What's new in emergencies, trauma and shock? Mechanical ventilation in trauma patients: A tight-rope walk!

A large proportion of trauma patients need mechanical ventilation. They are at a great risk of developing acute lung injury (ALI) and acute respiratory distress syndrome (ARDS).[1] Conditions like pulmonary contusion, shock, multiple transfusions, fractures, aspiration, near drowning, smoke inhalation, and fat embolism predispose trauma patients to development of ARDS.[1] Sometimes mechanical ventilation may be needed in the absence of lung injury, for example, a brain injured patient may require intubation and ventilation for airway protection or for prevention of secondary brain injury.

Lung protective ventilation strategy using lower tidal volumes and plateau pressures with high positive end-expiratory pressure (PEEP) and permissive hypercapnia has become widely accepted for management of ALI and ARDS and the same is advocated for the majority of chest trauma patients. This strategy is aimed at preventing ventilator associated lung injury (VALI), which results from high lung volumes (volutrauma), high airway pressures (barotrauma), and repetitive opening and closing of collapsed lung units (atelectrauma). Lung injury can propagate proinflammatory cytokine cascade resulting in biotrauma.[2] All these problems can be avoided by adopting lung protective ventilation. However, the management becomes tricky when lung injury occurs in concurrence with head injury. ARDS develops in up to 20% patients suffering from severe head injury.[3] The ventilatory management goals for lung and brain injury are in conflict with each other. High PEEP and hypercapnia, major components of lung protective ventilatory strategy, can increase intracranial pressure (ICP) and reduce cerebral perfusion pressure (CPP), thus worsening the brain injury. Therefore one needs to use a very cautious and balanced approach to achieve good outcome in trauma patients.

In this issue of JETS,. Arora, et al. have highlighted various ventilatory strategies used in trauma patients.[4] They have described common injuries to different organ systems and their management principles. Independent lung ventilation has an important role in cases of massive chest trauma where isolation of lungs is required. In most of the patients with traumatic injuries of chest, protective lung ventilatory strategy remains the basic principle. Some other ventilatory modes based on open lung concept, for example, airway pressure release ventilation (APRV), high frequency oscillatory ventilation (HFOV), and high frequency percussive ventilation (HFPV) also help in preventing and managing VALI. Extracorporeal membrane oxygenation (ECMO) provides rest to the lungs by facilitating gas exchange outside the body. Other newer techniques like closed loop ventilation seem to have potential for further improving ventilatory management of critically ill patients.

APRV appears to be a promising technique that improves lung recruitment, oxygenation, end-organ blood flow, pulmonary vasoconstriction, and sedation requirements.[5] However, any reduction in mortality has not been proven; therefore trials directly comparing APRV with standard lung protective ventilation in ARDS/ALI patients are needed. There have been encouraging results from a recent animal study demonstrating that early application of APRV stabilizes alveoli, reduces alveolar edema, and thus prevents development of ARDS.[6]

HFOV has been found to decrease mortality when compared with conventional ventilation. However, it is yet to be seen if it has any advantage over current lung protective ventilatory strategies.[7] Some retrospective studies have shown good results with HFPV in trauma patients having ARDS with or without head injury. This technique produced significant improvement in oxygenation with reduction in ICP during first 16 hours, when applied in head injured patients with ARDS.[3] However, further trials are required to evaluate the long-term outcome and reduction in mortality, if any.

In patients with traumatic brain injury and ALI/ARDS, a strategic ventilatory approach, maintaining a balance between principles for brain injury management and protective lung ventilation is required. High levels of PEEP have a potential to increase intrathoracic pressure, thus decreasing cerebral venous drainage and in turn compromising cerebral perfusion. However, in case of noncompliant lungs, as is the situation in patients with ALI/ARDS, transmission of intrathoracic pressure to cranium and thus effect on cerebral perfusion is lesser.[8] Thus PEEP can be safely applied in this group of patients, provided the volume status and mean arterial pressure are maintained and the level of PEEP provided is lower than ICP.[8] The lowest level of PEEP that maintains oxygenation should be applied. Head elevation with avoidance of extreme neck rotation and tight endotracheal tube ties around the neck are quite helpful. However, arterial carbon dioxide tension plays an important role in determining cerebral blood volume, intracranial compliance, and ICP.[8] Hence hypercapnia should be avoided and all efforts should be made to maintain normocarbia.

Thus the outcome of trauma patients can be improved by following principles of ventilatory management according to the organ systems involved. Further studies using newer ventilatory strategies are needed to find optimal solutions to the conflicts faced in polytrauma patients.