Hypothermia, a condition where the body’s core temperature drops below 95°F (35°C), significantly complicates internal bleeding. Internal bleeding is the loss of blood from vessels within the body that is not externally visible, and its severity varies by location. When hypothermia and internal bleeding occur together, the bleeding becomes more severe and harder to control, increasing adverse outcomes.
Impaired Blood Coagulation
Cold temperatures directly hinder the body’s natural blood clotting mechanisms. Platelets, small cell fragments that form the initial plug at an injury site, become less efficient and active as body temperature drops. Below 33°C (91.4°F), platelet aggregation and adhesion are significantly reduced, impairing their ability to clump and seal a wound.
The complex coagulation cascade, involving numerous enzymes, is also impaired under hypothermic conditions. These enzymes require specific temperatures to function optimally; colder temperatures slow or stop their activity. This delays thrombin generation, a central step in clot formation.
Ultimately, the final step of forming a stable fibrin clot, which provides the structural meshwork for a strong blood clot, is impaired. Hypothermia affects fibrin clot formation, leading to weaker or absent clots that cannot effectively stop bleeding. It can also decrease fibrinogen synthesis, leading to a deficit in this crucial clotting protein.
Altered Vascular Response
Hypothermia affects the body’s blood vessels, further contributing to increased bleeding. The immediate response to cold is peripheral vasoconstriction, where blood vessels near the skin surface narrow to conserve heat. While this reduces heat loss, it can paradoxically worsen internal bleeding by shunting blood toward the core and increasing pressure in larger, injured vessels.
As hypothermia becomes more severe, the body’s ability to maintain sustained vasoconstriction can fail, leading to paradoxical vasodilation. This widespread relaxation and widening of blood vessels allows more blood to flow to injured areas, intensifying internal bleeding.
Cold temperatures can also damage the delicate inner lining of blood vessels, called the endothelium. This damage makes capillaries more permeable and fragile, allowing fluid and blood to leak more easily into surrounding tissues. Such leakage contributes to overall blood loss and swelling.
Metabolic Acidosis
Hypothermia contributes to metabolic acidosis, an imbalance where the blood becomes too acidic, further hindering the body’s ability to stop bleeding. Lower body temperatures slow cellular processes, including oxygen delivery and waste removal.
When tissues lack sufficient oxygen due to slowed circulation and impaired metabolism, they switch to anaerobic metabolism. This less efficient energy production method produces lactic acid as a byproduct.
Lactic acid accumulation lowers the blood’s pH, resulting in metabolic acidosis. This acidic environment directly inhibits the function of clotting factors and platelets, creating a detrimental feedback loop. Low pH levels reduce both clot formation and firmness, impairing the body’s clotting machinery.
The Vicious Cycle of Worsening Bleeding
Hypothermia, impaired blood coagulation, altered vascular responses, and metabolic acidosis are interconnected elements that create a dangerous, self-perpetuating cycle in internal bleeding. Hypothermia directly triggers impaired clotting and problematic vascular responses, while also leading to metabolic acidosis.
Each consequence exacerbates the others, forming a “vicious cycle” often termed the “lethal triad” in trauma. Increased bleeding leads to further heat loss, worsening hypothermia. Ongoing bleeding also contributes to shock, which can deepen acidosis due to poor tissue perfusion.
This cycle means that as the body gets colder, it bleeds more, becomes more acidic, and its circulation further deteriorates, making it less capable of stopping the hemorrhage. Recognizing and actively managing hypothermia is important for improving patient outcomes in situations involving significant blood loss.