Plunging into cold water during hot weather can cause heart attacks even in young, fit and healthy individuals, according to new research yesterday. Scientists are warning that entering cold water suddenly, without taking time to acclimatize, may cause abnormal heart rhythms that can be fatal. The research comes after several people died after going into water during the recent heat wave, although the cause of death in these cases is not yet known.  Professor Mike Tipton, who runs the Extreme Environments Laboratory at the University of   Portsmouth, southern England, said entering cold water should be done with caution. He said: “As the recent sad spate of immersion deaths confirm, we have entered the most dangerous time of the year for water-related deaths.  “As air temperatures rise dramatically, people start to go into water that remains dangerously cold. “The body’s responses to immersion in cold water are profound, uncontrollable and can result in drowning and heart problems within seconds.” In the study, published in the Journal of “Physiology”, Prof Tipton and Prof Mike Shattuck of King’s College London explain how rapid submersion in cold water, combined with holding one’s breath, automatically activates two powerful responses in the body which may interact and cause conflict for the heart. The body’s cold shock response speeds up the heart rate and causes hyperventilation which can conflict with the diving response, which does the opposite and which acts to conserve oxygen.  This can lead to autonomic conflict, causing the heart to go into abnormal rhythms and, on occasions, causing sudden death. Prof Tipton said: “Those wanting to enter the water should do so in a slow and controlled fashion to minimize these hazardous responses. “Individuals should also realize the water they felt comfortable in at the end of last year is colder, and they are less prepared for it at the start of the summer. “The prevalence of heart problems on immersion in water tends to be underestimated because electrical disturbances to the heart are undetectable post-mortem”.