Why does orthostatic hypotension happen?

Author: SCIRE Community Team | Reviewer: Darryl Caves | Published: 9 April 2018 | Updated: ~

Blood pressure changes in response to changes in body position.²

When the body moves into an upright position, the blood naturally flows downward because of gravity. If the body does not respond in any way to help push the blood back up, it pools in the blood vessels of the lower body, causing a drop in blood pressure in the upper body. If blood pressure is low, not enough blood can reach the heart and brain, which causes symptoms like dizziness, fainting and tiredness.

Under normal circumstances, when we move upright, the body’s blood pressure control mechanisms kick in to maintain circulation. The main changes include:

• The blood vessels narrow (called vasoconstriction), to prevent pooling and maintain blood pressure.
• The heart works harder by pumping more quickly to help push blood through circulation.

The combined narrowing of blood vessels and increase in heart rate help maintain blood pressure against the force of gravity in standing. These changes are controlled by the autonomic nervous system.

Orthostatic hypotension is common after SCI, particularly in the early period after injury. There are several reasons that orthostatic hypotension happens after SCI.

During the early phase after injury, bed rest is often required for healing of injuries. However, long periods of bed rest and reduced movement can cause the heart and blood vessels to become less responsive to position changes and changes in blood pressure.

During bed rest, there is a shift in the distribution of fluids within the body which causes a loss of fluid through urination (called diuresis). This leads to reduced blood volume (the volume of blood in circulation), which affects the body’s ability to adapt to changes in blood pressure. This likely contributes to orthostatic hypotension in the early phase after SCI.

The body may also produce more of a chemical called nitric oxide as a result of bed rest following SCI, which causes widening (relaxing) of the blood vessels and further contribute to orthostatic hypotension.

The main reason that orthostatic hypotension happens after SCI is because of altered control of the autonomic nervous system.

The autonomic nervous system controls largely unconscious bodily processes such as blood pressure, heart rate, breathing rate, body temperature, digestion, bladder, bowel and sexual function. It has two divisions:

• The sympathetic nervous system prepares the body for stressful or emergency situations. It is often called the ‘fight or flight’ system, because it prepares the body for action. For example, it increases heart rate and constricts blood vessels.
• The parasympathetic nervous system prepares the body for normal, non-emergency situations. It is often called the ‘rest and digest’ system, because it allows the body to restore itself. For example, it slows heart rate and relaxes blood vessels.

The sympathetic and parasympathetic systems have different (and often opposite) effects on the organs and work together to control bodily functions according to the situation.

The autonomic nervous system controls various body functions.^3-6

Most of the nerves that control the sympathetic nervous system arise from the thoracic spinal cord. Because the sympathetic nerves are responsible for narrowing (constricting) the blood vessels and increasing heart rate when changing positions, SCI in these areas can affect blood pressure control. Without the sympathetic nervous system, the blood vessels and heart rate do not respond appropriately to changes in position and blood can pool in the legs and abdomen, leading to orthostatic hypotension.

Loss of muscle activity in the legs and trunk after SCI may also contribute to orthostatic hypotension. Normally, when the muscles tense, they act like small pumps that squeeze the blood vessels to help push blood back up to the heart. Muscle activity in the legs and abdomen helps to maintain blood pressure in standing. This is sometimes called the skeletal muscle pump.

If there is paralysis in the leg muscles, the muscles do not help to pump blood back up to the heart, which can lead to blood pooling and orthostatic hypotension. This is why sometimes just sitting for a long time after a position change to upright can lead to symptoms of orthostatic hypotension. The effects of the loss of muscle activity seem to have a greater impact earlier after injury, before the body develops strategies to compensate for blood pressure changes.

During the early phase after injury, bed rest is often required for healing of injuries. However, long periods of bed rest and reduced movement can cause the heart and blood vessels to become less responsive to position changes and changes in blood pressure. This likely contributes to orthostatic hypotension in the early phase after SCI.

The body may also produce more of a chemical called nitric oxide as a result of bed rest following SCI, which causes widening (relaxing) of the blood vessels and further contributes to orthostatic hypotension.

The body regulates blood volume (the amount of blood in circulation) through water and salt levels in the blood. Spinal cord injury can disrupt this balance and lead to low blood volume, low blood pressure and orthostatic hypotension. Low salt levels in the blood are also common after SCI, which can also contribute to orthostatic hypotension. A diet that is low in salt or fluids can also contribute to orthostatic hypotension.

Warm environments can increase the likelihood of experiencing orthostatic hypotension. The body responds to heat by widening the blood vessels, which lowers blood pressure. Excessive sweating in warm environments may also lead to dehydration, which further contributes to orthostatic hypotension.

Many medications can trigger orthostatic hypotension. These include medications used to treat conditions such as high blood pressure, heart disease and erectile dysfunction.

Consuming alcohol and caffeine can worsen orthostatic hypotension by affecting the constriction of blood vessels during position changes and contributing to dehydration.

After eating, more blood is sent to the intestines for digestion, which may lower blood pressure. Postprandial hypotension is a form of orthostatic hypotension experienced after eating large meals. It is more common among older people.

People with complete cervical SCI may experience orthostatic hypotension during and after exercise, even if they are well-hydrated and have taken steps to avoid overheating. This happens because the heart and blood vessels do not respond adequately to compensate for the increased blood flow to the muscles. This can cause symptoms like dizziness or nausea while exercising. To adjust for this, individuals may start their exercise programs with a slow progression or may need to use other strategies to enable them to exercise safely.