Embolism

Definition 

 An embolus is a detached intravascular solid, liquid or gaseous mass that is carried by blood to sites distant from its point of origin. After traveling via the blood, the embolus can obstruct a vessel.   

Causes of embolism:

 An embolus can arise from:

-   Thrombus (99% of emboli arise from a thrombus. Such an embolus is called thromboembolus)

-  Platelets aggregates 

-   Fragment of material from ulcerating atheromatous plaque 

-   Fragment of a tumour 

-   Fat globules 

-   Bubbles of air 

-   Amniotic fluid 

-   Infected foreign material 

-   Bits of bone marrow 

-  Others. 

 Unless otherwise specified, the term embolism should be considered to mean thromboembolism. This is because thromboembolism is the commonest form of embolism.

Next, we will discuss it in more detail.

Thromboembolism

 Based on its sites of origin & impaction, thromboembolism can be divided into:

a) Pulmonary thromboembolism (PTE)

-   PTE is refers to the impaction of an embolus in the pulumonary arteries & their branches. Such an embolus is derived from a thrombus in the systemic veins or the right side of the heart.

b) Systemic thromboembolism

-   Systemic emboli arise from the left side of the heart or from thrombi & atheromatous debris in large arteries. And they impact in the systemic arteries.

c) Crossed embolism (Paradoxical embolism)

-   This occurs in the presence of patent foremen ovale when an embolus is transferred from the right to the left side of the heart, then into the systemic circulation.

Now, we will elaborate the first two.

 a) Pulmonary thrombeomblism (PTE) 

95% of PTE arise from thromi in the deep leg veins. The thromboembolus will travel long with the venous return & reach the right side of the heart. From there, it will go into the pulmonary trunk & pulmonary arteries. Depending on the size of the embolus and on the state of pulumonary circulation, the pulmonary embolism can have the following effects:

1.  If the thrombus is large, it may block the outflow tract of the right ventricle or the bifurcation of the main pulumonary trunk (saddle embolus) or both of its branches, causing sudden death by circulatory arrest. Sudden death, right side heart failure (cor pulmonale), or cardiovascular collapse occurs when 60% or more of the pulumonary circulation is obstructed with emboli.

2.  If the embolus is very small (as in 60-80% of the cases), the pulmonary emboli will be clinically silent. Embolic obstruction of medium sized arteries manifests as pulmonary haemorrhage but usually does not cause infarction because of dual blood inflow to the area from the bronchial circulation.

3.  If the cardiorespiratory condition of the patient is poor (i.e., if the patient previously had cardiac or pulmonary disease), then obstruction of a medium sized pulmonary artery by a medium-sized embolus can lead to pulmonary infarction.  

4.  Recurrent thromboembolism can lead to pulmonary hypertension in the long run. A patient who has had one pulmonary embolus is at high risk of having more.  

b) Systemic thromboembolism 

•  Systemic thromboembolism refers to emboli  travelling within arterial circulation & impacting in the systemic arteries.

•  Most systemic emboli (80%) arise from intracardiac mural thrombi. In turn, two thirds of intracardiac mural thrombi are associated with left ventricular wall infarcts and another quarter with dilated left atria secondary to rheumatic valvular heart disease. 

•  The remaining (20%) of systemic emboli arise from aortic aneurysm, thrombi on ulcerated athrosclerotic plaques, or fragmentation of valvular vegetation. 

•  Unlike venous emboli, which tend to lodge primarily in one vascular bed (the lung), arterial emboli can travel to a wide variety of sites. The major sites for arteriolar embolization are the lower extremities (75%) & the brain (10%), with the rest lodging in the intestines, kidney, & spleen. The emboli may obstruct the arterial blood flow to the tissue distal to the site of the obstruction. This obstruction may lead to infarction. The infarctions, in turn, will lead to different clinical features which vary according to the organ involved.

 Next, we will briefly touch upon some rare forms of embolism.

 Fat Embolism 

 Fat embolism usually follows fracture of bones and other type of tissue injury. After the injury, globules of fat frequently enter the circulation. Although traumatic fat embolisms occur usually it is as symptomatic in most cases and fat is removed. But in some severe injuries the fat emboli may cause occlusion of pulmonary or cerebral microvasculature and fat embolism syndrome may result. Fat embolism syndrome typically begins 1 to 3 days after injury during which the raised tissue pressure caused by swelling of damaged tissue forces fat into marrow sinsosoid & veins. The features of this syndrome are a sudden onset of dyspnea, blood stained sputum, taccycardia, mental confusion with neurologic symptoms including irritability & restlessness, sometimes progress to delirium & coma. 

5. Air embolism 

Gas bubbles within the circulation can obstruct vascular flow and cause distal ischemic injury almost as readily as thrombotic masses. Air may enter the circulation during: 

• Obstetric procedures 

•  Chest wall injury 

•  In deep see divers & under water construction workers. 

•  In individuals in unpressurized aircraft

Neck wounds penetrating the large veins 

•  Cardio thoracic surgery. 

•  Arterial catheterisation& intravenous infusion. 

• Etc.

Generally, in excesses of 100cc is required to have a clinical effect and 300cc or more may be fatal. The bubbles act like physical obstructions and may coalesce to form a frothy mass sufficiently large to occlude major vessels. 

 Amniotic fluid embolism 

It is a grave but un common, unpredictable complication of labour which may complicate vaginal delivery, caesarean delivery and abortions. It had mortality rate over 80%. The amniotic fluid containing fetal material enters via the placental bed & the ruptured uterine veins. The onset is characterized by sudden severe dyspnea, cyanosis, hypotensive shock followed by seizure & coma of the labouring mother. If the patient survives the initial crisis, pulmonary oedema typically develops & 50% of the cases will develop DIC due to activation of the coagulation cascade by fetal material. 

References

Bezabeh ,M. ; Tesfaye,A.; Ergicho, B.; Erke, M.; Mengistu, S. and Bedane,A.; Desta, A.(2004). General Pathology. Jimma University, Gondar University Haramaya University, Dedub University.