Defibrillation threshold (DFT) is the minimum energy level required to successfully terminate ventricular fibrillation (VF) with a defibrillation shock. In the context of implantable cardioverter defibrillators (ICDs), it refers to the amount of energy the device must be able to deliver to reliably convert VF back to a normal rhythm.
Why it matters at ICD implantation
When an ICD is implanted, the cardiologist needs to confirm that the device can deliver enough energy to terminate VF reliably. A safety margin is required: the ICD must be able to deliver a shock significantly above the DFT to provide confidence that it will work when needed in real life, when conditions may differ from the controlled setting of the implant procedure.
DFT testing
Historically, DFT testing was performed routinely at ICD implantation by deliberately inducing VF under anaesthetic and confirming that the device terminates it. This practice has declined in most centres because modern ICDs deliver high-energy shocks (typically 30 to 40 joules) that are effective in the vast majority of patients without the need for testing. DFT testing is still performed in specific circumstances, such as when a high DFT is suspected or when the device has been positioned unusually.
High defibrillation threshold
A small proportion of patients have a high DFT, meaning the standard ICD shock may not reliably terminate VF. This can be managed by repositioning the device, adding a subcutaneous electrode (shocking coil), using a different waveform, or switching to a higher-energy device. Antiarrhythmic medications such as amiodarone can paradoxically increase the DFT in some patients.
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