Gas Embolism

Gas emboli may involve either or both the venous and arterial systems. In most instances, the gas is air, though, in some diagnostic situations, it could be carbon dioxide, nitrous oxide or nitrogen.1 In the venous system, death from air embolism depends on the size of the bolus and the rate of delivery. In arterial embolism, these factors are not as important, because only a small number of air bubbles occluding a coronary artery or a cerebral vessel can result in death. In venous air embolism, between 75 and 250 cm3 of air delivered rapidly is necessary to cause death.

Venous air embolism may occur during therapeutic or diagnostic procedures secondary to trauma, during childbirth or abortion, and during oralgenital sex in a pregnant woman when her partner blows air into the vagina. Arterial air embolism occurs secondary to cardiopulmonary bypass, arterial catherization, surgical procedures involving arteries, or injury to the pulmonary veins after chest trauma. One might also incur a paradoxical air embolism, that is, air crossing from the venous to the arterial circulation.

Air entering the venous system is carried to the heart and pulmonary arteries, with resultant mechanical occlusion of the pulmonary arterial vas-culature by air bubbles. This is followed by a transient vasoconstriction. Obstruction of the pulmonary blood flow results in churning of the blood and air, producing the frothy appearance of the blood seen at autopsy. This churning can result in the development of complexes of air bubbles, fibrin, platelet aggregates, erythrocytes, and fat globules, thus further occluding the vasculature.2 Death is caused by obstruction of the pulmonary blood flow secondary to obstruction of the pulmonary arterial system by the air bubbles, the pulmonary vasoconstriction, and the cellular aggregates. With a very large bolus of air, the obstruction occurs not only in the pulmonary vasculature but also in the right ventricle.

Paradoxical air emboli occur when air or gas that has entered the venous system crosses over to the arterial system. Typically, these have been described in association with septal defects of the heart. These permit air to go from the right side of the heart to the left without passing through the pulmonary vasculature. If a large air embolism is carried to the heart, the sudden rise in the right-sided heart pressure may result in a right-to-left shunt through a probe patent, but physiologically closed, foramen ovale. Increased right-sided heart pressure also causes air to be forced into the epicardial veins on the surface of the heart. Paradoxical emboli can also occur secondary to arteriovenous anastomoses in the lung. In addition, with sufficiently high pressures and delivery of large quantities of air, the ability of the lungs to filter out air can be exceeded and bubbles of gas may traverse the pulmonary circulation and enter the left atrium. This has been demonstrated by Butler and Hills in experiments on dogs.3 Air entering the arterial circulation causes death by occluding the cerebral or coronary arteries. Only a very small quantity of air is necessary.

Forensic pathologists will encounter air emboli most commonly in knife wounds of the neck, and secondary to surgical procedures. Air enters an open vein whenever there is a negative pressure gradient between the vein and the right atrium. This is facilitated by the negative intrathoracic pressure generated during inspiration. The higher the open vein is above the right atrium, the greater the pressure gradient and the more likely air is to enter the vessel. This is why wounds to the neck can result in air emboli. Thus, in individuals incurring stab or incised wounds of the neck with injury to the veins, the prosector may want to explore the possibilities of air embolism at the time of autopsy.

The true incidence of venous air embolism during surgical and diagnostic procedures is unknown, with one exception — craniotomy in the sitting position. Here, air emboli occur in 21 to 29% of all craniotomies and 40% of all occipital craniotomies.4,5 Air emboli have also been reported in a host of other therapeutic and diagnostic procedures. Any surgical procedure that causes a negative pressure gradient between the right side of the heart and a vein is a potential risk for venous air embolism. Individuals have been seated or prone, supine, in the lithotomy position, and in the lateral knee-chest position at the time they incurred air emboli.6

Air emboli were more common when blood and other fluids were delivered in glass bottles rather than the present collapsible plastic bags. Even with plastic bags, there is the possibility, though rare, of an air embolus.7 Air emboli have also occurred following cesarean section, placenta previa, and subclavian venipuncture.810 With a large-bore channel to a vein, a fatal amount of air can pass quickly into a vessel. Theoretically, 100 cm3 of air per second can enter through a 14-gauge needle with a 5-cm water pressure drop across it.10 Fortunately for people with subclavian lines, this is more theoretical than actual.

Homicides secondary to injection of air into the venous system using a syringe are rare because of the large quantity of air one has to introduce in a bolus (100-250 cm3), the expertise necessary to administer the injection intravenously, and the necessary passivity of the patient. Individuals with established intravenous lines, such as hospital patients, will, of course, be easier to kill in this way.

When abortions were, for the most part, illegal, occasional deaths caused by air emboli were encountered. This occurred following dilatation of the cervical os, with resultant tears of the margins of the placenta or the cervical veins. In some cases, there was a delay in the air embolus. Thus, in one case, after having an illegal abortion, the woman left the premises, only to collapse and die 2 h later. At autopsy, air could be seen in the inferior vena cava, right atrium, and right ventricle.

