Venous Air Embolism 

Venous air embolism (VAE), a subset of gas embolism, is an entity with the potential for severe morbidity and mortality. Venous air embolism is a predominantly iatrogenic complication^{[1, 2]} that occurs when atmospheric gas is introduced into the systemic venous system^[3] . In the past, this medical condition was mostly associated with neurosurgical procedures conducted in the sitting position.^{[4, 5]} More recently, venous air embolism has been associated with central venous catheterization,^{[3, 6, 7]} penetrating and blunt chest trauma,^{[8, 9]} high-pressure mechanical ventilation,^[3] thoracocentesis,^[1] hemodialysis,^{[3, 7]} and several other invasive vascular procedures.

Venous air embolism (VAE) has also been observed during diagnostic studies, such as during radiocontrast injection for computerized tomography.^{[10, 11]} The use of gases such as carbon dioxide and nitrous oxide during medical procedures and exposure to nitrogen during diving accidents can also result in VAE.^[2] Many cases of VAE are subclinical with no adverse outcome and thus go unreported. Usually, when symptoms are present, they are nonspecific, and a high index of clinical suspicion of possible venous air embolism is required to prompt investigations and initiate appropriate therapy.

Two preconditions must exist for venous air embolism to occur: (1) a direct communication between a source of air and the vasculature and (2) a pressure gradient favoring the passage of air into the circulation.^{[12, 4]}

The key factors determining the degree of morbidity and mortality in venous air emboli are related to the volume of gas entrainment, the rate of accumulation, and the patient’s position at the time of the event.^{[1, 6, 11]}

Generally, small amounts of air are broken up in the capillary bed and absorbed from the circulation without producing symptoms. Traditionally, it has been estimated that more than 5 mL/kg of air displaced into the intravenous space is required for significant injury (shock or cardiac arrest) to occur.^[1] However, complications have been reported with as little as 20 mL of air^[7] (the length of an unprimed IV infusion tubing) that was injected intravenously. The injection of 2 or 3 mL of air into the cerebral circulation can be fatal.^[13] Furthermore, as little as 0.5 mL of air in the left anterior descending coronary artery has been shown to cause ventricular fibrillation.^{[13, 9]} Basically, the closer the vein of entrainment is to the right heart, the smaller the lethal volume is.^[1]

Rapid entry or large volumes of air entering the systemic venous circulation puts a substantial strain on the right ventricle, especially if this results in a significant rise in pulmonary artery (PA) pressures. This increase in PA pressure can lead to right ventricular outflow obstruction and further compromise pulmonary venous return to the left heart. The diminished pulmonary venous return will lead to decreased left ventricular preload with resultant decreased cardiac output and eventual systemic cardiovascular collapse.^{[1, 4, 6]}

With venous air embolism (VAE), resultant tachyarrhythmias are frequent, but bradyarrhythmias can also occur.^{[4, 2]}

The rapid ingress of large volumes of air (>0.30 mL/kg/min) into the venous circulatory system can overwhelm the air-filtering capacity of the pulmonary vessels, resulting in a myriad of cellular changes.^[3] The air embolism effects on the pulmonary vasculature can lead to serious inflammatory changes in the pulmonary vessels; these include direct endothelial damage and accumulation of platelets, fibrin, neutrophils, and lipid droplets.^[1]

Secondary injury as a result of the activation of complement and the release of mediators and free radicals can lead to capillary leakage and eventual noncardiogenic pulmonary edema.^{[1, 7, 3]}

Alteration in the resistance of the lung vessels and ventilation-perfusion mismatching can lead to intra-pulmonary right-to-left shunting and increased alveolar dead space with subsequent arterial hypoxia and hypercapnea.^{[1, 4, 11]}

Arterial embolism as a complication of venous air embolism (VAE) can occur through direct passage of air into the arterial system via anomalous structures such as an atrial or ventricular septal defect, a patent foramen ovale, or pulmonary arterial-venous malformations. This can cause paradoxical embolization into the arterial tree.^{[1, 4, 9, 2, 3]} The risk for a paradoxical embolus seems to be increased during procedures performed in the sitting position.^{[1, 5]}

Air embolism has also been described as a potential cause of the systemic inflammatory response syndrome (case report), triggered by the release of endothelium derived cytokines.^[12]

Frequency

United States

The nonspecific nature of the signs and symptoms of venous air embolism (VAE) as well as the difficulty in documenting the diagnosis does not allow the true incidence of VAE to be known. Interventional radiology literature reports an incidence of venous air embolism of 0.13% during the insertion and removal of central venous catheters despite using optimal positioning and techniques.^[14] The frequency of venous air embolism with central venous catheters based on a reported case series has also ranged from 1 in 47 to 1 in 3000.^{[15, 2]} The neurosurgical procedure-related complications of venous air embolism have been estimated to be between 10-80%.^{[16, 2, 17]} Reports of venous air embolism in the setting of severe lung trauma have been estimated between 4-14%.^{[13, 8, 9, 18, 17]}

Mortality/Morbidity

The potentially life-threatening and catastrophic consequences of venous air embolism (VAE) are directly related to its effects on the affected organ system where the embolus lodges. VAE may be fatal and frequently carries high neurologic, respiratory, and cardiovascular morbidity. Catheter-associated VAE mortality rates have reached 30%.^[2] In a case series of 61 patients with severe lung trauma, the mortality rate associated with concomitant VAE was 80% in the blunt trauma group and 48% in the penetrating trauma group.^{[8, 18, 17]} The morbidity and mortality associated with traumatic VAE, as with nontraumatic VAE, depends not only on associated injuries but also on the volume and rate of air entry, underlying cardiac condition, and the patient's position.

Race

No racial predilection exists for venous air embolism.

Sex

No gender predilection exists for venous air embolism.

Age

No specific age predilection exists for venous air embolism.