Thus, a small volume of air trapped in a coronary artery during bypass surgery or introduced into the cerebral arterial circulation by neurosurgery performed in an upright “sitting position” can occlude flow, with dire consequences. Small venous gas emboli generally have no deleterious effects, but sufficient air can enter the pulmonary circulation inadvertently during obstetric or laproscopic procedures or as a consequence of a chest wall injury to cause hypoxia, and very large venous emboli may arrest in the heart and cause death.
A particular form of gas embolism called decompression sickness is caused by sudden changes in atmospheric pressure. Scuba divers, underwater construction workers, and persons in unpressurized aircraft who undergo rapid ascent are at risk. When air is breathed at high pressure (e.g., during a deep sea dive), increased amounts of gas (particularly nitrogen) become dissolved in the blood and tissues. If the diver then ascends (depressurizes) too rapidly, the nitrogen expands in the tissues and bubbles out of solution in the blood to form gas emboli, which cause tissue ischemia. Rapid formation of gas bubbles within skeletal muscles and supporting tissues in and about joints is responsible for the painful condition called the bends (so named in the 1880s because the afflicted person arches the back in a manner reminiscent of a then-popular women’s fashion pose called the Grecian bend). Gas bubbles in the pulmonary vasculature cause edema, hemorrhages, and focal atelectasis or emphysema, leading to respiratory distress, the so-called “chokes”. Bubbles in the central nervous system can cause mental impairment and even sudden onset of coma. A more chronic form of decompression sickness is called caisson disease (named for pressurized underwater vessels used during bridge construction), in which recurrent or persistent gas emboli in the bones lead to multifocal ischemic necrosis; the heads of the femurs, tibiae, and humeri are most commonly affected.
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Placing affected persons in a high-pressure chamber, to force the gas back into solution, treats acute decompression sickness. Subsequent slow decompression permits gradual gas resorption and exhalation so that obstructive bubbles do not re-form.
Anembolusisasolid,liquid,orgaseousmasscarriedbythebloodtoasitedistantfromitsorigin;mostaredislodgedthrombi.
Pulmonaryembolideriveprimarilyfromlower-extremitydeepveinthrombi.Theireffectsdependmainlyonthesizeoftheembolusandthelocationinwhichitlodges.Consequencesmayincluderight-sidedheartfailure,pulmonaryhemorrhage,pulmonaryinfarction,orsuddendeath.
Systemicembolideriveprimarilyfromcardiacmuralorvalvularthrombi,aorticaneurysms,oratheroscleroticplaques;whetheranemboluscausestissueinfarctiondependsonthesiteofembolizationandthepresenceorabsenceofcollateralcirculation.
Fatembolismcanoccuraftercrushinginjuriestothebones;symptomsincludepulmonaryinsufficiencyandneurologicaldamage.Amnioticfluidembolismmayfollowchildbirthandcangiverisetofatalpulmonaryandcerebralmanifestations.Airembolismcanoccuruponrapiddecompression,mostcommonlyindivers;itresultsfromsuddenbubblingofnitrogendissolvedinbloodathigherpressures.