Hypothermia

Recognition

Hypothermia is defined as a core temperature less than 35°C (Steedman 1994).

Mild to moderate hypothermia is defined as a core temperature of 30-32°C and severe hypothermia as a core temperature of below 30°C (Guly & Richardson 1996).

• If there is rapid cooling prior to the development of hypoxia, decreased oxygen consumption and metabolism may precede the cardiac arrest and reduce organ ischaemia (Larach 1995). Therefore becoming cold very quickly can exert a protective effect on the brain and other vital organs during cardiac arrest (Larach 1995).

• Using a tympanic thermometer will give a fast reading to confirm the diagnosis. Oesophageal temperature monitoring may be used in a critical care area.

It is imperative to record the patient's temperature as soon as possible as it will directly affect the management of the cardiac arrest. If the first three shocks in pulseless ventricular tachycardia or ventricular fibrillation (VF/VT) are unsuccessful, then further attempts at defibrillation should be deferred until the core temperature is greater than 30°C (Resuscitation Council UK 2000a), as they are very unlikely to be successful and will simply cause further damage to the myocardium.

In PEA the ECG may show evidence of J waves. These are seen at the junction of the QRS complex and ST segments. They are thought to be due to hypothermia-induced ion fluxes causing delayed left ventricular depolarisation or early repolarisation (Steedman 1994).

Drugs should also be withheld until the core temperature is above 30°C, as administration will simply lead to pooling of the drugs in the peripheral circulation without any direct action on the heart.

Response

Once the diagnosis is established, passive rewarming methods such as using warm blankets will be ineffective if the patient has lost their cardiac output (Larach 1995). Active and rapid rewarming is essential in order to reverse the cause of the cardiac arrest (Carson 1999). Active rewarming includes:

• the administration of warmed (42-46°C) and humidified oxygen;

• administering warmed (40-43°C) intravenous fluids;

• catheterising the patient and carrying out a bladder washout with warmed fluids;

• peritoneal lavage with warmed fluid (which must be potassium free);

• the gold standard is extracorporeal blood warming (by-pass) but only a small percentage of hospitals can offer this treatment. It would only be available in centres where cardiac by-pass operations or haemodialysis are offered (Wollenek et al. 2002).

It is important to remember that during a prolonged arrest, a patient who was normothermic may become hypothermic (Resuscitation Council UK 2000a).

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