Death caused by air embolism in association with pregnancy may also occur secondary to cesarean section and placenta previa.8-10 Just as in an abortion, there can be a time delay prior to the onset of the fatal air embolus. In deaths of pregnant females during sexual intercourse, one should always suspect air emboli. This occurs during oral-genital intercourse, with the partner blowing air into the vagina during cunnilingus.

Suicide from an air embolus is rare. The authors have seen it in individuals who have slit their throats, severing their jugular veins, as well as in patients hooked up to dialysis machines who disconnected their tubing, with resultant massive air embolus.

The broad spectrum of deaths caused by air embolism is illustrated by the following cases: The first involved a 22-year-old woman, 34 weeks pregnant, who was having intercourse with her husband. Her husband blew into her vagina and she suddenly "passed out." She was dead on arrival at the emergency room. At autopsy, there was air in the right atrium and ventricle. The epicardial veins had a beaded appearance caused by air bubbles. Bubbles of air were also present in the pelvic veins. The blood of the heart had a frothy appearance.

The second case involved an obese 40-year-old man who was prone in the semi knee-chest position for a lumbar laminectomy. Five hours into the procedure, he developed bradycardia and an agonal rhythm. The autopsy revealed air in the right ventricle, with beading of the epicardial veins by air bubbles.

The third case was a 39-year-old woman who went into premature labor. She presented with vaginal bleeding. On admission to the hospital, she had a complete central placenta previa and a breech presentation. The child was delivered by cesarean section at 8:51 in the evening. The operation was over at 9:30 p.m. She then was returned to the recovery room. At midnight, she was taken to her room. As she was being placed into bed, she had a grandmal seizure and went into cardiopulmonary arrest. Attempts at resuscitation were unsuccessful. At autopsy, there was air in the right atrium and ventricle with beading of the epicardial veins by massive numbers of air bubbles.

In a living patient, the diagnosis of venous air embolism to the heart can be made by auscultation of the mill-wheel murmur or by detection of intracardiac air using dopler ultrasonography or transesophageal echocardiography. In the deceased individual, to make the diagnosis of air embolism, one must consider the diagnosis prior to the autopsy. The first step may be a chest X-ray to look for air in the heart (Figure 19.1). A Y-shaped incision can then be made into the skin and musculature of the chest and the skin and muscle retracted. Instead of then removing the chest plate in the ordinary way, a "window" should be cut in the sternum and ribs overlying the heart. The bony plate should then be retracted very carefully, so as not to introduce air into the venous system. The pericardial sac can then be cut open and the heart visualized. The epicardial veins should be examined for the presence of air. One or two bubbles in an epicardial vein do not make a diagnosis of air embolism.

Air Embolism
Figure 19.1 Massive air embolus with air in the right atrium and ventricle. Note outline of right ventricular wall.

With air embolism, the epicardial veins usually have a beaded appearance, with numerous air bubbles the length of these vessels (Figure 19.2). The pericardial sac should then be flooded, the plunger removed from a 50-cc syringe, a large-bore needle attached, and the syringe filled with water. The tip of the needle can then be pushed into the right ventricle. If there is air in this chamber, it will be seen to bubble up through the water in the syringe. The water should then be removed and the right side of the heart opened. The blood typically has a frothy appearance. Following this, the incision can be extended through the anterior abdominal wall to the pubic area. If the air entered in the pelvic region, the inferior vena cava is examined for air bubbles. One should be careful about interpreting one or two bubbles in this vessel as evidence of air embolism. If the deceased has been vigorously resuscitated with a thoracotomy and internal cardiac massage, it is usually impossible to make the diagnosis of air embolus based on the autopsy, because the air observed in vessels could be caused by resuscitation. Air in the coronary arteries cannot be identified at autopsy because air bubbles cannot be seen through the wall of these vessels. In the brain, the process of removing the skull cap, cutting through the dura, and putting traction on the brain to see the cerebral circulation might introduce air bubbles into the circulation. Thus, the presence of a few air bubbles in the cerebral circulation

Pictures Air Embolism
Figure 19.2 Air bubbles in epicardial veins of heart in death caused by air embolism.

does not necessarily indicate an embolus. If individuals survive the initial insult of air in the coronary and cerebral circulation, they might develop myocardial or cerebral infarcts.

The technique described to demonstrate air in the heart using a 50-cc syringe is very basic and very crude. A much better and more sophisticated method is to use an aspirometer. This device not only demonstrates the presence of air but measures the amount and stores it for subsequent analysis by gas chromatography.11 Embolized air differs from atmospheric air in that CO2 is less than 15%; N2 is higher than 70% and O2 is reduced, usually measuring between 8 and 15%.10 Detection of CH4 and H2 indicates that decomposition has begun. Most individuals feel that decomposition, with its formation of gases of putrefaction, precludes the diagnosis of air emboli. Bajanowski et al. disagree.12 They analyzed gas recovered from the ventricles of 15 individuals, exhumed after 10-45 weeks burial, who were believed to have been killed by injection of air. They found that analysis in 8 of the 15 cases gave results typical of air embolism, with only small amounts of the gases of putrefaction present.

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