The human body is uniquely adapted to functioning while in an upright position, sometimes, for more than 16 hours in a day. Usually, sleep in the supine position for up to 8 hours per day is well tolerated by the body. However, research is still ongoing on the effects of prolonged bed rest (exceeding 24 hours) on the various body systems.
Referencing a 2009 work by Knight J. et al; (Effects of bed rest 1: cardiovascular, respiratory and hematological systems. Nursing Times; 105: 21), the diuresis associated with bed rest causes a gradual reduction in plasma volume. After a person has spent a week in bed, around 10% of plasma volume is lost, increasing to around 15% after four weeks. In the early stages of bed rest, the total red cell mass remains relatively constant, but, as plasma volume is lost, there is an increase in the hematocrit (packed red cell volume), leading to a significant increase in blood viscosity.
Furthermore, prolonged bed rest activates all three factors of Virchow’s triad and it has been estimated that up to 13% of patients in bed for long periods may develop DVT (Virchow’s triad refers to a combination of three factors – venous stasis, hypercoagulability and blood vessel damage – which, when present together, dramatically increase the chances of deep vein thrombosis developing).
Venous stasis: As the skeletal muscle pump becomes less efficient, blood flow within the veins of the lower limbs can become sluggish. In some veins, blood flow may cease completely, leading to the pooling of blood and venous stasis.
Hypercoagulability: Because blood is pooling in the veins of the lower limbs, clotting factors are not cleared as quickly by the liver. This, together with reduced plasma volume and the increased hematocrit seen in bedridden patients, increases the viscosity of the blood and further increases the likelihood of clot formation (thrombosis).
Blood vessel damage: The inner endothelial lining of arteries and veins is only one layer of cells thick and is extremely delicate. It rests on top of a layer of collagen-rich connective tissue and is incredibly smooth to minimize drag and resistance, and maintain a free flow of blood. The continual weight of the supine body compresses blood vessels and can cause damage to the vulnerable endothelium, especially if patients are not turned regularly.
This mechanical damage, often made worse by the pooling of blood and venous stasis, leads to the death of endothelial cells, exposing the collagen-rich tissue beneath. Platelets rapidly stick to the exposed collagen fibers and become activated, prompting the formation of blood clots.
This pattern of DVT is common not only after prolonged bed rest but also with immobility of any kind. Cramped economy seats on long-haul aeroplane flights put passengers at risk of DVT formation in much the same way, a situation that is often referred to as economy-class syndrome.
The entire article is an excellent review of the effects of prolonged bed rest on the various body systems and I highly recommend it. You may access the entire article via this link.
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