Table 4 Main physiological effects of severe hypothermia

CARDIOVASCULAR [79, 230, 267]

• Initial vasoconstriction (effect blocked by ethanol). Vasoconstriction fails <24 °C [268].

• Failed vasoconstriction means the patient becomes poikilothermic i.e. dependent on ambient temperature.

• Cardiac conduction is affected by cold and changes in pH and PaO₂ [79]. Initial tachycardia due to shivering [79] subsides as temperature drops due to decreased spontaneous depolarization of pacemaker cells leading to linear fall in pulse rate (~50 % at 28 °C) [79]. Any ECG rhythm is possible. Commonly at <32 °C, sinus bradycardia, prolonged QTc. J waves (not pathognomonic for hypothermia) best seen in leads I & V6 [79, 269–272]. Likelihood of VF is high <28 °C [267].

• Bradycardia is atropine unresponsive [79].

• A “relative” tachycardia inconsistent with patient’s temperature means something else is going on e.g. occult trauma.

• Be prepared for any rhythm but expect it to be resistant to treatment until the heart rewarms.

• Normal rhythm resumes on rewarming.

• Cardiac output falls to 45 % at 25 °C [79].

• Hypotension is the norm.

• After rewarming, mean arterial pressure, contractility, and cardiac output are decreased, especially if alcohol ingested before cooling [273].

• More prolonged depression of cardiac function after rewarming

CENTRAL NERVOUS SYSTEM

• Reflexes become increasingly sluggish as body temperature falls and become absent ≈ 28–30 °C [230, 274].

• Pupils become dilated and cease reacting to light at ≈ 28 °C [230].

• EEG shows burst suppression ≈ 22 °C and becomes isoelectric ≈ 18–20 °C [79, 275].

• The level of consciousness should be consistent with the core temperature. A significant discrepancy suggests an alternative diagnosis.

• All the effects of hypothermia make it very hard to diagnose death by the usual criteria while the patient is still cold

RESPIRATORY

• Tidal volume, respiratory rate, pulmonary compliance and thoracic elasticity decrease [230]. The respiratory rate may only be five breaths per minute when the body temperature is <30 °C [79]. Sensitivity to CO₂ is attenuated, although the hypoxic drive is maintained to deeper levels of hypothermia [230]. Cough reflex is obtunded, ciliary activity is reduced and secretions are more viscous.

• An irregular respiratory pattern can be mistaken for agonal breathing leading to premature institution of CPR.

• The likelihood of a chest infection is increased.

• Oxygen consumption and carbon dioxide production fall by about 50 % at 30 °C [230]

• Reduced CO₂ production means it is easy to inadvertently hyperventilate hypothermic patients. Hyperoxia is also possible.

RENAL & METABOLIC

• Cold diuresis, partly due to the relative central hypervolaemia resulting from peripheral vasoconstriction [79], but also from a reduction in ADH release and resistance to its effects [230]. Alcohol will further increase the diuresis.

• Severely hypothermic patients are dehydrated. This becomes particularly important during rewarming as the consequent opening up of the peripheral circulation will lead to a rapid fall in BP.

• Hyperglycaemia is common due to catecholamine-induced glycogenolysis, decreased insulin release and inhibition of insulin transport [79, 267].

• Hyperglycaemia can exacerbate the diuresis.

• Glomerular filtration rate falls as cardiac output and hence renal blood flow fall [230]. At low temperatures, tubular capacity for H⁺ secretion is reduced, and hence there is a renal component of the acidosis [230].

• This makes the interpretation of acid-base more complex.

• Hypokalaemia commonly occurs with hypothermia [230].

• If potassium replacement is given excess to the losses, hyperkalaemia may occur on rewarming [276, 277].

• Severe initial hyperkalaemia is a marker of acidosis and cell death and is therefore a sign of poor prognosis [8]

HAEMATOLOGY

• Haematocrit increases by about 2 % for every 1 °C decline in temperature [250].

• A normal haematocrit in a moderately or severely hypothermic patient suggests pre-existing anaemia or blood loss [230].

COAGULATION

• Platelet function and coagulation enzyme activity are reduced [278].

• Coagulopathy is likely and increases with decreasing core temperature. At temperatures below 33 °C coagulopathy significantly increases mortality in patients with concomitant trauma [279].