An embolus is a foreign substance which forms in, or enters the blood circulation: It can be a thrombo-embolus or blood clot, air or gas, fat, amniotic fluid or even a bullet.
Blood clots or thrombi often develop in the veins of the lower legs of individuals who are immobile, for example when sitting in one position for a long time (as in the so-called economy-class syndrome as described in air passengers). It can also develop in bedridden persons after an operation or due to illness. Some people may also have a genetic predisposition for developing blood clots.
If this blood clot (thrombus) detaches from the blood vessel wall, it can travel in the venous system to the lungs, where a lung embolism can develop. This presents with symptoms such as shortness of breath and coughing up of blood. If the blood clot is large it can cause death.
Certain measures can prevent the formation of thrombi and consequently embolism, inter alia moving the legs and contracting and relaxing leg muscles, elevating the foot of the bed, and allowing patients to become mobile as soon as possible.
Air embolism is the presence of air in the blood circulation.
Air embolisms can form in the arterial and/or venous systems, and therefore in either the systemic blood vessels (eg the blood vessels of the brain) or the pulmonary (lung) vessels. The terminology of veins and arteries is often confusing, and you are referred to the basic anatomy as described in study unit 2.
The following general principles apply: a vein always carries blood towards the heart, while an artery carries blood away from the heart. Arteries usually contain oxygen-rich blood, and veins oxygen-poor blood. The two major blood vessels of the lungs can cause confusion. The pulmonary artery distributes oxygen-poor blood from the heart to the lungs; it is an artery as it carries the blood away from the heart. The pulmonary vein brings the oxygen-rich blood from the lungs to the heart.
If an air bubble enters the circulation, it forms an air lock as the air is compressed and therefore prevents forward movement of blood. The amount of air necessary to create such an airlock depends on the diameter of the blood vessel. An arterial vessel, for instance the carotid and coronary arteries, can be blocked by as little as one millilitre air. Venous emboli are usually larger, and approximately one hundred millilitre (100 ml) air is needed to obstruct pulmonary outflow from the right ventricle.
The pathological features seen in cases of arterial air embolism are similar to those seen in ischaemic damage, including infarctions in the brain and heart. If the patient dies instantly, no changes, except for the presence of air in vessels, will be noted.
Certain factors will determine whether air will enter a blood vessel after damage to the vessel. With a venous air embolism the negative pressure in the venous system during inspiration is the major factor. This is often seen in cases where the defect in the blood vessel is at a higher level than the heart, for instance in cases where a person remains standing after being stabbed in the jugular vein. The size of the defect and the distance to the heart is also important. Some veins also dilate due to the action of the adjacent muscles during inspiration, for instance the subclavian vein.
A number of small emboli formed over time is not necessarily fatal. Air can also enter the blood circulation but only spread to other organs after some time (delayed air embolism).
· Penetration of the jugular vein in the neck (eg due to a stab wound) and especially when the person remains in the upright position. The air will go via this vein to the superior vena cava and ultimately to the right side of the heart.
· Criminal abortions. In some cases air may enter the uterus inadvertently, especially if a syringe is used during the procedure. The placenta will detach from the uterine wall. The air will enter the venous system through the numerous blood vessels in the placental bed, and will then spread via the pelvic veins to the inferior vena cava and eventually to the right side of the heart.
· Neurosurgical procedures, especially with surgery in the occipital region and the patient in the upright position.
· Iatrogenic or doctor-associated causes, for example intravenous lines. This includes defective dialysis equipment.
Barotrauma, where pressure changes or pressure waves may rupture or lacerate the lungs allowing air to move from the alveolar spaces into the lung blood vessels. Mostly seen in two situations:
· explosions, especially in confined spaces
· diving accidents, where a diver surfaces without exhaling while surfacing.
The basic principle is that the volume of any gas, including air, is inversely proportionate to the pressure. A given volume of air will therefore occupy a small volume under water compared with at the surface, due to the higher pressure under water. If the diver does not gradually exhale while surfacing, the air in the lung will expand and the lung will rupture.
Stab wounds in the lung, where the lung tissue is damaged and the blood vessels come into contact with air.
Stab wounds in the neck, with damage to the carotid arteries. Air, and sometimes even fat, may be sucked into the vessel through the so-called Venturi-effect.
