Cause of Device-Related Incident
Device factors

Clinical Specialty or Hospital Department
Cardiology / Cardiac Catheterization; Clinical/Biomedical Engineering

Device Factors
Device interaction

Document Type
User Experience Network (UEN) reports

External Factors
*Not stated

Mechanism of Injury or Death
Failure to deliver therapy

Support System Failures
*Not stated

Tampering and/or Sabotage
*Not stated

User Errors
*Not stated

UMDNS
Apnea Monitors [12-575]; Cardiac Output Units, Impedance [17-496]; Pacemakers, Cardiac, Implantable [12-913]; Physiologic Monitoring Systems, Acute Care [12-647]; Polysomnographs [17-458]

Thoracic Impedance Measurements Can Interfere with Impedance-Based Rate-Responsive Pacemakers



User Experience Network™  [Health Devices Sep-Oct 1997;26(9-10):393-4]

Hospital

After an electrocardiograph (ECG) cable from a physiologic monitor's ECG/respiratory rate module was connected to a patient being paced by a minute-ventilation rate-responsive implantable pacemaker, we noted that the patient's heart rate increased from 60 to 120 beats per minute (bpm). We canceled the respiratory rate function on the monitor, but the patient's heart rate remained at 120 bpm. We are concerned that the monitor's respiratory rate function is interfering with the pacemaker.

ECRI

Causes

The interference experienced in this instance occurred because two devices—the pacemaker and the monitor—were passing electric signals through the patient to measure thoracic impedance. The pacemaker was using these measurements to set the pacing rate, while the monitor was using them to determine the respiratory rate. Although the hospital indicated that the change in pacing rate occurred with a specific brand of implantable pacemaker, similar results could occur with any impedance-based rate-responsive pacemaker. (Note that non-impedance-based rate-responsive pacemakers, such as accelerometer-based and piezoelectric-based models, are not affected by other devices' thoracic impedance measurements.)

When programmed to do so, impedance-based rate-responsive pacemakers adjust the pacing rate based on the patient's physical exertion levels. To determine when such adjustments are needed, these pacemakers generate periodic, low-amplitude electric pulses that identify changes in thoracic impedance during breathing. The pacemaker uses this information to calculate minute ventilation, which is considered a measure of the patient's physical exertion level. The pacemaker then adjusts the pacing rate based on the minute ventilation value.

Some physiologic monitors, like the one used in the reported incident, measure thoracic impedance in a similar fashion—by passing low-amplitude, high-frequency signals through surface ECG leads. These monitors then use the change in thoracic impedance to determine respiratory rate. The monitor's signals can interfere with a pacemaker's impedance-measuring pulses if the pacemaker is being operated in an impedance-based rate-responsive mode. The pacemaker may then misinterpret the interference as an increase in minute ventilation and thus an increase in physical exertion level. In response, the pacemaker will increase the pacing rate to its upper rate limit to accommodate the perceived increase in physical exertion level.

Physiologic monitors are not the only devices that may interfere with the thoracic impedance measurements of these pacemakers. Some apnea monitors and polysomnographs (used for sleep studies) likewise use thoracic impedance measurements to determine respiratory rate. Furthermore, impedance-based non-invasive cardiac output units use thoracic impedance measurements to determine cardiac output. Any of these devices can interfere with impedance-based rate-responsive pacemakers.

Consequences

The type of pacemaker interference described in this report typically results in upper-rate pacing that can cause patient discomfort but that would not likely adversely affect the patient over a short duration. The interference does not damage the pacemaker or cause inappropriate inhibition of pacing, and normal pacing resumes when the patient is disconnected from the interfering device—in this case, the ECG/respiratory rate module of the physiologic monitor.

Corrective Actions

As the reporting hospital found out, canceling the respiratory rate function on the monitor (i.e., so that respiratory rate is not displayed on the screen) may not eliminate the pacemaker interference. This is because the monitor may continue to generate a signal to determine respiratory rate even though the rate is not being displayed. Other steps, however, can be taken to address the problem. For example, the manufacturer of the pacemaker in question recommends temporarily reprogramming the pacemaker to a non-rate-responsive mode (e.g., DDD, VVI) so that its minute-ventilation rate-response function is inactive. Alternatively, if immediate reprogramming is not possible, the patient can be temporarily placed on a monitor that does not have the respiratory rate function. These alternatives are discussed in more detail in the Recommendations, below.

Recommendations

  1. If a patient has an impedance-based (e.g., minute ventilation) rate-responsive pacemaker, temporarily reprogram the pacemaker to a non-rate-responsive mode before connecting the patient to a monitoring device that uses thoracic impedance measurements—for example, physiologic monitors, apnea monitors, or cardiac output units. Reprogramming the pacemaker will prevent upper-rate pacing without affecting appropriate function of the pacemaker. Remember that the pacemaker will have to be reprogrammed to the original rate-responsive mode once the patient has been removed from the monitor.
  2. If the pacemaker cannot be reprogrammed to a non-rate-responsive mode immediately, then an alternative monitoring device that does not employ thoracic impedance measurements should be used (e.g., a physiologic monitor that does not have the respiratory rate function) until the pacemaker can be reprogrammed.

UMDNS Terms

  • Apnea Monitors [12-575]
  • Cardiac Output Units, Impedance [17-496]
  • Pacemakers, Cardiac, Implantable [12-913]
  • Physiologic Monitoring Systems, Acute Care [12-647]
  • Polysomnographs [17-458]

Cause of Device-Related Incident

Device factor: Device interaction

Mechanism of Injury or Death

Failure to deliver therapy 


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