Category: Pacemaker Assessment

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.

Two fundamental mechanisms that enable pacemakers to function effectively are sensing and capture. A thorough understanding of these mechanisms is essential for healthcare professionals to manage patients with pacemakers and accurately interpret EKGs. This article offers an analytical overview of sensing and capture in pacemakers, exploring their functions, underlying mechanisms, and clinical implications.

Sensing in Pacemakers

What does it mean when a pacemaker is sensing?

Sensing refers to a pacemaker’s ability to detect the heart’s intrinsic electrical activity. By monitoring electrical signals through its leads, the pacemaker determines whether it needs to deliver a pacing pulse.

Function of Sensing

The primary function of sensing is to ensure that the pacemaker provides electrical stimulation only when the heart’s natural rhythm is insufficient. If the pacemaker detects an intrinsic heartbeat, it inhibits its own output to prevent unnecessary pacing, thereby conserving battery life and maintaining physiological heart function.

Chambers Involved in Sensing

Sensing can occur in different heart chambers:

• Atrial Sensing: Detects electrical activity in the atria (upper chambers).

• Ventricular Sensing: Detects electrical activity in the ventricles (lower chambers).

• Dual-Chamber Sensing: Monitors both atria and ventricles.

The specific chambers where sensing occurs are indicated by the second letter in the pacemaker’s code. For example, “A” stands for atrial, “V” for ventricular, and “D” for dual.

Purpose of Sensing

Sensing allows the pacemaker to work in harmony with the heart’s natural activity, providing support only when needed. This synchronization helps maintain an appropriate heart rate without interfering unnecessarily with the heart’s own rhythm.

Abnormalities related to sensing

Undersensing

Undersensing happens when the pacemaker fails to detect intrinsic cardiac activity that is present. Consequently, it delivers unnecessary pacing pulses, potentially causing competition between the pacemaker and the heart’s natural rhythm.

Mechanisms of Undersensing

• Inadequate Sensitivity Settings: The pacemaker’s sensitivity may be set too low to detect smaller electrical signals.

• Electrode Dislodgment: The pacing lead may have shifted, reducing its ability to detect cardiac signals.

• Interference: External or internal electrical signals can mask the heart’s activity.

• Lead Damage: Fractured or damaged leads can impair signal detection.

Clinical Implications of Undersensing

Undersensing can lead to inappropriate pacing and arrhythmias. Recognizing undersensing on an EKG allows for timely interventions, such as reprogramming the pacemaker or correcting hardware issues.

Oversensing

Oversensing occurs when the pacemaker detects electrical signals that are not true cardiac activity, leading it to withhold necessary pacing pulses.

Mechanisms of Oversensing

• Electromagnetic Interference: External devices can emit signals that the pacemaker misinterprets.

• Muscle Potentials: Electrical activity from skeletal muscles can be sensed erroneously.

• T-Wave Oversensing: The pacemaker mistakes the T-wave for a QRS complex.

Clinical Implications of Oversensing

Oversensing can result in bradycardia or pauses in the heart rhythm, causing symptoms like dizziness or syncope. Identifying oversensing is crucial for adjusting the pacemaker’s sensitivity or addressing contributing factors.

Capture in Pacemakers

What does it mean for a pacemaker to capture?

Capture refers to the successful depolarization of cardiac tissue following a pacemaker’s electrical impulse, resulting in a heart contraction. On an EKG, capture is indicated by a pacemaker spike immediately followed by the appropriate waveform—a P-wave for atrial capture or a QRS complex for ventricular capture.

Types of Capture

• Atrial Capture: A pacing spike followed by a P-wave indicates the atria have “captured” the pacemaker spike resulting in atrial depolarization.

• Ventricular Capture: A pacing spike followed by a QRS complex indicates the ventricles have “captured” the pacemaker spike resulting in ventricular depolarization.

• Dual Capture: Spikes precede both the P-wave and the QRS complex in dual-chamber pacing.

Abnormalities related to capture

Loss of Capture

Loss of capture occurs when a pacemaker’s electrical impulse fails to depolarize cardiac tissue, evident as a pacing spike not followed by the expected waveform on an EKG.

Mechanisms of Loss of Capture

• Lead Dislodgment: The pacing lead may have moved, losing effective contact with the heart muscle.

• Battery Depletion: Low battery power can reduce the energy of pacing impulses.

• Increased Pacing Threshold: The heart muscle may require a higher energy to depolarize due to factors like ischemia.

• Lead Integrity Issues: Damage to the lead can impair impulse delivery.

Clinical Implications of Loss of Capture

Loss of capture can lead to inadequate heart rates and reduced cardiac output, causing symptoms like fatigue or syncope. It requires immediate evaluation to adjust pacemaker settings or address hardware problems.

Cardiac pacing is a pivotal intervention in cardiology that involves providing electrical impulses to prompt the heart’s activity when its natural electrical system is unable to sustain an adequate rate or rhythm. Pacemakers, the devices that fulfill this role, are indispensable for patients whose conditions hinder the heart’s capability to beat efficiently on its own. For healthcare professionals overseeing the care of these patients, a thorough understanding of pacing mechanisms and the ability to interpret EKG (electrocardiogram) changes are vital.

The Purpose of Cardiac Pacing

The main goal of cardiac pacing is to ensure the heart maintains a proper rate and rhythm to facilitate efficient blood circulation. When the heart’s natural electrical system is impaired due to issues like bradycardia (an abnormally slow heart rate), heart block, or other rhythm disorders, a pacemaker can step in to deliver the necessary electrical impulses. This intervention helps prevent symptoms associated with insufficient heart rates, such as dizziness, fatigue, and fainting, while also lowering the risk of more serious complications.

