Among the most important casualties of a heart attack seems to be the victim's sex life, doctors are finding, often without good reason. A recent study of 100 heart attack patients in England found that a year after leaving the hospital, a third refrained from all sexual activity and most of the others reported decreased frequency. Reasons included impotence, cardiac symptoms during intercourse and loss of desire. Consultation with physicians about sex had little effect, the English study found. Indeed, an American study found that medical consultation about sex with wives of coronary patients actually increased their fears rather than alleviating them. Based on these and other findings, doctors are trying to work out an effective counseling program to help recovered cardiac patients, most of whom needn't fear resumption of a normal sex life. According to Dr. Chris Papadopoulos of South Baltimore General Hospital, some individuals may experience chest pain or other symptoms during intercourse, but these can be avoided most times with medication. "Just because a person experiences symptoms the first time or two, that doesn't mean it forever is a no-no," he says. The staff of Chicago's Rush-Presbyteran-St. Luke's Medical Center has even devised a short slide show and booklet to help patients. The message is: sexual activity does tax the heart, but no morre than a brisk walk or climbing two flights of stairs. It is normal for a person to lose interest in sex after a heart attack, but this needn't be a permanent condition, In most cases, sex can be resumed about two months after a heart attack.