Measuring Capture and Sensing Treshold on PACE Model 203

Summary

The video demonstrates how to measure sensing and capture thresholds and set pacing parameters on the PACE Model 203 temporary pacemaker. Both measurements are performed for the atrium and ventricle. The demonstration focuses on atrial pacing in AAI mode, commonly used after cardiac surgery when the AV node is intact and only backup atrial stimulation is needed.

Manual sensing-threshold measurement

1. Set the pacing rate 10–20 pulses per minute below the patient’s intrinsic heart rate so the heart’s own rhythm can be observed.
2. Reduce the stimulation amplitude to its minimum so any pacing pulses are ineffective.
3. Adjust the sensitivity setting while watching the ECG and sensing indicator. A higher millivolt setting means lower sensitivity; a lower setting means the pacemaker can detect smaller signals.
4. If sensitivity is too low, intrinsic beats may not be detected (undersensing). If it is too high, interference may be mistaken for heart signals (oversensing).
5. Find the threshold where the pacemaker just detects intrinsic signals, then verify it by adjusting in the opposite direction. Set the operating sensitivity to about half to one-third of the measured threshold.

In the example, the atrial sensing threshold is 3.0 mV, and 1.2 mV is given as a suitable setting.

Manual capture-threshold measurement

1. Set the pacing rate at least 10 pulses per minute above the intrinsic heart rate so the pacemaker consistently stimulates.
2. Starting from a low amplitude, increase stimulation step by step while observing the ECG until a pacing pulse produces a heart contraction. This is the capture threshold.
3. Set the stimulation amplitude to two to three times the measured threshold for a safety margin. In the example, a 1.5 V capture threshold leads to a suggested setting of 4.6 V.

Thresholds can change during recovery or with prolonged pacemaker use, so sensing and capture should be checked periodically—and at least daily during prolonged use.

Automatic sensing options

• Pause function: Pressing and holding Pause stops pacing while the device records intrinsic cardiac signals and displays P- and R-wave amplitudes. The operator can use those readings to estimate a safety-margin setting and enter the sensitivity manually.
• AutoSense: The pacemaker measures the sensing threshold, calculates a safety margin, and sets sensitivity automatically. It can also adapt to changing P- or R-wave amplitudes. Select it through Main menu → Auto → AutoSense, then choose the atrial and/or ventricular channel and initialize the function. AutoSense must be reactivated if sensing is switched off or the pacemaker is restarted.

The demonstration connects the PACE 203 to the CoraSim interactive heart simulator, whose ECG display shows the effects of changing pacemaker settings.

Sources and speakers: An instructional narrator and device demonstration from Osypka Medical; the ECG simulation is shown using CoraSim.