The Performance Gap
Air embolism (AE) is defined as the presence of air (or other gas) in either the arterial or venous circulation. In the hospital setting, this is usually the result of inadvertent injection of air into the venous system. However, it can also result from traumatic injuries, surgical procedures, or exposure of venous access systems (e.g., central venous lines) to the open air. Inadvertent air injections can be sudden, as from an air-filled syringe or pumping system, or gradual, as through a continuous IV infusion. Outside of the hospital, air embolism can occur during SCUBA diving or with blast injuries.
In adults with normal anatomy (no right-left shunts), venous air embolism will enter the pulmonary circulation and become trapped in the lungs. Small amounts of pulmonary air embolism may be well tolerated – even up to 50 ml or more in a healthy adult. However, at some point the air load in the lung capillaries will impede the pulmonary circulation, resulting in pulmonary hypertension and eventually right-heart failure (cor pulmonale). This can lead to circulatory collapse and death.
The problem of venous AE becomes more immediately serious in the presence of any form of right-to-left (or transpulmonary) shunt, such as patent foramen ovale (PFO), atrial septal defect (ASD) or patent ductus arteriosus (PDA). PFO has an incidence of 25-30% in otherwise healthy adults, and most of these are asymptomatic and undiagnosed \cite{HAGEN_1984}. The presence of one of these forms of R-L shunt allows venous AE to bypass the lungs and enter the arterial circulation as a “paradoxical embolism,” where even small amounts of air can block circulation to vital organs. The brain is particularly vulnerable to arterial air embolism, where even a few milliliters of air can cause a major stroke. Because of the high incidence of undiagnosed PFO in adults, we usually do not know which patients are at risk. For any patient with a known diagnosis of potential right-left shunt, the increased risk of AE must be documented in the EMR, and clearly explained to all care-team members. The incidence of right-left shunts is much greater in newborns than adults; hence all infants should be treated as high risk for venous AE entering the arterial circulation.
Signs and symptoms of air embolism in conscious patients can include chest pain, dyspnea, shortness of breath, decreased level of consciousness, unconsciousness, sudden cardiac arrest, or neurological deficit from transient ischemic attack (TIA) or stroke. (See also signs and symptoms of VTE, in APSS #12A above.) Slower infusions of venous air may be asymptomatic until serious damage to the pulmonary circulation has occurred. The patient’s ability to tolerate and compensate for air embolism is variable, depending on general health status and presence of specific diseases (e.g., cerebrovascular).
The literature on the various types of venous or arterial air emboli seems to agree on one important point: most of these should be considered “never events” – potential disasters that should never occur if proper safeguards, precautions, and procedures are followed. A retrospective case study by Albin showed that air embolism occurred in 100 of 400 patients who underwent craniotomy in the sitting position – an incidence of 25% \cite{Albin2011}! Other surgical procedures that create high risk for air embolism include cardiopulmonary bypass, in which there are many reports of fatal cases \cite{van2014,Robich2017}, as well as intrathoracic surgery, major joint surgery, Cesarean section, eye surgery \cite{Gayer2016}, pacemaker placement\cite{Xiao2016}, and major trauma. An excellent review of venous air embolism during surgery is found in \cite{Palmon1997}.
AE can also occur when any type of intravascular cannula is used. This includes standard peripheral intravenous catheters, central venous catheters, pulmonary artery catheters, dialysis catheters, and arterial catheters – in other words, with any external cannulation of the circulation for any reason. Pressurized intravenous infusion systems create a particularly serious risk of massive venous air embolism. One-liter plastic bags of intravenous crystalloid, such as Lactated Ringer’s Solution, contain up to 150 cc of air. If this air is not carefully removed before the fluid bag is placed in a pressurized device, it can be forcefully pumped into the patient’s vein. There have been a number of published case reports of fatal or near-fatal AE from this mechanism \cite{Adhikary1998,Aldridge2005}. Central circulation catheters (CVP, PA, “triple lumen”, etc.) pose an even higher risk. If such a catheter becomes disconnected and exposed to atmosphere in a sitting, spontaneously breathing patient, the negative intrathoracic pressure during inspiration can rapidly suck massive amounts of air directly into the heart, with fatal results \cite{Ploner1991}.
An excellent review and bibliography of the diagnosis and treatment of all of these types of air embolism can be found in \cite{Mirski2007}. Annual death rates from AE are difficult to document, because of the wide variety of causes and clinical settings of these cases. The serious nature of this problem is evidenced by the fact that there have been over 4,000 publications on the topic in the past 30 years (Mirski 2007).