What Causes Low Blood Pressure After Surgery?

The first couple of hours following surgery is a critical time period. As a person begins to recover from anesthesia and surgery, changes in blood pressure are common. Blood pressure -- the outward force exerted by blood against the walls of blood vessels -- may be either higher or lower than normal. In the early postoperative period, blood pressure is measured at frequent intervals to promptly identify changes and begin appropriate treatment, if necessary. Low blood pressure has 3 main causes: decreased volume of blood, heart problems, and dilated blood vessels.

Decreased Blood Volume

A reduced amount of blood in the body, called hypovolemia, is one of the most common reasons for low blood pressure after surgery. Sometimes this is due to excessive bleeding during the operation. The amount of bleeding is typically small during minor surgery, but it can be extensive during major surgery. Bleeding may also continue after the operation, although the amount is generally small, even after major surgery. A large amount of postoperative bleeding can be life-threatening and may require immediate additional surgery.

Even when surgery does not cause excessive bleeding, blood volume can decrease because of evaporation. During surgery, water evaporates from the surface of tissues in the area being operated on. This loss of fluid leads to a decrease in plasma -- the liquid portion of blood in which blood cells are suspended. The larger the incision, the larger the area being operated on, and the greater the amount of fluid lost in this manner. Low blood pressure due to hypovolemia is usually treated with fluids or blood administered through tubing into a vein.

Heart Problems

When the heart is unable to pump an adequate amount of blood to the rest of the body, the blood pressure will fall. This sometimes occurs because the heart rate is too slow. A slow rate can be caused by certain medications, such as beta-blockers that are used for heart disease or high blood pressure. Some anesthetic medications and painkillers may also produce a slow heart rate. A number of medical conditions may reduce the heart rate as well, such as pre-existing heart disease, changes in certain electrolyte levels in the blood and low oxygen levels in the body.

Low blood pressure may also occur because the heart muscle is weakened and cannot pump effectively. Myocardial infarction, in which an area of the heart muscle dies because of inadequate blood supply, is one of the most common causes of a weakened heart muscle. In some people, poor heart muscle function occurs for the first time during or after surgery. In others, the heart muscle is weak due to pre-existing heart disease and the stress of surgery may further reduce its ability to pump blood.

Dilated Blood Vessels

When blood vessels expand to become unusually wide -- called dilated blood vessels or vasodilation -- the pressure on the wall of the vessels decreases, causing low blood pressure. Vasodilation may be produced by a number of medications, especially those used to treat heart disease or high blood pressure. It may also be a sign of a serious allergic reaction, which may occur with exposure to any medication or other substance around the time of surgery. A spinal or epidural anesthetic produces vasodilation in the lower part of the body. This can cause the blood pressure to be low after surgery until the anesthetic wears off.

Any increase in body temperature will cause blood vessels in the body to dilate, which may lower the blood pressure. Life-threatening low blood pressure occurs with sepsis. In this condition, a severe infection enters the bloodstream and causes inflammation throughout the body. Although extremely rare after minor surgery, sepsis may occur after major surgery, especially when an infection is present in the area being operated on or the person has an impaired immune system, as may occur with HIV or chemotherapy for cancer.

Additional Considerations

With minor surgery, low blood pressure is most likely to occur in the first couple of hours after the operation. At this time, you will probably be cared for in a post-anesthesia care unit, where appropriate treatment will be quickly administered. You will not be allowed to go home until your blood pressure returns to normal and you are able to move about without feeling dizzy or lightheaded. If you had major surgery, the possibility of low blood pressure may persist for a couple of days or longer.

Regardless of whether you are still in the hospital or have been discharged to home, notify a medical professional immediately if you notice excessive bleeding, shortness of breath, chest pain, an increased temperature, extreme weakness, decreased urination, dizziness or lightheadedness, or confusion, as these may indicate that your blood pressure is low.

Reviewed by: Mary D. Daley, MD

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