Failure to sense is a type of malfunction in implantable cardiac devices, such as pacemakers and implantable cardioverter-defibrillators (ICDs). Proper sensing is fundamental for these devices to deliver therapy only when the heart requires assistance, preventing unnecessary interventions. When a device fails to sense, it can lead to either under-pacing or over-pacing, both of which can compromise heart rhythm management.
Understanding Device Sensing
Cardiac devices like pacemakers and ICDs are designed to continuously monitor the heart’s electrical signals. These devices “listen” for the heart’s intrinsic electrical activity through small wires, called leads, which are typically placed inside the heart chambers. The leads contain electrodes that detect specific electrical impulses, such as P-waves (atrial depolarization) and R-waves (ventricular depolarization), which indicate the heart’s natural contractions.
Accurate sensing allows the device to distinguish between the heart’s own beats and periods when it needs support. If the heart beats on its own, the device should ideally remain inactive, or “inhibited,” to avoid competing with natural rhythms. Conversely, if the device does not detect a natural beat within a set time, it then delivers an electrical impulse to stimulate the heart, ensuring a regular rhythm.
Factors Leading to Impaired Sensing
Several issues can cause a cardiac device to improperly sense the heart’s electrical activity. Problems with the device’s leads are common culprits. For instance, if a lead becomes dislodged from its intended position within the heart, it may no longer be in optimal contact with the heart tissue to pick up signals effectively. Damage to the lead, such as a fracture in the wire or a break in its insulation, can also disrupt the transmission of electrical signals back to the device.
The heart’s own electrical signals can also change, becoming too weak or small for the device to consistently detect. This can happen due to changes in the heart tissue itself, such as the formation of scar tissue, or as a side effect of certain medications that alter cardiac electrical properties. If the heart’s signals fall below the device’s programmed sensitivity threshold, the device may simply miss them.
Device programming plays a role in sensing performance. If the device is programmed with a “less sensitive” setting, it requires a stronger electrical signal from the heart to register a beat. This can lead to the device missing weaker, but otherwise normal, heart signals. While less common with modern devices, strong external electromagnetic interference (EMI) from sources like certain medical equipment or industrial machinery can sometimes disrupt the device’s ability to sense accurately.
Recognizing Impaired Sensing
When a cardiac device fails to sense properly, individuals may experience a range of symptoms because the device either paces unnecessarily or fails to provide pacing when needed. Common symptoms can include dizziness, lightheadedness, and unusual fatigue. Some individuals might also feel palpitations or experience shortness of breath. In more severe cases, fainting spells can occur if the heart rate becomes critically slow or irregular.
Diagnosing impaired sensing typically involves a thorough review of the device’s performance data during routine clinic visits. During these “interrogations,” healthcare professionals use specialized equipment to download information stored in the device, which provides a detailed record of sensed cardiac events and delivered therapies. Electrocardiograms (ECGs) or continuous monitoring devices, such as Holter monitors, may also be used to correlate any reported symptoms with the device’s actual behavior over time.
Managing Impaired Sensing
Addressing impaired sensing often begins with reprogramming the device. Adjusting the device’s sensitivity settings, or other parameters, can frequently resolve the issue by allowing the device to better detect the heart’s intrinsic signals.
If a physical problem with a lead is identified, such as dislodgement or damage, surgical intervention might be necessary. This procedure, known as a lead revision or replacement, involves repositioning the existing lead or implanting a new one to ensure proper contact and signal transmission. In some instances, changes to a person’s medications, particularly those affecting heart signals or tissue characteristics, may be considered to improve the amplitude of the heart’s electrical activity.
Regular follow-up appointments and device checks are important for individuals with cardiac implants. These routine evaluations help healthcare providers monitor the device’s ongoing function and identify any emerging issues with sensing or other parameters.