The most common reason for a drop in blood pressure during surgery is a reaction to anesthesia, the drugs administered to put the patient to sleep. As a side effect of the anesthesia, blood vessels dilate throughout the body. Dilation of the blood vessels causes them to grow larger in diameter, which in turn increases the flow of blood but decreases the pressure at which that blood flows. This loss in blood pressure may self-correct during surgery as the body reacts to the cutting and other invasive acts of surgery by producing hormones which raise blood pressure. If not, the anesthesiologist will administer drugs to correct the problem.
Losing blood occurs in any surgery, though the amount lost depends upon the type of surgery performed. The very act of cleaning the wounds causes blood loss, as does the cutting and shifting of organs. As the body loses blood, the pressure of the remaining blood flow decreases. This drop in blood pressure is corrected through drugs, fluids and sometimes blood transfusions if the blood loss is severe enough.
Outside influences, such as medications and cardiovascular condition, can also cause a drop in the blood pressure of a surgery patient. These reactions can be more difficult to treat during surgery, which is why it is important to alert the medical staff of any preexisting conditions or medications, particularly if they are over-the-counter medications.