Air Emboli

Definition

Gas bubbles within the vascular system are termed gas emboli or air emboli. When venous air emboli enter the arterial circulation, they are termed paradoxic air emboli. Venous air emboli or paradoxic air emboli from gases dissolved in solution are released through effervescence, or they may enter the bloodstream from outside through insufflation or entrainment.

The amount of gas dissolved in a liquid is a function of temperature and pressure. A sudden increase in the temperature of a gas-containing liquid can release gas bubbles from solution through effervescence. This can occur during rapid rewarming after hypothermic cardiopulmonary bypass or by rapidly warming cold intravenous fluids or blood products. It also happens in divers who experience a too-rapid decompression (the “bends”).

More commonly, gas is introduced into the bloodstream by insufflation (e.g., during laparoscopy, thoracoscopy, arthroscopy) or delivered with fluids or blood products by pressurized delivery systems. Veins that do not easily collapse can also entrain air—for example, venous sinuses in bone; open, large central veins; and open veins that are well above the level of the heart. For entrainment to occur, the vein opening must be sufficiently above the level of the heart to exceed central venous pressure (e.g., sitting craniotomy). Venous and paradoxic air emboli can occur in the supine, prone, or lateral position. The risk of such entrainment is increased by low venous pressure or negative intrathoracic pressure, as occurs during spontaneous respiration.

Small children are at special risk for venous air emboli. Significant blood loss may occur rapidly, and a small amount of blood may constitute a large portion of a child’s blood volume. This is a particular concern during craniotomies, because the calvaria is very thin. Further, the head is relatively large in proportion to body size, frequently resulting in the surgical site’s being elevated above the heart level during a supine or prone craniotomy. Finally, owing to the high prevalence of intracardiac shunts (patent foramen ovale), amounts of venous air emboli that might be insignificant in an adult can result in paradoxic air emboli and be disastrous for a neonate.

Recognition

Awake patients may experience dyspnea and coughing as a result of venous air emboli. During anesthesia, changes in vital signs occur late and usually only after the entrainment of large amounts of air. Monitoring methods to detect venous air embolism, in decreasing order of sensitivity, include the following:

• Echocardiography or Doppler ultrasonography

• End-tidal carbon dioxide (ETCO ₂ ) decrease or new appearance of end-tidal nitrogen (ETN ₂ )

• Pulmonary artery pressure elevation

• Central venous pressure elevation

• Blood pressure reduction

• Electrocardiogram (ECG) changes (e.g., right ventricular strain, ischemia, arrhythmias)

• Audible cardiac or “mill-wheel” murmur

Echocardiography and Doppler monitoring are exquisitely sensitive. They can detect even microbubbles from routine intravenous injections and minor entrainment of air. Air emboli detected with echocardiography and Doppler monitoring should alert the clinical team but must be interpreted cautiously, taking into account the severity of detected air (amount, duration, and associated clinical signs) and the clinical situation (e.g., craniotomy). ECG changes are more ominous, and an audible cardiac or “mill-wheel” murmur is least sensitive; however, when associated with echocardiographic or Doppler evidence of venous air embolism, they suggest that a significant amount of air has been entrained.

Echocardiography

Transthoracic or transesophageal echocardiography (TEE) enables the recognition of discrete air bubbles and the relative quantification of larger volumes (i.e., the density of snowstorm pattern). Further, TEE localizes emboli to the right or left side of the heart and detects cardiac anomalies (septal defects) that increase the risk of paradoxic air emboli ( Fig. 127.2 ). TEE has been used in neonates who weigh as little as 2.5 kg. Limitations to its widespread use include the following:

• High cost

• Requirement for a separate, highly trained observer during anesthesia and surgery

• Risk of injury to the pharynx, larynx, and esophagus

• Possible displacement of the endotracheal tube, especially during manipulation in small infants

Consequently, although TEE is a very sensitive technique for detecting venous air emboli, it is currently not practical in many institutions and may not be necessary as a routine monitor.