Any procedure where the operative site is higher than the right atrium and where the vasculature is exposed in a surgical field carries a risk of air embolism. The addition of a pressure gradient between the surgical site and the right heart substantially increases the risk. A pressure difference as small as 5 cm of water will allow 100 ml of air entrainment per second via a 14-gauge cannula. This is the rational for head down position during central venous cannulation. Hypovolaemia and negative pressure associated with spontaneous respiration will also increase the pressure difference and hence the risk.
Additional risks include interventions accessing the circulation such as central venous cannitional risks include interventionulation (causing air embolism) or using carbon dioxide insufflation during laparoscopic surgery (causing CO2 embolism). Air may also be inadvertently injected via a vascular access device. The risk factors for air embolism can be broadly categorised into patient factors, surgical factors and anaesthetic factors, as detailed in Figure:
The use of Positive End Expiratory Pressure (PEEP) during patient’s ventilation to minimise the risk of air embolism is controversial. The concept that raising central venous pressure through the use of PEEP (>5 cm H2O) would minimise the risk of air entrainment seems logical. However, it has been implicated as a risk factor for paradoxical air embolism in patients with patent foramen ovale. Moreover, PEEP may also have an exaggerated effect on reducing preload due to reduced venous return secondary to increased intra-thoracic pressure. Sudden release of PEEP may also increase the rate of air entrainment in open venous beds within the surgical field.
Source: http://www.wfsahq.org
