Tuesday, January 12, 2010

Mechanism of Pulmonary Embolism

Looking back on 2009, our records show that, once again, pulmonary embolism ranked as the most common medical malpractice issue litigated by our clients. These cases involve blood clots formed in the veins that break off and travel to the chest, through the heart to become lodged in the lungs causing respiratory distress. The popularity of this topic encourages me to provide my readers with a bit of background information. Although the basic mechanism of a pulmonary embolism is straightforward, there are associated issues and terminology that you should be familiar with before you encounter one of these common cases in your future practice.

First of all, why are pulmonary embolism cases so common? Most likely this is because pulmonary emboli can be secondary conditions resulting from prolonged inactivity such as surgical procedures, bed rest or even lengthy airline travel. Therefore, there are a large number of situations that might put a person at risk for this dangerous complication. Generally litigation revolves around the failure to prevent, diagnose or properly treat these emboli.

To understand the mechanism of a pulmonary embolism, we must first understand the anatomy of the veins. The veins are the system of vessels throughout the body that return blood from the various body regions to the heart. Unlike arteries, the veins do not have high internal pressure to keep the blood moving along but actually require assistance from the muscle contractions involved in normal daily activity. This problem of moving the blood along is most pronounced the further the veins are from the heart, and since gravity also acts against venous flow the flow of blood is slowest in the lower extremities (legs and feet). This slowing of blood flow in the veins is a key component in the mechanism of pulmonary embolism. In situations when the body is immobilized due to surgery, convalescence or prolonged inactivity there are no muscle contractions to assist in speeding blood flow. In these cases, flow can slow to such an extent that the normal clotting factors within the blood begin to react and form blood clots. This is often seen at the site of a valve in the vein where blood can pool within the eddy behind the leaflet of the valve. This clot that remains at the site in which it originally formed is referred to as a thrombus. The formation of these clots is called deep vein thrombosis (DVT).

Once a thrombus is formed within a deep vein, it will continue to grow as additional clotting factors collect and adhere themselves to the original clot. But this thrombus is not completely stable. The real problem lies in the fact that pieces can break off from the thrombus and join in the blood stream to be carried upward toward the heart and lungs. Once a clot joins the blood stream and begins to move, we no longer call it a thrombus. It is now called an embolus. To reach the heart and lung from the legs, the embolus must travel up through the inferior vena cava, a large vein in the posterior abdomen. Eventually the embolus will reach the heart where it will travel through the left atrium and ventricle and be pumped out into the pulmonary arteries that lead to the lungs. This embolus will continue to travel within the pulmonary vasculature, which divides again and again into smaller and smaller arteries within this tree-like network, until the embolus reaches a vessel too small to pass through. At this point the embolus will become lodged forming a dam that will block further blood flow through this artery. This blockage of blood flow and the resulting reduction in lung function is called an embolism.

Generally, the smaller the embolus, the less severe the effects will be since a smaller percentage of lung volume will be affected by the blockage of a smaller pulmonary vessel. But often there are a number of emboli released at once or over time leading to a "shower" of emboli that can cause multiple blockages to one or both lungs. These multiple blockages can be as severe as one larger blockage resulting in a higher percentage of total lung function being lost. The most severe form of embolism is referred to as a saddle embolus. This type of embolus is so large that it cannot pass beyond the bifurcation (division) of the main pulmonary trunk into the left and right main pulmonary arteries and therefore causes a sudden blockage of blood flow into both lungs. This condition can often be almost instantly fatal.

The healthcare provider's first responsibility is prevention of deep vein thrombosis. During prolonged surgery, compression stockings and pneumatic compression devices can be used to increase circulation and prevent the pooling that leads to thrombosis. Medications can also be given to thin the blood and prevent clotting. If prevention of clotting in the legs is unsuccessful, medication or even surgery may be necessary to break up and remove the clots from the deep veins. Also, mechanical filters can be placed within the inferior vena cava beneath the heart to prevent emboli from reaching the lungs.

So, as you can see, these cases are generally rather straightforward. A thrombus forms in the deep veins due to inactivity. Pieces of the thrombus break of and join the bloodstream as emboli. These emboli travel through the inferior vena cava and heart to the lungs. These emboli block blood flow through pulmonary arteries creating an embolism that can lead to respiratory distress and possibly death. Those are the basics, and if you can appreciate the basic mechanism, it should be easy to understand any slight variations that may arise in any unique case.

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