Cardiac Rhythm Management

ICD Shock Vectors and High DFTs

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ICD Shock Vectors and High DFTs

Nasir Nawaz, MD


Lecture Abstract

Defibrillation threshold (DFT) is the minimum amount of energy that is necessary to defibrillate the heart. Testing for DFT is often done at the time of ICD implantation. Because of growing number of ICD implantation; high DFT is being increasingly encountered in clinical practice. The management is tailored to specific clinical situations and needs a complete understanding of the various underlying mechanisms. The shock vector is a three-dimensional orientation of the distribution of energy delivered by the coil system. It is crucial that the energy encompassing the left ventricle be uniformly distributed to ensure successful defibrillation. Several clinical, biochemical and echocardiographic parameters have been implicated in the literature to cause high DFT(s). Numerous drugs are known to affect the threshold and must be considered when managing patients with ICDs. The management involves both invasive and non-invasive strategies. Medications, electrolytes, acid base disturbances and medical therapy for heart failure need to be optimized before testing the DFT.

During the procedure; if on testing, a high DFT is encountered; one should be able to identify hypoxia, hypercarbia, pneumothorax, pleural effusions and prolonged myocardial depression as the potential underlying etiologies. Amongst invasive management strategies; shock vector alteration by shock polarity reversal, configuration change, addition of SVC coil and subcutaneous arrays can help reduce the DFT to a safe margin. Recognition of high DFT and improvement of safety margin for high defibrillation thresholds can help prevent large number of sudden deaths in patients with ICDs.



Nasir Nawaz, MD
Department of Cardiovascular Medicine
Crozer-Keystone Health System
Upland, PA


Dr. Nasir Nawaz reports no conflicts of interest.


A special thanks to my mentor and teacher, David Kleinman MD

Cardiac Rhythm Management

Suggested Reading

The Case of an Unusual Defibrillation Vector in the Management of High Defibrillation Threshold

Cardiac Rhythm Management