Mediastinal Air Entrapment During Extravascular ICD Implantation: A Preventable Cause of Elevated Shock Impedance.

INTRODUCTION: Elevated shock impedance during extravascular implantable cardioverter-defibrillator (EV-ICD) implantation may preclude defibrillation threshold (DFT) testing despite appropriate retrosternal lead position. We describe suspected mediastinal air entrapment as a reversible mechanism and a simple preventive strategy.

METHODS AND RESULTS: During early EV-ICD implantation experience, two patients developed shock impedance > 200 Ω immediately after retrosternal lead placement, with associated sensing abnormalities. Lead repositioning did not normalize impedance, no pneumothorax was identified, and DFT testing was deferred. Shock impedance normalized spontaneously within 1-2 days, permitting successful DFT testing without lead revision. Transient mediastinal air entrapment along the defibrillation vector was suspected. A procedural modification was adopted in which 5-10 cm H2O positive end-expiratory pressure was applied immediately before retrosternal introducer removal, followed by prompt EV-ICD lead advancement. Among five consecutive subsequent implantations using this approach, initial shock impedance remained within the expected range, permitting immediate DFT testing without lead repositioning or delay.

CONCLUSION: Transient mediastinal air entrapment may cause elevated shock impedance during EV-ICD implantation. Positive end-expiratory pressure during introducer removal may mitigate this complication and facilitate reliable intraoperative DFT testing.