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A vasopressor agent is any medication that tends to raise reduced blood pressure. Some antihypotensive drugs act as vasoconstrictors to increase total peripheral resistance, other sensitize adrenoreceptors to catecholamines - glucocorticoids, and the third class increase cardiac output - dopamine, dobutamine.
If hypotension is due to loss of blood, then preparations increasing volume of blood circulation—plasma-substituting solutions such as colloid and crystalloid solutions (salt solutions)—will raise the blood pressure without any direct vasopressor activity. Packed RBCs, plasma or whole blood should not be used solely for volume expansion or to increase oncotic pressure of circulating blood. Blood products should only be used if reduced oxygen carrying capacity or coagulopathy is present. Other causes of either absolute (dehydration, loss of plasma via wound/burns) or relative (third_space losses) vascular volume depletion also respond, although blood products are only indicated if significantly anemic.