Pathophysiology

Haemostasis is a group of complex and inter-related processes maintaining integrity of blood vessels both in normal conditions as well as after vessel damage. The role of haemostasis in case of vessel wall damage is formation of clot, followed by fibrinolysis and restoration of normal flow.

Proper functioning of haemostasis is enabled by plasma components, mainly platelets, blood vessel wall (vascular endothelium), as well as coagulation and fibrinolysis systems. In normal conditions, balance between coagulation and fibrinolysis systems is maintained.

Coagulation system, particularly in its plasmatic aspect, is a series of enzymatic reactions vitally dependent on temperature and pH. The optimal functioning of the system coincides with physiological body temperature, hence drop in body temperature entails significant disturbances in the process.

Coagulopathy in hypothermia is a result of decrease in activity number of platelets, reduction in activity of plasma coagulation factors, and enzymatic processes [1–3]. Relationship between drop in blood temperature and occurrence of coagulopathy, however, is not linear. It is accepted that decrease of 1oC of body temperature causes decrease of function of coagulation system by 10%.

Hypothermia in the range 37–33°C causes mainly platelets adhesion disorders without disturbing significantly neither their activation nor enzymatic activity of plasma factors [1–4]. Furthermore, an increased sensitivity of blood platelets to pro-coagulation factors is observed, what translates into increased capacity to form clots in peripheral parts of the body, more exposed to potential injury [4]. Only in temperature lower than 33°C significant haemostasis disorders are observed, related mainly to reduced activity of thrombocytes and enzymatic activity [1, 2, 5].

Paradoxically, in chronic hypothermia (with polyuria and dehydration) haemoconcentration with increase of hematocrit by 2% for each °C < 34°C may occur [6, 7], as well as vasoconstriction, release of tissue thromboplastins, and increase in fibrinogen concentration. These may lead to clotting and occurrence of embolism.