Improved management of clinical hypothermia based upon related biochemical studies

Physiological Impact of Hypothermia: The Good, the Bad and the Ugly

Hypothermia is defined as a core body temperature of < 35°C, and as body temperature is reduced the impact on physiological processes can be beneficial or detrimental. The beneficial effect of hypothermia enables circulation of cooled experimental animals to be interrupted for 1-2 h without creating harmful effects, while tolerance of circulation arrest in normothermia is between 4 and 5 min. This striking difference has attracted so many investigators, experimental as well as clinical, to this field, and this discovery was fundamental for introducing therapeutic hypothermia in modern clinical medicine in the 1950's. Together with the introduction of cardiopulmonary bypass, therapeutic hypothermia has been the cornerstone in the development of modern cardiac surgery. Therapeutic hypothermia also has an undisputed role as a protective agent in organ transplantation and as a therapeutic adjuvant for cerebral protection in neonatal encephalopathy. However, the introduction of therapeutic hypothermia for organ protection during neurosurgical procedures or as a scavenger after brain and spinal trauma has been less successful. In general, the best neuroprotection seems to be obtained by avoiding hyperthermia in injured patients. Accidental hypothermia occurs when endogenous temperature control mechanisms are incapable of maintaining core body temperature within physiologic limits and core temperature becomes dependent on ambient temperature. During hypothermia spontaneous circulation is considerably reduced and with deep and/or prolonged cooling, circulatory failure may occur, which may limit safe survival of the cooled patient. Challenges that limit safe rewarming of accidental hypothermia patients include cardiac arrhythmias, uncontrolled bleeding, and "rewarming shock".

Surgical hypothermia

Induced hypothermia is an interesting and useful adjunct to therapy in many areas of surgery and medicine. To paraphrase Professor Swan (1973), clinical hypothermia 'has a past and some promise for the future'.

Microwave heating of intravenous fluids

Microwave heating of intravenous (IV) fluids is a viable alternative to heating by conventional means, such as by blood warmer(s) or an on-site warming oven, for administration to hypothermic patients and trauma victims. Three 1-l bags each of lactated Ringer's solution, normal saline solution, 1/2 normal saline solution, and 5% dextrose in water were packaged in parenteral containers and heated in a microwave oven from room temperature (21 degrees C) to 40-42 degrees C in 3 minutes. Little difference between temperatures of the four solutions was detected at each of five intervals up to two hours after heating for 3 minutes. Samples were taken before and after heating to assess any potential alterations in sodium, potassium, chloride, calcium, glucose, and lactate levels; differences were within the range of variation of the methods used. Though the plasticizer in the polyvinyl chloride containers is stable to microwave heating, data on other components is incomplete.

Recurrent thrombocytopenia, erythroid hypoplasia and sideroblastic anaemia associated with hypothermia

A distinct and previously undescribed haematological picture was noted in three patients with hypothermia. During hypothermia there was anaemia with reduced normoblastic erythropoiesis, marked sideroblastic change and thrombocytopenia in the presence of a normal number of megakaryocytes. In two patients as the body temperature returned to normal these changes were slowly reversed. Evidence both from observations in patients and from animal experiments is given why the haematological changes are due to the hypothermia.

Simple deep hypothermia for open heart surgery in infancy

Results of open cardiac surgery under deep simple hypothermia in 121 infants with body weight of less than 10 kg are reported. Deep ether anaesthesia combined with large quantities of ganglion blocking agents (triflupromazine 3 mg/kg) constitutes the anaesthetic management of choice for deep surface-induced hypothermia. The mean lowest oesophageal temperature was 20.8 degrees C, and 18.9 degrees C rectally. The mean circulatory arrest time was 40 minutes. Seventeen infants (14.0 per cent) died post-operatively. There were no operative deaths attributable to failure of cardiac resuscitation. This technique widens the scope of open heart surgery in small infants. Most of the surgically correctable malformations should be operable by this method. More than the potential hazards of hypothermia, which we believe are solved by our technique, the major problem posed by surgery in these small infants is the trans and post-operative respiratory management.

Accidental hypothermia: core rewarming with partial bypass

Three patients with profound hypothermia were treated by rewarming on partial bypass. Two surivived and have normal mental and metabolic functions. The resuscitation of the hypothermic patient should be approached with enthusiasm since the outcome is often much better than expected from initial vital signs and neurologic examination. To avoid ventricular fibrillation the patient should be handled gently and an effort should be made to keep the patient well oxygenated and the pH normal. Blood gases should be measured often and corrected for temperature. The potassium concentration and hydration status of the patient should also be monitored closely. The rewarming of profoundly hypothermic patients can readily be accomplished with a pump oxygenator and heat exchanger. The indications for this method are not established from our small experience and the few cases reported in the literature. Certainly ventricular fibrillation is a compelling indication. Patients with frozen extremities might also benefit from this method since theoretically tissue salvage would be increased. Finally, those patients who do not respond rapidly to external rewarming may be at less risk of ventricular fibrillation if rewarmed on bypass.

Surgical treatment of artrial septal defects in children

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