Polycythemia is an increase in the circulating red blood blood cell count or a higher than normal concentration of hemoglobin in the blood. In the true sense of the word, polycythemia is not a high level of hemoglobin on its own but a consequence of either an elevated red blood cell count or low plasma levels.
Polycythemia may be physiological or pathological – meaning that it may occur as a means for the body to maintain a normal level of functioning in the absence of a disease (physiological) or due to a disease process (pathological). The causes of polycythemia can be classified as primary or secondary where a disease process triggers an actual increase in red blood cell production or it arises secondary to some other disease process, usually hypoxia.
The most common cause of physiological polycythemia is seen in people who live in high altitudes. The lower oxygen density in the atmosphere in these areas triggers the body to produce more red blood cells. In this way the blood has a greater oxygen-carrying capacity and hypoxia is overcome.
Another common cause of polycythemia is cigarette smoking. Here the hypoxic state related to daily smoking stimulates the body to produce more red blood cells.
Types of Polycythemia
There are broadly two types of polycythemia – true and relative. The hemoglobin levels are higher than normal in both types of polycythemia.
In true polycythemia there is an actual increase in the number of red blood cells. This means that the red blood cell count and the hemoglobin concentration is higher than normal.The blood plasma volume is normal and this is relevant when comparing it to relative polycythemia (below).
In relative polycythemia, the red blood cell count is normal but the plasma volume is low. Blood plasma is the watery fluid part of the blood that contains no cells. So in relative polycythemia, the blood cells are concentrated due to less fluid and not more cells. This type of polycythemia is also known as apparent polycythemia and is seen in severe dehydration often related to the use of diuretics or excessive alcohol consumption.
Effects of Polycythemia
Since red blood cells are the most abundant cells in the bloodstream, an increase in the number of red blood cells will increase the viscosity (thickness) of the blood. The same applies for a low plasma volume as is seen in relative polycythemia.
Hypervisocity slows the flow of blood through vessels as there is increased peripheral resistance. In very small vessels and capillaries, stasis may occur. This significantly increases the risk of blood clot formation – thrombus. If polycythemia occurs in a person with other risk factors for thrombosis, the chance of a blood clot formation leading to a significant narrowing of the artery is high.
Sluggish circulation slows the flow of blood and this allows for greater deoxygenation. In the superficial (skin) circulation, this can lead to a slightly bluish tinge to the skin.
Since the circulation is sluggish as a result of increased peripheral resistance, the amount of blood returning to the heart is reduced (venous return). At the same time however, the cardiac output is increased due to the viscosity of the blood. These two factors cancel each other out so that there is no significant increase in cardiac output or rise in blood pressure in most cases of polycythemia. However in severe polycythemia, arterial blood pressure may be significantly elevated.