Patient tolerance of pain from discharges of implantable cardioverter defibrillators (ICD's) is highly variable. It can be influenced by psychological factors, physical condition and the number of shocks delivered.

It has been shown that cutaneous pain perception can be reduced by delivery of a weak , low voltage pulse (prepulse)prior to the delivery of a stronger shock.

A prepulse delivered before a high voltage ICD shock will decrease discomfort compared to a shock of equal energy without a prepulse.

Inclusion Criteria:

18+ Approved indication for implantation of an ICD Able and willing to provide informed consent Independently complete and understand the Mini Mental State Evaluation

Exclusion Criteria:

Pregnancy Cannot understand English or French Contraindicated for defibrillation testing because of risk of thromboembolism Deaf or hard of hearing, making the acute testing difficult to conduct having pain or anxiety to the extent that, in the opinion of the investigator, the results would be confounded.

Having a Class I bradycardia pacing indication, requiring ongoing pacing support.

Visual problems which preclude reading the computer screen for the VAS test Severe musculoskeletal disorder that makes it difficult to undergo testing in the recumbent position Do not achieve a score >24/30 on a Mini Mental State Exam