Blood Loss and the Lethal Triad

Symptoms of blood loss:

500 mL - well tolerated, may produce slight tachycardia, equivalent to a typical                       blood donation volume.

1000 mL - tachycardia over 100

1500 mL - changes in mental status, weak radial pulse, persistant tachycardia,                         tachypnea

2000 mL - confusion, lethargy, weak radial, tachycardia over 120, tachypnea over                     35, might be fatal if not managed properly

2500 mL - unconsciousness, no palpable radial pulse, tachycardia over 140,                             tachypnea over 35, fatal without intervention

 In patients with blood loss, the "Lethal triad"- is a self-reinforcing cycle of acidosis, hypothermia, and coagulopathy.

Acidosis: Reduced circulating blood volume leads to shunting of blood from the periphery to vital organs in the core. Peripheral tissues resort to anaerobic metabolism, which creates lactic acid as a byproduct. This can be worsened by administration of large volumes of non-oxygen-carrying, acidic fluids, such as normal saline (pH 5.5).  

Hypothermia: Develops easily and rapidly in trauma patents, even in warm conditions. Anaerobic metabolism, immobility, and other physiological responses to blood loss reduce heat production. Evidence shows that even small drops in body temperature (to 36C / 96.8F) can significantly increase mortality in trauma and burn patients. At core temperatures below 30C / 86F, patients stop shivering and cannot warm up without application of external heat, even if they are well-insulated. It is far easier to prevent hypothermia than to correct it. 

Coagulopathy: Clot formation depends on a complex series of pH- and temperature-dependent chemical reactions. Loss of clotting factors due to bleeding, acidosis, and hypothermia all produce coagulopathy, which in turn further exacerbates blood loss, acidosis, and hypothermia. 

Preventing the Lethal Triad cycle is crucial; once established, it is difficult to interrupt. Field treatment of patients with significant blood loss should include oxygen, insulation from the ground, covering with blankets/space blankets/ready-heat systems, and placement in a heated environment if possible.

Trauma’s Lethal Triad of Hypothermia, Acidosis & Coagulopathy Create a Deadly Cycle for Trauma Patients