Mechanisms of Pacing and EKG Changes

Pacemakers function by delivering electrical impulses to specific chambers of the heart, initiating depolarization and subsequent contraction. The mechanism of pacing and its effect on the heart’s electrical activity depend on which chamber is being stimulated and the patient’s underlying cardiac condition. These interventions result in characteristic changes on an EKG, a tool that records the electrical activity of the heart.

When a pacemaker stimulates the heart, it can alter the normal conduction pathways. For instance, ventricular pacing often leads to a widened QRS complex on the EKG due to the delayed spread of the electrical impulse through the ventricular muscle. Understanding these changes is essential for interpreting EKGs in patients with pacemakers and for identifying any potential issues with the device or the heart itself.

Types of Cardiac Pacing

1. Atrial Pacing

Atrial pacing involves delivering electrical impulses to the atria, the heart’s upper chambers, when the natural atrial activity is insufficient. This type of pacing is beneficial for patients with sinoatrial node dysfunction, where the heart’s natural pacemaker fails to generate impulses at an adequate rate.

EKG Interpretation: On an EKG, atrial pacing is indicated by a pacing spike preceding the P-wave, which represents atrial depolarization. The presence of this spike followed by a P-wave shows that the atria have responded to the pacemaker’s stimulus. The subsequent conduction through the atrioventricular node and ventricles should be normal if the conduction system is intact.

2. Ventricular Pacing

Ventricular pacing is used when the ventricles, the heart’s lower chambers, require direct stimulation. This scenario often occurs in patients with atrioventricular block, where the electrical impulses from the atria do not reach the ventricles.

Mechanism and EKG Changes: In ventricular pacing, the pacemaker delivers impulses directly to the ventricles, typically via a lead in the right ventricle. This direct stimulation causes the ventricles to depolarize in a manner that does not follow the normal conduction pathways, resulting in a widened QRS complex on the EKG. The pacing spike appears just before the QRS complex. Because the impulse starts in the right ventricle, it spreads to the left ventricle in a way that mimics a left bundle branch block (LBBB) pattern.

Clinical Considerations: Diagnosing myocardial infarction (heart attack) in patients with ventricular pacing can be challenging due to these EKG changes. The modified Sgarbossa criteria are used to identify acute myocardial infarction in paced rhythms by assessing specific EKG patterns.

3. Dual Chamber Pacing

Dual chamber pacing involves pacing both the atria and the ventricles, allowing for coordinated contraction and maintaining the natural sequence of heartbeats. This type of pacing is particularly useful for patients who need support in both chambers to optimize cardiac output.

EKG Interpretation: In dual chamber pacing, pacing spikes may appear before both the P-wave and the QRS complex. The pacemaker can sense the heart’s intrinsic activity and only deliver impulses when necessary. If a normal P-wave is detected, the device may inhibit atrial pacing but still pace the ventricles if needed. This flexibility helps maintain a more natural heart rhythm and improves the efficiency of the heart’s pumping action.

4. Biventricular (BiV) Pacing

Biventricular pacing, also known as cardiac resynchronization therapy (CRT), involves simultaneous pacing of both the right and left ventricles. This approach is used in patients with severe heart failure and intraventricular conduction delays that cause the ventricles to contract out of sync, reducing the heart’s efficiency.

Mechanism and EKG Changes: By pacing both ventricles at the same time, biventricular pacing helps synchronize their contractions, improving cardiac output and alleviating heart failure symptoms. On the EKG, this type of pacing may show unique patterns, such as QS or QR complexes in lead I and R or rS patterns in lead V1. These changes reflect the altered depolarization pathways due to the simultaneous stimulation of both ventricles.

Understanding Pacing Spikes on the EKG

Pacing spikes are sharp, vertical lines on the EKG that represent the electrical impulses delivered by the pacemaker. The presence and location of these spikes provide valuable information about which chamber is being paced.

• Atrial Pacing Spike: Appears just before the P-wave, indicating atrial stimulation.

• Ventricular Pacing Spike: Occurs immediately before the QRS complex, indicating ventricular stimulation.

• Dual Chamber Pacing Spikes: Both atrial and ventricular spikes are present, each preceding their respective depolarization waves.

The morphology of the subsequent waves helps assess the effectiveness of the pacing and the heart’s response. Abnormalities in the spikes or the associated waves can signal issues such as lead displacement, battery depletion, or pacemaker malfunction.

Mechanisms of Changes on an EKG in Pacing

Understanding the mechanisms behind the changes seen on the EKG with different pacing modes is crucial for accurate interpretation and patient management.

• Altered Conduction Pathways: Paced electrical impulses may not follow the heart’s normal conduction pathways, leading to changes in the duration and shape of the EKG waves. For example, ventricular pacing causes the impulse to spread through the ventricular muscle rather than the specialized conduction fibers, resulting in a widened QRS complex.

• Bundle Branch Block Patterns: Ventricular pacing, especially from the right ventricle, often produces an EKG pattern similar to a left bundle branch block. Recognizing this pattern is important to avoid misdiagnosing conduction system diseases.

• Assessment of Ischemia: In paced rhythms, traditional criteria for detecting ischemia or infarction may not apply due to the altered EKG patterns. The modified Sgarbossa criteria help identify acute myocardial infarction in these patients by evaluating specific concordant and discordant changes in the ST segments relative to the QRS complexes.

• Pacemaker Malfunction Detection: Abnormalities such as failure to capture (the heart does not respond to the pacemaker’s impulse) or failure to sense (the pacemaker does not detect intrinsic heart activity) can be identified by analyzing the EKG. This analysis is essential for timely intervention and correction of any device-related issues.