7.3.6 Gas embolisms

 Bubbles formed during ECC can embolise into the body and cause myocardial ischaemia, neurological disorders or organ dysfunction. Depending on their size and origin, two types coexist [1,3].

• Miliary microbubbles, tiny and low contrast; due to cavitation or thermal variations, they have no impact on the postoperative neurological status.
• Small, shiny shuttles that dance in the heart chambers and accumulate in large sheets in overhanging areas. They are caused by air being sucked into the circuit or by the opening of the heart chambers; their embolisation in the coronary arteries or the brain alters the functioning of the organ.

Their identification and localisation on TEE allows for more adequate emptying, and thus may improve the neurological prognosis of patients. Several phenomena contribute to the creation of these bubbles [2].

• The solubility of gases in a liquid increases when the temperature drops or the pressure rises. Microbubbles can therefore form as soon as the blood heats up locally, or as soon as the pressure drops due to cavitation (suction against resistance by the pump or vortex in swirls). These bubbles embolise easily, because the bubble detectors placed on the arterial circuit do not detect elements smaller than 0.5 mL.
• When opening the left chambers (e.g. mitral valve surgery), the air must be purged before closing the heart chambers; this purging is never complete, and volumes of air accumulate in the overhanging areas (see Figure 7.44). This air will embolise in the form of small bubbles (Air Video). If the patient has respiratory movements while the OG or LV is open, the air is drawn into the pulmonary veins, where it is inaccessible. Bathing the operating field with carbon dioxide by diffusing 1-2 L/min of CO₂ through a small hose replaces air with a gas that is more solvable in blood and therefore less emboligenic.
• If venous drainage is assisted by a pump, it may cause the circuit to collapse and result in air being drawn in when flow is restored.
• Air can come from the cardioplegia cannula if it is not closed properly.
• Two situations can lead to massive embolisation and haemodynamic collapse:
  • A reversal of the arterial and venous circuits at the start of bypass suddenly drains blood out of aorta ; it fills with air when the system is stopped.
  • Air entrainment is possible if the venous reservoir suddenly empties and the roller pump sucks in air; this is prevented easily through pump retrocontrol of blood level in the reservoir.

Airflow is closely monitored on the arterial line. The filter has a bubble trap in the form of a continuous purge (100-200 mL/min) that draws blood from the top of the filter and returns it to the venous reservoir (see Figure 7.10). The presence of air in the arterial cannula is a catastrophic event that requires immediate action: stopping the pump, forced Trendelenburg position to reduce the risk of air progressing into the carotid arteries, debulking of the aorta, hypothermic (20-24°) retrograde cerebral perfusion (1-2 L/min) through the superior vena cava to perfuse the brain a retro and drain the air that has infiltrated the arterial side Resumption of the bypass after complete air drainage, brain protection with mannitol and methylprednisolone (see What to do in case of an acute problem? ).

 © CHASSOT PG, GRONCHI F, April 2008, last update December 2019

References

BLAUTH CI. Macroemboli and microemboli during cardiopulmonary bypass. Ann Thorac Surg 1995; 59:1300-3

DAVIS RF, THOMPSON J. Technology, pathophysiology and pharmacology of cardiopulmonary bypass. In: THYS DM, et al Eds. Textbook of cardiothoracic anesthesiology. New York, McGraw-Hill Co, 2001,354-75

HOGUE CW, PALIN CA, ARROWSMITH JE. Cardiopulmonary bypass management and neurologic outcomes: an evidence-based appraisal of current practices. Anesth Analg 2006; 103:21-37