Air embolism: an unusual cause of delayed death following gunshot wound to the chest

Introduction

Mechanisms of death following lethal gunshot wounds vary depending on the region of the body that has been traumatized, the calibre of the projectile and the type of weapon used. For example, gunshot wounds to the head often result in rapid death due to severe injuries ranging from skull fractures with cerebral laceration to almost complete parenchymal disruption. ¹ Lethal gunshot wounds to the chest and abdomen usually cause haemorrhagic shock due to vascular and/or organ injuries. Air embolism may also occur in any of these cases if the integrity of the walls of large veins or venous plexuses have been compromised. However, this usually occurs soon after projectile impact. ² Delayed death following gunshot injury usually results from multiorgan failure, sepsis or hypoxic ischaemic encephalopathy. A rare case is reported of delayed death due to air embolism resulting from gunshot wounds to the chest that did not involve direct major organ or blood vessel laceration.

Case report

A 29-year-old man who sustained two bullet wounds to the chest was conscious and was transferred to hospital for assessment and treatment. Three bullet wounds were noted in the right side of the chest and one in the right upper arm. Chest X-ray showed a fragmented bullet in the right shoulder with ultrasound revealing that neither projectile had entered the chest cavity. This was confirmed on computerized tomography of the chest, which also showed extensive contusion of the right lung. Major vessels were intact and there was no significant blood loss. The patient's condition was considered stable with improving oxygenation. Five hours after his admission, however, he suffered an unexpected cardiac arrest.

An emergency thoracotomy was performed which showed massive contusion of the middle lobe of the right lung. A right middle lobectomy was performed. There were no injuries to the heart and there was no evidence of a haemopericardium. It was noted that while the right ventricle was contracting normally, the left ventricle was dilated and poorly contracting with air observed in the left coronary artery system. Resuscitative measures were not successful.

At autopsy two gunshot wounds were identified. The first projectile had entered the right anterior chest, passing upwards, backwards and to the right through the soft tissues of the chest wall to terminate in the right scapula. The second gouged the right upper arm and then entered the lateral aspect of the right side of the chest passing through the soft tissues of the anterior aspect of the chest to exit to the right of the midline. Neither projectile had entered the chest cavity or caused significant vascular damage. Examination of the surgically excised middle lobe of the right lung confirmed marked contusion with extensive intra-alveolar haemorrhage. No lethal injuries were demonstrated at autopsy. Toxicology on admission blood revealed a high but not lethal level of methylamphetamine, and a high level of morphine (the latter in keeping with medical administration). Death was therefore attributed to air embolus complicating gunshot wounds to the chest. Air embolism was not checked for at autopsy given the presence of the thoracotomy and the history of prolonged external cardiac massage.

Discussion

An embolus is a foreign body, solid, liquid or gas, which ‘migrates from one region of the body through the vascular system to another, resulting in damage when it lodges at a bifurcation’. ³ Systemic air or gas embolism results when air enters the venous system embolizing to the heart, or to the arterial system when there is paradoxical embolization due to right to left intracardiac shunting. ^4–6 Early detection of air in the vasculature is important given the potential that exists for significant morbidity and mortality.

Air embolism may be iatrogenic arising from cervical surgical procedures or from endoscopy, laparoscopy, cardiac catheterization, obstetric or gynaecological procedures (including caesarean section), lung biopsy, vascular or orthopaedic surgery, the use of positive pressure ventilation, or through vascular access devices. Air embolism may also complicate diving, air travel and sexual activity during pregnancy. In forensic practice it is most commonly encountered when there has been traumatic disruption of internal organs from extensive blunt trauma, or when superficial vessels have been incised. ^4,5,7–13 Gas will enter the pulmonary veins when pulmonary venous pressure is low or when there is increased airway pressure. ⁸ The incidence of systemic air embolism after severe lung trauma is 4–14% with a mortality rate of 80% after blunt trauma and 48% after penetrating injury. ²

Air embolism to the coronary arteries may result in rapid collapse due to myocardial ischaemia from trapping of gas bubbles within the coronary microvasculature. Even a small amount of gas entering the arterial system can occlude end arteries, with as little as 0.5–1 mL of air injected into the pulmonary vein causing cardiac arrest as a result of coronary air embolism and resultant ischaemia. ^2,8,14 Administration of 0.02 mL/kg of gas into the coronary arteries in a dog model was associated with a mortality rate of 28%, and it has been estimated that for an embolus to pass through capillaries of 6 µm diameter a blood pressure of 200 mmHg is required. ⁶

In the reported case there was no injury to major vessels or significant blood loss, with the victim appearing initially stable. Problems arose however due to lung parenchymal damage from passage of the two projectiles close to the chest cavity, with injuries arising from the same mechanisms as in explosions where significant energy transfer derives from shock waves. This was thought to have resulted in disruption of the capillary–alveolar interface with subsequent leakage of air into the pulmonary venous circulation. ^8,15 Given that cardiac arrest preceded surgery it is unlikely that surgery could have contributed to the air embolism.

This case demonstrates, therefore, a very rare complication of chest wall trauma in an individual who had no other lethal injuries. Fortunately for the determination of the cause of death there had been assessment of cardiothoracic function during surgery. Given that air embolism is a difficult diagnosis to make at autopsy (most often by cardiac puncture under water), imaging studies may provide useful information if the chest cavity has not been previously opened. ^16–18 Air embolism should be considered in all cases where there has been penetrating trauma, or where there may have been blast type injuries to the lungs. Although rare cases of air embolism have been reported following tangential gunshot wounds to the chest, symptoms are usually immediate. ² It is important to recognize, however, that death may be delayed for some time following the initial trauma, and that it may also occur unexpectedly in an individual who is considered clinically stable.