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Researchers say many heart disease patients, who have been given a tiny implantable device that automatically delivers a big electrical jolt to their heart if it develops a dangerous rhythm, may suffer nightmares in which they imagine the device has gone off.

The phenomenon, reported in Thursday's New England Journal of Medicine, may add to the psychological problems some people suffer when they are kept alive by the devices, which restore the beat of a heart that is pulsating too rapidly or too erratically to pump blood effectively.A group led by Dr. Peter Kowey of the Lankenau Hospital and Medical Research Center in Wynnewood, Pa., found that four of 84 patients given the devices have regularly received "phantom" shocks while they slept.

All had experienced one real shock while awake, an experience likened to having a mule kick you in the chest.

"In each case, the patient was awakened abruptly from a sound sleep with the sensation of having received a defibrillator discharge, but no memories of any dreams," they said. In some cases, the sleepers were seen to behave as if they had been jolted and they cried out.

"A few patients have even described a soreness in the chest immediately after they recovered consciousness," according to the researchers.

But when the counter on each device was checked with an electronic monitor, the doctors discovered that in every case there was no indication that a shock had been delivered.

A 1988 study of people who had received the devices, known as ICDs, found that 59 percent were afraid of them, either because of the pain caused by the shocks or because they lost control during the jolt. The fear can produce anxiety, depression and sleep disorders.

The 1988 study found that 11 percent said that, if it were possible, they would want the devices removed.

But often that isn't possible. ICDs are the implantable version of the familiar paddles placed on the chest to restore the heart's rhythm. ICDs, about the size of a transistor radio, are usually given to people who have survived one heart attack but have a high risk of developing an abnormal beat.

Kowey and his colleagues said two of their four patients have required psychiatric treatment because the "phantom" shocks have added to the difficulty of adjusting to the devices.

They say doctors should warn patients that nightmare shocks may occur.

The findings reinforce "the need for intensive counseling before and after insertion of the device," they said.