Air can enter the venous system in a number of situations (Table 1). The best known example is where air enters through a catheter into a peripheral or central vein.
This can occur during insertion or removal, or when the catheter is in situ if venous access is left open to the atmosphere. Venous air embolism may be caused by either blunt or penetrating chest trauma. Many surgical procedures, either involving the lung parenchyma directly or extrathoracic in origin, may precipitate venous emboli formed from air or other gases. Barotrauma may be iatrogenic (positive-pressure ventilation) or due to a blast injury when significant venous air embolism can occur if extra-alveolar air is driven along a pressure gradient into the systemic veins. Ingestion of hydrogen peroxide is a rare cause of venous air embolism.
Table 1 Causes of venous air embolism
Arterial air emboli (Tabled) can occur as a result of any of the causes discussed above. Air enters the venous circulation and can pass into the systemic circulation via a functioning right to left shunt (usually a patent foramen ovale). Even in the absence of a clinically apparent shunt, pulmonary arteriolar to pulmonary venous air spill-over occurs once the finite capacity of the lungs to filter out air bubbles is exceeded.
Direct entry of air into the arterial circulation can also occur via indwelling arterial catheters, penetrating injuries, and during surgical procedures. However, self-contained underwater breathing apparatus (scuba) diving is the most common cause of patients presenting to emergency departments with arterial air embolism (dysbaric arterial air embolism).
Dysbaric arterial air embolism is caused by gas bubbles entering the systemic circulation from ruptured alveoli and pulmonary veins. According to Boyle's law, the volume of a fixed mass of gas is inversely proportional to pressure, and therefore compressed air can expand into surrounding tissues. During ascent after diving, delicate alveoli and pulmonary veins are damaged by increasing volumes of gas as pressure decreases. This is marked when high intrapulmonary pressures are reduced when divers surface and remove their masks, and is exacerbated if the divers inadvertently fail to exhale fully on ascent so that gas can pass rapidly into the circulation.
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