Pacemaker Coding System
Understanding the pacemaker coding system is essential for accurately interpreting EKG strips and evaluating the performance of the device. Before determining if a pacemaker is functioning correctly, it is important to comprehend its intended operation. The pacing mode is typically represented by a four-letter code, which provides critical information about the pacemaker’s functions.
Pacing Chamber(s)
Indicates which chambers can be paced
• A: Atrial
• V: Ventricular
• D: Dual (both atrial and ventricular)
Sensing Chamber(s)
Indicates which chambers are sensed
• A: Atrial
• V: Ventricular
• D: Dual
Response to Sensed Event
Indicates what happens when the pacemaker senses depolarization.
I: Inhibited
An inhibited response means that the pacemaker will withhold its pacing impulse when it detects an intrinsic electrical signal from the heart.
For example, if a ventricular pacemaker detects a natural ventricular depolarization, it will not deliver a pacing spike. This prevents unnecessary competition between the pacemaker and the heart’s intrinsic rhythm, ensuring efficient cardiac function.
e.g., In a VVI pacemaker, the device is programmed to pace the ventricles and sense ventricular activity. If the pacemaker detects a QRS complex indicating that the ventricles have depolarized naturally, it will inhibit ventricular pacing, avoiding redundant or excessive stimulation.
T: Triggered
A triggered response means that the pacemaker delivers a pacing impulse in reaction to a sensed event. This setting is used in scenarios where an electrical stimulus is needed to ensure proper cardiac conduction following a specific sensed activity. Triggered pacing is less common but can be essential in certain clinical situations.
For instance, in an atrial-triggered mode, the pacemaker senses intrinsic atrial activity and responds by triggering a ventricular pacing impulse. This maintains proper timing between atrial and ventricular contractions, ensuring synchronized heart function.
D: Dual (can both inhibit and trigger)
A dual response incorporates both inhibited and triggered modes, allowing the pacemaker to respond dynamically to sensed events in both the atria and the ventricles. This is most often seen in dual-chamber pacemakers, such as the DDD mode, which paces and senses activity in both chambers.
Function:
• If the pacemaker senses intrinsic atrial activity, it inhibits atrial pacing and triggers a ventricular pacing impulse after a programmed atrioventricular (AV) interval. This mimics the natural sequence of atrial and ventricular contraction.
• If intrinsic ventricular activity occurs before the programmed AV interval ends, the ventricular pacing is inhibited to prevent over-pacing.
Clinical Relevance: The dual response capability is particularly useful in patients with intermittent conduction abnormalities. For example, it ensures that the pacemaker steps in to maintain rhythm only when the heart’s natural conduction system fails, while minimizing unnecessary pacing when normal function is present.
Rate Modulation
• R: Rate-responsive (adjusts pacing rate based on physiological needs).
• Absence of a letter indicates fixed-rate pacing
Rate-Responsive Pacing
Some pacemakers adjust the pacing rate based on the patient’s physiological needs, known as rate-responsive pacing.
Mechanism
- Sensors: The pacemaker uses sensors (e.g., accelerometers, minute ventilation sensors) to detect physical activity.
- Adjustment: The pacing rate increases during activity and decreases at rest.
Clinical Significance
- Enhances quality of life by accommodating varying activity levels.
- Important to recognize on EKGs, as the pacing rate may vary appropriately.
Examples of Pacemaker Types
VVI Pacemaker
• Paces: Ventricles
• Senses: Ventricles
• Response: Inhibits pacing if intrinsic ventricular activity is sensed
• Rate Modulation: Fixed-rate (no “R” in the code)
DDD Pacemaker
• Paces: Both atria and ventricles
• Senses: Both atria and ventricles
• Response: Inhibits atrial output if atrial activity is sensed; triggers ventricular output after a set atrioventricular (AV) interval unless inhibited by sensed ventricular activity
• Rate Modulation: Can be rate-responsive if an “R” is added (DDDR)
Analyzing Paced EKG Strips
When interpreting a paced EKG strip, consider the following key questions:
1. What is the Nature of Pacing?
• Identify if the pacing is atrial, ventricular, or dual-chamber.
• Look for pacing spikes before the P wave (atrial pacing), before the QRS complex (ventricular pacing), or both (dual-chamber pacing).
2. Is the Pacemaker Functioning Correctly?
- Assess for appropriate sensing and pacing. Is there any evidence of:
- over-sensing: Absence of pacing spikes when they are needed, leading to pauses or bradycardia.
- under-sensing: Pacing spikes appear at inappropriate times, disregarding the heart’s intrinsic activity.
- Check for the presence of pacing spikes and corresponding cardiac depolarizations. This would help detect failure to capture.
Interpreting Specific Pacemaker Functions on EKG
VVI Pacemaker Interpretation
Normal Function: Regular pacing spikes before wide QRS complexes. Also, consistent pacing rate if fixed-rate; variable if rate-responsive.
Malfunctions:
- Under-sensing: Pacing spikes ignore intrinsic QRS complexes.
- Over-sensing: Missing pacing spikes leading to bradycardia.
- Failure to capture: There is a pacemaker spike but no QRS follows it.
DDD Pacemaker Interpretation
Normal Function: Pacing spikes before P waves and/or QRS complexes and it maintains AV synchrony (p-waves followed by a delay followed by a QRS complex), mimicking normal physiological conduction. So, if it does not sense atrial depolarization, it will pace the atria and wait for a bit. If no ventricular depolarization occurs, it then paces the ventricles.
Malfunctions:
- Under-Sense Atrial Activity: Leads to inappropriate atrial pacing.
- Over-Sense Atrial Activity: Leads to no atrial pacing when warranted.
- Failure to capture at the atria: an atrial pacer spike does not lead to atrial depolarization (so no P-wave after the spike)
- Under-Sense Ventricular Activity: Leads to inappropriate ventricular pacing.
- Over-Sense Ventricular Activity: Leads to no ventricular pacing when warranted.
- Failure to capture at the ventricles: a ventricular pacer spike does not lead to ventricular depolarization (so no QRS complex after the spike)
Troubleshooting Steps
When to suspect?
- symptomatic patient (dizziness from bradycardia, palpitations from extra beats, etc.)
- abnormal EKG
What to do?
- Device Interrogation: Use a programmer to check settings and battery status.
- Lead Evaluation: Imaging or testing to assess lead position and integrity.
- Adjusting Sensitivity: Modify device settings to optimize sensing without over-sensing.
Leave a Reply