Phrenic Nerves and Pacemakers: Comprehensive Guide

1. Phrenic Nerves and Leadless Pacemakers

The relationship between phrenic nerves and leadless pacemakers involves an important potential complication: phrenic nerve stimulation (PNS).

The Issue

Leadless pacemakers (like the Medtronic Micra) are implanted directly into the right ventricle. The right phrenic nerve runs along the right side of the heart, relatively close to where these devices are positioned. If the pacemaker is placed too close to this nerve, the electrical impulses intended for the heart muscle can inadvertently stimulate the phrenic nerve.

What Happens with Phrenic Nerve Stimulation

When the phrenic nerve is stimulated, it causes the diaphragm to contract, which can result in:

• Hiccups
• Diaphragmatic twitching
• Uncomfortable rhythmic contractions synchronized with the pacemaker
• In some cases, shortness of breath

Clinical Management

This is why during leadless pacemaker implantation:

1. Testing is performed at high outputs to check for phrenic nerve capture before final deployment
2. Device positioning is carefully adjusted if PNS is detected
3. The device can be repositioned if needed before being released from the delivery catheter

2. Comparison with Traditional Pacemaker Leads

Incidence of Phrenic Nerve Stimulation

Traditional Pacemakers:

• RV leads: 1-3% (similar to leadless)
• RA leads: Rare, but can occur
• LV leads (CRT): 10-25% - this is much more common because the left phrenic nerve runs very close to the lateral/posterolateral cardiac veins commonly used for LV lead placement

Leadless Pacemakers:

• 1-2% in clinical trials
• Only right phrenic nerve at risk (device only in RV)

Key Differences

Management Options

Traditional Pacemakers - More flexibility:

1. Reprogramming: Change to bipolar pacing, reduce output, change polarity vector
2. LV lead specific: Phrenic nerve stimulation management algorithms, pacing vector changes (with quadripolar leads offering 10+ pacing configurations)
3. Lead revision: Surgical repositioning if programming fails

Leadless Pacemakers - More limited:

1. Lower output: Reduce voltage (if adequate safety margin)
2. Device retrieval: If cannot be managed with programming
3. No alternative pacing vectors available

Clinical Advantages

Leadless Pacemakers:

• ✓ Testing is more straightforward before final deployment
• ✓ Lower overall incidence since no LV lead option
• ✓ Once positioned correctly, very stable (no lead migration)

Traditional Pacemakers:

• ✓ More programmability to manage PNS without reoperation
• ✓ Can use alternative pacing vectors (especially with quadripolar LV leads)
• ✓ Lead repositioning is an option, though invasive

3. LBBAP and Phrenic Nerve Stimulation

Left Bundle Branch Area Pacing (LBBAP) adds an interesting dimension to this discussion.

Incidence with LBBAP

LBBAP: Typically <1-5% in most studies

• Generally lower than traditional LV epicardial CRT (10-25%)
• Similar to or slightly lower than RV apical pacing (1-3%)
• Often lower than His bundle pacing

Anatomical Considerations

With LBBAP, the lead is screwed deep into the interventricular septum (typically 1.5-2.0 cm). This creates unique PNS characteristics:

Which phrenic nerve is affected:

• Right phrenic nerve - most common (approaching from RV side)
• Left phrenic nerve - can occur with very deep penetration or high outputs
• Depends on depth of lead penetration and capture threshold

Comparison Table

Why LBBAP Has Lower PNS Risk

1. Septal location - far from both phrenic nerves compared to lateral LV wall
2. Deep penetration - insulated by myocardium
3. Lower capture thresholds - typically need less energy (0.5-1.5V)
4. Selective capture - can capture left bundle without capturing surrounding tissue at proper depth

LBBAP-Specific PNS Causes

1. Insufficient septal penetration - lead tip not deep enough, capturing RV endocardium near right phrenic nerve
2. Excessive penetration - too deep, approaching LV endocardium/left phrenic territory
3. High output programming - especially during threshold testing
4. Anodal capture - from the ring electrode

Management Options for LBBAP

During implantation:

• Adjust depth of lead penetration (screw in more or back out slightly)
• Reposition to different septal location
• Test at high outputs (10V) before fixation

After implantation:

• Reduce output - LBBAP typically has low thresholds with good safety margin
• Change to unipolar pacing (if bipolar was causing anodal capture)
• Lead revision - if programming unsuccessful

Key Advantage of LBBAP

The low baseline capture thresholds (often 0.5-1.0V) provide excellent safety margins, allowing significant output reduction if PNS occurs. This makes PNS more manageable compared to His bundle pacing, which often requires higher outputs.