Hypothermia is defined mild when the body temperature is 32-37°C, moderate when the temperature is 29-32°C and severe when it is below 27°C. The diagnosis of severe hypothermia is made by measuring the body temperature using specific sensors for low temperatures present in some digital and mercury thermometers. Passive systems (e.g. blankets) are sufficient to treat mild hypothermia, whereas active systems (e.g. mats containing circulating hot water) are needed for moderate and severe hypothermia. Patients with mild hypothermia may survive for about 24 hours, whereas when the body temperature is below 27°C for more than 12 hours the prognosis is poor.
The main mechanisms by which the body generates and retains heat are metabolism, vasoconstriction, movement, piloerection and shivering; in the colder months, the stimulated increase in hair and fat deposits contribute to retaining the heat produced. During hypothermia the body reacts by reducing conduction (transfer of heat between the body and surfaces in contact with the body), evaporation, convection (loss of heat due to movements of fluid within the body) and radiation, first with peripheral vasoconstriction, which greatly decreases the dispersion of heat and then, if the vasoconstriction is not sufficient, by reducing the heart rate, cardiac output and renal perfusion and by stimulating adrenergic activity.
Hypothermia occurs when the above-mentioned mechanisms of producing and preserving heat are compromised or when there are major losses of heat as a result of increased convection or conduction because of exposure to low temperatures. The severity of the hypothermia depends on the factors that have caused it, on the continued exposure to these factors and on the response of the body. Puppies and elderly animals are more prone to hypothermia because of an imbalance in these groups of animals between metabolic requirements to maintain the body temperature (very high in puppies) and the thermoregulatory capacity (thermoregulatory mechanisms are immature in puppies and impaired in elderly animals). Hypothermia is often also found in severely ill or very debilitated animals because of their incapacity to produce heat and prevent its dissipation. Small animals are more prone to develop hypothermia because of the high ratio between body surface and body volume. Healthy, adult cats and dogs (especially the latter) tolerate cold environments well; in these animals hypothermia is usually the consequence of an impairment, albeit temporary, in the central mechanisms of thermoregulation and peripheral vasodilatation caused by the administration of sedative drugs or anaesthetics. Cases of hypothermia following drowning or exposure to cold environments are rarer1 than the cases due to altered thermoregulation.
AETIOLOGY
The most common causes2 of hypothermia are:
- toxic substances: anaesthetics (particularly barbiturates), carbon monoxide, alcohol;
- renal impairment, uraemic syndrome;
- sepsis, severe sepsis, septic shock, multiple organ dysfunction syndrome (MODS);
- severe haemorrhage;
- severe trauma;
- head injury;
- brain tumours;
- hypothalamic lesions;
- cerebrovascular lesions;
- encephalopathies;
- hypoglycaemia;
- hypothyroidism;
- pituitary failure;
- hypoadrenocorticism;
- shock (hypovolaemic, distributive and cardiogenic);
- anorexia, malnutrition, prolonged fasting;
- prolonged exposure to low temperatures.
SIGNS AND SYMPTOMS OF HYPOTHERMIA
The signs and symptoms of mild hypothermia (26-34°C) tend to worsen with the degree of hypothermia. The most common findings are listed below:
- muscle tremors;
- slow respiratory rate (in the initial phase, tachypnoea may occur);
- bradycardia;
- hypotension;
- altered state of consciousness (obfuscation, stupor, coma);
- altered metabolism;
- altered muscle contraction;
- polyuria.
The possible findings in subjects with moderate hypothermia (30-32°C) are:
- hypovolaemic shock;
- metabolic acidosis;
- electrolyte imbalances;
- hypoglycaemia;
- reduced muscle contractions;
- loss of thermoregulation.
Severe hypothermia (20-28°C)may cause:
- arrhythmias with a high risk of ventricular fibrillation and cardiac arrest;
- mydriasis;
- pulmonary oedema;
- reduced perfusion;
- reduced oxygen availability (DO2).
MANAGEMENT OF THE HYPOTHERMIC PATIENT
History
It is useful to collect the following information when taking the history:
- duration of exposure to cold;
- any previous episodes of hypothermia;
- debilitating chronic diseases;
- ongoing treatments or intake of any toxic substances;
- circumstances in which the symptoms occurred and their duration.
