other problems with external cooling include overshoot of temperature (20% of people will have overshoot), slower induction time versus internal cooling, increased compensatory response, decreased patient access, and discontinuation of cooling for invasive procedures such as the cardiac catheterization. if therapy with water blankets is given along with two litres of cold intravenous saline, people can be cooled to 33 °c (91 °f) in 65 minutes. most machines now come with core temperature probes. when inserted into the rectum, the core body temperature is monitored and feedback to the machine allows changes in the water blanket to achieve the desired set temperature. in the past some of the models of cooling machines have produced an overshoot in the target temperature and cooled people to levels below 32 °c (90 °f), resulting in increased adverse events. they have also rewarmed patients at too fast a rate, leading to spikes in intracranial pressure. some of the new models have more software that attempt to prevent this overshoot by utilizing warmer water when the target temperature is close and preventing any overshoot. some of the new machines now also have 3 rates of cooling and warming; a rewarming rate with one of these machines allows a patient to be rewarmed at a very slow rate of just 0.17 °c (0.31 °f) an hour in the "automatic mode," allowing rewarming from 33 °c (91 °f) to 37 °c (99 °f) over 24 hours. cool caps there are a number of non-invasive head cooling caps and helmets designed to target cooling at the brain. a hypothermia cap is typically made of a synthetic material such as neoprene, silicone, or polyurethane and filled with a cooling agent such as ice or gel which is either cooled to a very cold temperature, −25 to −30 °c (−13 to −22 °f), before application or continuously cooled by an auxiliary control unit. their most notable uses are in preventing or reducing alopecia in chemotherapy, and for preventing cerebral palsy in babies born with hypoxic ischemic encephalopathy. in the continuously cooled iteration, coolant is cooled with the aid of a compressor and pumped through the cooling cap. circulation is regulated by means of valves and temperature sensors in the cap. if the temperature deviates or if other errors are detected, an alarm system is activated. the frozen iteration involves continuous application of caps filled with crylon gel cooled to −30 °c (−22 °f) to the scalp before, during and after intravenous chemotherapy. as the caps warm on the head, multiple cooled caps must be kept on hand and applied every 20 to 30 minutes. history hypothermia has been applied therapeutically since antiquity. the greek physician hippocrates, the namesake of the hippocratic oath, advocated the packing of wounded soldiers in snow and ice. napoleonic surgeon baron dominique jean larrey recorded that officers who were kept closer to the fire survived less often than the minimally pampered infantrymen. in modern times, the first medical article concerning hypothermia was published in 1945. this study focused on the effects of hypothermia on patients with severe head injury. in the 1950s, hypothermia received its first medical application, being used in intracerebral aneurysm surgery to create a bloodless field. most of the early research focused on the applications of deep hypothermia, defined as a body temperature of 20–25 °c (68–77 °f). such an extreme drop in body temperature brings with it a whole host of side effects, which made the use of deep hypothermia impractical in most clinical situations. this period also saw sporadic investigation of more mild forms of hypothermia, with mild hypothermia being defined as a body temperature of 32–34 °c (90–93 °f). in the 1950s, doctor rosomoff demonstrated in dogs the positive effects of mild hypothermia after brain ischemia and traumatic brain injury. in the 1980s further animal studies indicated the ability of mild hypothermia to act as a general neuroprotectant following a blockage of blood flow to the brain. this animal data was supported by two landmark human studies that were published simultaneously in 2002 by the new england journal of medicine. both studies, one occurring in europe and the other in australia, demonstrated the positive effects of mild hypothermia applied following cardiac arrest. responding to this research, in 2003 the american heart association (aha) and the international liaison committee on resuscitation (ilcor) endorsed the use of targeted temperature management following cardiac arrest. currently, a growing percentage of hospitals around the world incorporate the aha/ilcor guidelines and include hypothermic therapies in their standard package of care for patients with cardiac arrest. some researchers go so far as to contend that hypothermia represents a better neuroprotectant following a blockage of blood to the brain than any known drug. over this same period a particularly successful research effort showed that hypothermia is a highly effective treatment when applied to newborn infants following birth asphyxia. meta-analysis of a number of large randomised controlled trials showed that hypothermia for 72 hours started within 6 hours of birth significantly increased the chance of survival without brain damage. research ttm has been studied in several use scenarios where it has not been found to be helpful, or is still under investigation. stroke there is currently no evidence supporting targeted temperature management use in humans and clinical trials have not been completed. most of the data concerning hypothermia's effectiveness in treating stroke is limited to animal studies. these studies have focused primarily on ischemic stroke as opposed to hemorrhagic stroke, as hypothermia is associated with a lower clotting threshold. in these animal studies, hypothermia was represented an effective neuroprotectant. the use of hypothermia to control intracranial pressure (icp) after an ischemic stroke was found to be both safe and practical. traumatic brain or spinal cord injury animal studies have shown the benefit of targeted temperature management in traumatic central nervous system (cns) injuries. clinical trials have shown mixed results with regards to the optimal temperature and delay of cooling. achieving therapeutic temperatures of 33 °c (91 °f) is thought to prevent secondary neurological injuries after severe cns trauma. a systematic review of randomised controlled trials in traumatic brain injury (tbi) suggests there is no evidence that hypothermia is beneficial. neurosurgery as of 2015 hypothermia had shown no improvements in neurological outcomes or in mortality in neurosurgery. naegleriasis ttm has been used in some cases of naegleriasis.