Paradoxical air embolism. This is found in two situations, namely in newborn babies and in adults. In the unborn baby (foetus) there is a connection between the right and left heart chambers, the foramen ovale. It is not necessary for blood to flow through the lungs, because the baby is still intra-uterine and does not use its lungs.
In a newborn this opening is still open, and the circulation is also not yet optimal. A small volume of air entering the venous circulation can go through the foramen ovale to the left side of the heart, and from there into the arterial circulation. This may obstruct blood flow to the heart and the brain.
In approximately 20% of adults this potential right-to-left connection may continue to exist. However, this opening will usually be occluded by a membrane, as long as the pressure in the left heart chambers is higher than in the right heart chambers.
If the pressure in the right heart chambers rises higher than that in the left chambers, which happens when the patient exhales against a closed nose and mouth (the Valsalva manoeuvre), this potential connection may open due to the change in the pressure gradient, and blood (and air) may leak from right to left. An air embolus can then move from the venous system to the arterial circulation, with obstruction of the blood supply of the brain and heart.
In other heart defects where there are openings in the septum, air emboli can also occur.
In certain circumstances the possibility of an air embolism must be considered before the post mortem is performed, for example when there is a stab wound in the neck, or a young female dies suddenly and unexpectedly (criminal abortion). In these cases it is advisable to take X-rays so that air in the heart and blood vessels could be detected.
Often air is found in the tissues surrounding the defect, and this can result in a crackling sound with palpation, so-called crepitus. The tissue can also appear swollen, known as surgical emphysema.
If a young female in her fertile years dies suddenly and unexpectedly, it is important to examine the uterus in situ, that is before it is removed. Any signs of recent pregnancy must be noted, and the inferior vena cava must be examined for air bubbles.
The next step is to open the pericardial sac and to examine the coronary arteries for an air embolism. The pericardial sac is then filled with water and the right heart chambers and thereafter the left heart chambers opened under water with a knife or scissors. If there is an air embolism, air will bubble out of the heart.
The cerebral arteries must also be inspected for air bubbles. It is important to remember that air can be sucked into the cerebral veins during the removal of the skull, and that this does then not indicate an air embolism, but is due to human intervention. If decomposition has set in, air may either dissolve and disappear if there indeed was an air embolism, or decomposition gases can be produced, presenting as an air embolism. The gases then have to be analysed in order to determine what they are.
Fat embolism occurs when fat cells, and sometimes even bone-marrow tissue, enter the circulation.
Certain conditions cause fat cells to enter the venous circulation and they can then become trapped in the capillary vessels of the lung. This not only causes mechanical obstruction to the blood flow, but also activates the blood clotting system. The fat cells then enter the arterial circulation, and eventually the brain, after the vascular beds of the lungs had been saturated with fat.
Some of the causal conditions are:
· fractures of the shaft of long bones, for instance the femur
· soft tissue injuries
· burn wounds
Almost 90% of patients with severe skeletal injuries develop fat emboli, but only 1% will present clinically. The clinical presentation is called the fat embolism syndrome. It usually presents 24 to 72 hours after injury, and it is fatal in 10% of cases. It has the following clinical features:
· lung insufficiency: the patient feels short of breath
· neurological symptoms: the patient is irritated, restless and suffers from delirium
· decreased platelets (thrombocytopenia): the patient shows petechial haemorrhages
Microscopic examination of the lung and brain tissue in the deceased can indicate the presence of fat in the tissue. In a living person fat drops can be detected in the sputum.
Two conditions which are not caused by complications due to injuries but from part of the embolism spectrum, need to be mentioned. They are caisson disease or decompression sickness and an amniotic fluid embolism.
· Caisson disease or decompression sickness is the condition which is sometimes confused with air or gas embolism. It is seen in divers, when the nitrogen and other gases in the blood come out of solution and form minute air bubbles when decompression is too rapid. This is the cause of the so-called bends.
· Amniotic fluid embolism is a dangerous, but fortunately rare, complica-tion of pregnancy and labour. When the amniotic fluid enters the maternal circulation (via a mechanism similar to that of air embolism), the amniotic fluid does not only cause mechanical obstruction to the blood flow in the lungs, but it also contains certain substances which can interact with the blood clotting mechanism. It can also suppress cardiac function.