The factors that determine survival following life threatening injury

Survival following life threatening injury is determined by a number of factors that can be broadly classified as those related to the injury and those related to the host (patient factors). Therefore, the severity and nature of the tissue injury are key factors that determine survival. Patient factors that determine survival are age, concomitant comorbid medical diseases and physiological reserve. 

In humans the stress response to life threatening tissue injury is considered to be an important factor determining outcome from life threatening tissue injury. Whilst the stress response is an essential survival response that supports tissue healing and host recovery, this response is also associated with deleterious consequences following injury. The stress response may be considered as a double edged sword with both beneficial as well as adverse effects on the host. 

Adverse effects on the host from the stress response to injury may be categorised as sequelae arising firstly from failure to maintain homeostasis and vital organ function and secondly from deleterious effects of the immune response in response to tissue injury. The former category is linked to the serious clinical condition multiple organ dysfunction syndrome (MODS), whilst the latter group of sequelae is associated with impaired wound healing and the consequences of this outcome .

The double edged sword concept of the stress response to injury becomes more important with increasing severity of tissue injury. Increases in the severity of tissue injury activate a corresponding increase in the magnitude of the stress response necessary to achieve tissue healing and recovery. With increasing severity of tissue injury the host is required to direct a greater proportion of physiological reserves to wound healing and preserving vital organ function . This results in greater depletion of metabolic reserves as well as potentially greater tissue damage resulting from the actions of inflammatory mediators. Consequently increasing severity of tissue injury is associated with increased morbidity and mortality rates as well as greater long term health complications. 

The severity of the injury may be quantified by measuring the concentration of endocrine and inflammatory mediators in the bloodstream. Alternatively factors such as increases in the host metabolic rate or oxygen consumption also reflect the severity of the metabolic response to injury. Both high concentrations of the stress response mediators and increases in metabolic rate are associated with excess morbidity and mortality reflecting greater severity of injury.

Separating the adverse effects from the beneficial effects of the stress response to injury is a goal that has remained elusive to clinicians. Determining the component of overall morbidity and mortality that is attributable to the stress response to injury has been studied in different settings following life threatening injury. The clinical setting of tissue injury associated with surgery is a practical method of studying this important area of clinical practice. Attempts to block the surgical stress response either with pharmacological agents or with regional anaesthetic techniques have been shown to have only a limited effect on reducing mortality. Within surgical practice a strategy that minimises tissue injury has successfully improved mortality and morbidity through limiting tissue damage and reducing the magnitude of the stress response to injury. The introduction of minimally invasive surgical techniques into routine surgical practice has reduced postoperative complications following surgery and improved recovery back to baseline functional status.  However, whilst these surgical techniques reduce the surgical stress response and improve morbidity and mortality, this does not answer the question as to whether the surgical stress response is beneficial or detrimental to the host outcome following life threatening injury.   

Potentially life threatening complications following severe tissue injury are common and may include delayed wound healing, infection or vital organ dysfunction. Common adverse sequelae also include prolonged hospital stay, delayed recovery back to pre-injury baseline health and chronic health complications. Importantly the surgical recovery initiative Enhance Recovery after Surgery (ERAS) highlights the importance of the stress response to injury in determining the outcome following surgery. ERAS aims to minimise the stress response to injury by limiting tissue injury associated with surgery by using minimally invasive surgical techniques. This has transformed modern surgical techniques and justified investment in robotic and laparoscopic surgical techniques.