Subclinical entry of air into the circulation has been shown with Doppler techniques to occur in up to 60% of caesarean sections (possibly more if the head-down position is used or if the uterus is exteriorised), although the significance of circulating microscopic bubbles is uncertain. It has been suggested that chest pain or ST segment depression occurring during caesarean section under regional anaesthesia may be related to air in the coronary circulation, although both are frequently unaccompanied by Doppler demonstration of bubbles. Large amounts of air may cause cardiovascular impairment, but this is less common (up to 2%); the mechanism is obstruction of right ventricular output by the presence of compressible gas within the contracting ventricle. Volumes of 5 ml/kg are thought to be necessary to cause complete cardiovascular collapse. In addition, bubbles may lodge in the pulmonary circulation, increasing dead space, while paradoxical embolism may occur when there is a patent foramen ovale (a probe-patent foramen is found in about 30% of ‘normal’ hearts on routine autopsy) or other right-to-left shunt.
Although most cases are related to caesarean section, it should not be forgotten that air embolism may occur whenever open veins are above the level of the heart, such as when central venous lines are manipulated with the patient in the sitting position or when the arm in which a peripheral venous cannula has just been placed is held aloft to prevent spillage of blood; both are more likely to occur when staff are inexperienced in the management of intravenous lines, as may (unfortunately) occur on the labour ward. There have also been cases of air embolism associated with epidural catheter insertion, either through a loss-of- resistance technique using air, or through accidental cannulation of the epidural veins. Finally, the danger of accidental intravenous injection of air, for example when pressurising devices are used with air-containing bags of intravenous fluid or when bubbles are allowed into intravenous infusion lines, must not be forgotten. In patients with right-to-left shunts, even small bubbles may have disastrous systemic effects.
Problems and special considerations
The diagnosis may not be clear, especially at first. Clinical features are fairly nonspecific and include hypotension, tachycardia, reduced arterial saturation and reduced end-expiratory carbon dioxide concentration (during general anaesthesia), the last because of (1) reduced cardiac output and hence return of carbon dioxide from the tissues, and (2) increased pulmonary dead space. There may be an audible churning sound on cardiac auscultation, although this is usually only present in massive air embolism. Paradoxical embolism may result in systemic infarction of vital organs, especially heart and brain.
The differential diagnosis of air embolism therefore includes any cause of cardiovascular impairment or collapse, at least initially; a high index of suspicion is thus required. In particular, amniotic fluid embolism or thromboembolism may cause the same initial right ventricular outflow obstruction. If it occurs during regional anaesthesia, air embolism may mimic hypotension produced by anaesthesia-induced sympathetic blockade. It may be possible to aspirate bubbles from the right ventricle or atrium via a central venous catheter, but inability to do so does not exclude the diagnosis. Diagnosis may be aided by specialist monitoring, but this is unlikely to be used in obstetric cases. Precordial Doppler or transoesophageal echocardiogram are the diagnostic modalities of choice, but most units do not have the necessary equipment to hand.
Manoeuvres for preventing further embolism and managing the current embolism (as described below) may be difficult to carry out midway through a caesarean section, especially if the patient is awake and distressed. In addition, some of the traditional advice concerning positioning of the patient is self-contradictory (head-up for prevention of further embolism; head-down for its management).
Management options
Methods to reduce occurrence in the first place should be employed. These include minimising the time spent with the operative site above the level of the right atrium for a procedure such as exteriorising the uterus, careful use of intravenous and epidural catheters, and maintaining normovolaemia.
Prevention of further embolism is important as soon as the diagnosis is considered. This includes immediately informing the obstetrician, who should return the uterus to the abdomen if possible, flood the surgical field with saline and look for open veins (the ability to do this will obviously depend to some extent on the stage of surgery). Positioning the patient head-up is generally suggested to raise the level of the heart and increase venous pressure in the pelvis and abdomen.
Damage limitation is generally achieved by reducing the size of the bubble(s); this is done firstly by stopping any nitrous oxide that is being administered and secondly by attempting to remove air from the circulation, or more specifically from the right side of the heart. It may be possible to aspirate air from a routine central venous cannula or catheter; special wide-bore multi-perforated cannulae are manufactured specifically for this task but are not generally available on many labour wards. The head-down position is traditionally required for central venous cannulation, and the left lateral head-down position is advised for isolation of the bubbles away from the right ventricular outflow tract and easier aspiration of air. Both of these positions may compromise the advice given above, and moving to the left lateral position is at best awkward in the middle of surgery.
Further management consists of general supportive treatment (increased concentration of inspired oxygen, vasopressor/inotropic drugs, intravenous fluids) and basic resuscitative measures in an ‘ABC’ approach as appropriate. If the baby has not yet been delivered, the cardiovascular effects of air embolism will be exacerbated by aortocaval compression; thus lateral displacement of the uterus is especially important and delivery should be expedited.
It has been suggested that caesarean section should always be performed in the head-up position to reduce the incidence of air embolism; however, this has implications for the incidence and effects of hypotension following regional anaesthesia and for the spread of spinal blockade. It has also been suggested that Doppler or ultrasound monitoring (transthoracic or transoesophageal) should always be available during caesarean section, but this is hampered by the lack of equipment and expertise in its use.
Key points
• Subclinical air embolism occurs in up to 60% of caesarean sections as detected by Doppler studies.
• Air embolism may cause cardiovascular collapse if large.
• Management includes general resuscitation, preventing further embolism and removal of air already in the circulation.