Pathophysiology of lung injury
Injury results from MVC due to acceleration, deceleration and crush forces. There are three points of contact involved in MVCs: firstly the impact of the vehicle with another object; secondly the body impact with internal structures of the car; and thirdly the shifting of internal organs until they collide with other internal structures. A rib fracture occurs when the thoracic cage is unable to absorb force applied to the chest. Underling lung injury occurs in rib fractures because of the pliability of the bone. The ribs bend prior to fracturing which damages the lung tissue located underneath the ribs impacted. While less common, lung injury can occur in the absence of rib fractures, particularly in patients 25 years old or younger as their bones are more flexible and less susceptible to fracture.
The purpose of the breathing is to “supply oxygen to cells and remove carbon dioxide”. This is achieved through ventilation, the movement of air in and out of the lungs and respiration, the exchange of oxygen and carbon dioxide across the alveolar and peripheral capillary membrane. The combinations of injuries displayed in the case study impair ventilation and oxygen exchange by reducing vital capacity, weakening the patients’ ability to clear secretions and decreasing surface area available for oxygenation. This was clearly demonstrated by the patient's signs of respiratory compromise and subsequent pneumonia.
The presence of rib fractures recruit accessory muscles as injury increases workload and causes fatigue of respiratory muscles used in passive breathing. The ability of the lung to ventilate is impaired and lower airways collapse decreasing surface area available for respiration. The small haemothorax may have impaired lung function as the accumulation of blood in the pleural cavity compresses alveoli and prevents the lung from fully expanding. Contusion or bruising to the lung produces inflammation of the lower airways resulting in the collection of water and blood in the alveoli. This causes obstruction resulting in increased airway pressures and subsequently respiratory effort. Less oxygenated air moves in and out of the lungs reducing the surface area available for respiration to occur. Alveolar consolidation and collapse result, explaining the patient's impaired oxygenation.
Lung contusions may also result in hypovolaemic shock as seen in the case study manifested by hypotension, persistent tachycardia and drop in the haemoglobin. This is due to blood pooling into the lung tissue as a result of the injury. The development of respiratory failure may have also contributed to this which causes fluid to shift from alveoli capillaries into the lung, decreasing circulating blood volume. The blood pressure drops as a result of the decrease pressure in the intravascular space and heart rate increases as a compensatory mechanism. The combination of hypotension and hypoxia reduces cerebral perfusion and is responsible for the suppressed level of consciousness in the case study.