Once triage has been performed and the vital parameters measured (heart rate, pulse and its characteristics, respiratory rate, colour of the mucosae and blood pressure) the following parameters should be monitored at regular intervals (1-2 hours):
- rectal temperature;
- rate and type of breathing;
- state of consciousness and any neurological deficits;
- muscle stiffness;
- pulse and blood pressure;
- oxygen saturation.
Lavoratory examinations
The laboratory examinations to perform as soon as possible are:
- biochemistry (an initial hyperglycaemia may be followed by hypoglycaemia, hypoproteinaemia);
- blood-gas analysis (metabolic acidosis and uncompensated respiratory acidosis);
- electrolytes (at least Na+, K+, and Cl-);
- full blood count;
- coagulation tests (possible disseminated intravascular coagulation);
- urinalysis (e.g. casts).
Electrocardiographic changes
Electrocardiographic (ECG) monitoring should be performed in all patients because the following may be found:
- arrhythmias;
- prolongation of the P-R interval;
- alteration in the QRS complex (usually widened);
- inverted T waves;
- Osborne or J wave.
The J wave consists in the appearance of an anomaly in the final part of the QRS complex and the initial part of the ST segment (Fig. 1)3 and occurs in some hypothermic subjects. The J wave is slow and rounded and is named J (for Junction) because it occurs at the junction between the rapid ventricular complex and the subsequent isoelectric segment. The underlying electrophysiological mechanism is still controversial; its presence could indicate a late depolarisation or early repolarisation or a lesional electrical current due to an area of myocardial ischaemia.4
Diagnostic imaging
In cases of severe hypothermia the following should be performed:
- chest X-ray (because of possible pulmonary oedema);
- ultrasound of the abdomen (because of possible traumatic and pancreatic lesions).
Treatment
A large part of the patient’s body heat is dispersed by conduction (from the patient’s body to surfaces in contact with it). In order to reduce this phenomenon, the patient must be isolated from cold surfaces by using insulating blankets or foam rubber padding. To reduce convection and radiation to the surrounding air the patient can be isolated with insulating covers made for this purpose (e.g. aluminium sheets), ordinary synthetic or woollen blankets or, in an emergency and in the absence of other more appropriate material, plastic bags. It is important to dry the patient carefully since evaporation causes heat loss from the surface of the body.
In cases of moderate or severe hypothermia, the distal parts of the animal can be wrapped with aluminium sheet or bubble packaging to reduce heat loss from the extremities. When the body temperature falls below 30°C, active warming is needed. This can be achieved by using external sources of heat such as hot air generators, mats heated by the recirculation of hot water, or incubators. The hot air generators are considered the most effective and safest methods. The active warming systems also include infrared lamps; these must be powered through a potentiometer to regulate the intensity and the main beam must never be directed at the patient, particularly if it is unconscious or insensitive to pain (e.g. when receiving analgesics) in order to avoid burns, which may appear even a few days later. Thermal blankets, hot water containers and gel cushions (which change state through an exothermic reaction, becoming rigid, and maintain a constant temperature for 1-2 hours) must not be placed in direct contact with the patient but be separated by an isolating material (e.g. a cloth towel) in order to prevent burns in poorly vascularised areas. So that the heat produced by these systems is not dispersed, the patient can be put under a blanket next to the heat source (e.g. gloves containing hot water) but not in direct contact. Electric blankets or warming mats can cause exemia, because of the vasodilatation produced by the heat in the areas of contact, and second or third degree burns.
