trigger asynchronous pacing at the magnet rate and confirm each paced beat produces a corresponding QRS complex (ventricular) and P wave (atrial), then assess sensitivity by reducing the pacing rate below the intrinsic rhythm and confirming appropriate inhibition of pacing output when intrinsic cardiac activity is sensed.

Assessing Capture

Magnet Application Method

• Apply a magnet over the pacemaker generator to initiate asynchronous pacing at the programmed magnet rate, which overrides sensing functions and forces continuous pacing 1.
• Observe the ECG during magnet application to verify that each atrial pacing spike produces a visible P wave and each ventricular pacing spike produces a QRS complex 1.
• The magnet rate should be compared with previous values to detect any changes that might indicate battery depletion or other device issues 1.

Chamber-Specific Capture Assessment

• For atrial capture: Confirm that atrial pacing artifacts are followed by P waves with appropriate morphology and timing 1.
• For ventricular capture: Verify that ventricular pacing artifacts are followed by wide QRS complexes with appropriate morphology 1.
• Document the programmed pulse width and compare with previous assessments, as pulse width changes can indicate elective replacement indicators 1.

Assessing Sensitivity

Non-Magnet ECG Assessment

• Without magnet application, observe the baseline ECG to determine if the patient displays intrinsic rhythm or is being paced 1.
• If intrinsic rhythm is present, confirm that the pacemaker appropriately inhibits its output when sensing native atrial and ventricular activity 1.

Programmed Sensitivity Testing

• Reduce the pacing rate below the patient's intrinsic rate (if present) to allow native cardiac activity to emerge 2.
• Observe for appropriate sensing by confirming that intrinsic P waves and QRS complexes inhibit the corresponding pacemaker outputs 1.
• The atrial sensitivity should maintain at least a 2:1 safety margin between the measured P wave amplitude and the programmed sensitivity threshold 2.
• Similarly, ventricular sensitivity should maintain adequate safety margins to prevent undersensing 2.

Specific Considerations for DDD Mode

Dual-Chamber Synchrony

• Verify that sensed atrial activity triggers ventricular pacing after the programmed AV interval when AV block is present 1.
• Confirm that the AV interval changes appropriately during magnet application if this is a programmed feature of the device 1.
• Assess for appropriate AV synchrony by confirming that atrial events (sensed or paced) are followed by ventricular events (sensed or paced) at the correct timing 1.

ECG Recording Duration

• Obtain 6-9 second ECG strips both without and with magnet application, as this duration is sufficient to demonstrate all necessary pacing parameters 1.

Common Pitfalls and How to Avoid Them

Undersensing Detection

• Intermittent atrial undersensing occurs in approximately 20% of patients and may not be detected on brief ECG recordings 3, 4.
• Consider 24-hour Holter monitoring if routine testing shows appropriate parameters but clinical suspicion remains, as Holter has 97.5% sensitivity for detecting intermittent atrial undersensing compared to only 21.3% for brief 12-lead ECG 4.
• Deep respiration and changes in body position during ECG recording can help unmask intermittent sensing problems 4.

Oversensing Issues

• T-wave oversensing is the most common ventricular channel disorder, occurring in approximately 9% of patients even with apparently normal programmed parameters 3.
• Watch for inappropriate inhibition of ventricular output that could indicate oversensing of T waves or myopotentials 3.

Epicardial Lead Considerations

• Epicardial leads may have higher capture thresholds and lower signal amplitudes compared to endocardial leads, requiring more frequent threshold checks.
• Threshold testing should include assessment at multiple outputs to establish an adequate safety margin, typically programming output at least 2-3 times the measured threshold.

Device-Specific Features

• If the Osypka pacemaker has a Threshold Margin Test or similar automated threshold assessment feature programmed "on," analyze these results during the evaluation 1.
• Some devices provide P wave amplitude histograms or stored AV synchrony data that can supplement bedside testing, though these should not replace direct ECG assessment 4.