The non-haemorrhagic vagal response to trauma: a review of hypotensive and bradycardic responses to injury in the absence of bleeding

PURPOSE: Trauma has the potential to cause haemorrhage, tissue damage, pain, visceral manipulation and psychological distress. Each of these consequences of trauma can cause changes in autonomic outflow, which dictates a patient's vital signs. Patients who are hypotensive and bradycardic due to a vagally mediated parasympathetic response to pain, psychological distress and visceral manipulation may be confused with those who exhibit bradycardia and hypotension following significant blood volume loss. METHODS: This review summarises literature that describes specific stimuli, patterns of injury and patient characteristics that are associated with a non-haemorrhagic vagal response to trauma. RESULTS: Twenty-six records described predominantly parasympathetic responses to trauma (both blunt and penetrating) and surgery ("iatrogenic trauma"). Such a non-haemorrhagic vagal response occurs following a wide variety of injury patterns. Patient age and sex are poor predictors of the likelihood of a non-haemorrhagic vagal response. The development and resolution of a non-haemorrhagic vagal response occurs over a heterogenous time period. It is unclear whether speed of onset and resolution is linked to the pattern of injury or other factors causing a predominantly parasympathetic response following non-haemorrhagic trauma. CONCLUSION: The pattern of injury, patient demographic and speed of onset / resolution associated with the non-haemorrhagic vagal response to trauma may is heterogenous. It is therefore challenging to clinically distinguish between the hypotensive bradycardia due to hypovolaemia secondary to haemorrhage, or a parasympathetic response to trauma in the absence of bleeding.