Another way of raising the body temperature is to administer crystalloid solutions (e.g. Ringer’s lactate) warmed to 40-45°C. These solutions can be administered by the intravenous, intrapleural or intraperitoneal route. Intrapleural or intraperitoneal infusion is performed by placing a catheter for temporary peritoneal dialysis in the flank and infusing 10-20 ml/kg of Ringer’s lactate every cycle. The procedure is repeated every 30 minutes until the patient’s body temperature reaches 37.5°C; the body temperature should increase by about 1°C per hour. Patients can also be warmed by administering humidified air or a mixture of air and oxygen (FiO2 of 40%) through an orotracheal tube or a face mask. Patients that are breathing adequately should receive tepid, humidified oxygen therapy (FiO2 of 40%). If the animal is not ventilating adequately (paCO2 >50 mmHg or paO2 ≤60 mmHg despite oxygen therapy) orotracheal intubation and positive pressure ventilation are necessary. When the blood glucose level is below 60 mg/dl, 2.5% glucose should be added to the cystalloid solution and the glycaemia should be monitored every 1-2 hours until it stabilises.
Potential complications of active warming include disseminated intravascular coagulation, arrhythmias (with possible cardiac arrest), pneumonia, renal failure, central nervous system oedema and acute respiratory distress syndrome.
Hypothermia during general anaesthesia
Before starting prolonged general anaesthesia, all the strategies aimed at preventing conduction hypothermia should be used, such as insulating mats containing polystyrene which also prevent decubitus ulcers. The body temperature must be maintained between 36°C and 38°C,5 by heating the operating theatre adequately and using active warming systems (e.g. heated tables and recirculating hot water mats).
Preventing heat conduction is usually sufficient to prevent hypothermia. However, when surgery is prolonged and the body cavities (abdomen and thorax) are exposed, the dispersion of heat is increased because the cavities also lose heat through convection and radiation and compensatory mechanisms are affected by the anaesthetics. In these situations active warming systems must be used, such as hot air devices or recirculating hot water mats.
Another way to warm the body in the case of hypothermia is to irrigate the abdominal cavity with warmed physiological saline (38-40°C) which, besides supplying heat, also contributes to reducing the amount of evaporation. Hypothermia can also result from the inhalation of cold gases (e.g. oxygen) and evaporation; this contribution to the problem can be decreased by using anaesthetic circuits with gas recirculation (e.g. semi-closed circuits) which reduce dispersion of heat from the respiratory apparatus.
Once the general anaesthesia has finished, active heating systems must be used because the body may not be able to preserve its temperature in the first few hours of the post-operative period.
Therapeutic hypothermia
Hypothermia should not always be considered a negative phenomenon. For example, in human medicine it has been observed that the controlled use of hypothermia can protect nerve cells following a cardiac arrest.6 The hypothermia limits lesions in the brain, preventing cellular apoptosis.7 In veterinary medicine it has been seen that moderate hypothermia during cerebral and cardiopulmonary resuscitation increases the survival rate of patients and prevents organ dysfunction8. In the dog, hypothermia has also been used in the treatment of haemorrhagic shock.9
References
- Taylor PM Ipotermia. In Ettinger, feldman et al. Clinica medica veterinaria:malattie del cane e del gatto, ed. Elsevier Saunders, 2003 pgg 14-17
- OnckenAK, Kirby R, Rudloff E Hypotermia in critically ill dogs and cats.Comp Cont Educ Small Anim Pract 2001; 23:506
- Santilli RA, Perego M. Manuale di elettrocardiografia del cane e del gatto. Elsevier/Masson Milano 2007; pp 201-212
- Porciello F, Elettrocardiografia nel cane, nel gatto e nel cavallo Ed. Poletto MIlano, 2003; pp 64-66
- Armstrong SR, Roberts BK, Aronsogn M. Perioperative hypotermia J Vet Emerg Crit Care 2005; 15:32
- Kochanek PM, Drabek T, Tisherman SA Therapeutic Hypothermia: the Safar visionJ Neurotrauma 2009; 26(3): 417-420
- Curfman DG. Hypothermia to protet the brain. N Engl J Med 2002; 346:546
- Nozari A, Safar P, Stezoski SW. Mild hypothermia during prolonged cardiopulmonary cerebral resuscitation increases conscious survival in dogs.Crit Care Med. 2004; Oct;32(10):2110-6
- Tisherman SA, Safar P, Radovsky A, Peitzman A, Sterz F, Kuboyama K. Therapeutic deep hypothermic circulatory arrest in dogs: a resuscitation modality for hemorragic shock with ‘irreparable’ injury.J Trauma 1990 30(7):836-847