Blood pressure is measured as two numbers, systolic pressure over diastolic pressure, with normal blood pressure for most people being 120/80. Hypertension, or high blood pressure is defined as a systolic over 139 or a diastolic over 89. Most causes of hypertension are unknown. This is called primary, essential or idiopathic hypertension. Secondary hypertension is caused by a known process such as kidney disease. Rebound hypertension is caused by withdrawal from certain medications used to lower the blood pressure. As such, initiation and cessation of drug therapy should be done under the guidance of a healthcare professional.
Initiation of Medication
The course to rebound hypertension begins with the initiation of medications to lower blood pressure. Certain medications are more apt than others to cause rebound hypertension, one common such medication is clonidine. Once the drug is started, the blood pressure will start to lower. In the case of clonidine, the drug binds to receptors on nerves in the brain called alpha-2 receptors. This lowers the levels of calcium in the nerve and results in the nerve releasing less of a transmitter called norepinephrine. The lowered norepinephrine levels decrease the blood pressure.
Escalation and Tolerance
As the drug is administered, it is often started at a lower dose and then escalated. One reason for this is tolerance. The body tends to regulate itself to stay within its own set range. If a low dose of clonidine is given, it binds to the alpha-2 receptors on the nerve cells. The nerve cells respond by adding more alpha-2 receptors, bringing the blood pressure back towards normal. When more receptors are added, they are said to be "up-regulated." The dose of the drug is elevated, binding to more of the alpha-2 receptors. The nerve adds more receptors until it cannot add enough to counteract the drug, then the blood pressure will be lowered, achieving the intended therapeutic effect.
Withdrawal and Rebound
Withdrawal and rebound occur when the drug is stopped suddenly. All of those receptors that were added to counteract the drug are suddenly available when the drug is stopped. This drives the blood pressure up, potentially even higher than when the drug was first initiated. This is called rebound hypertension. The body recognizes that it is receiving more signals from the receptors than usual and ends up moving in the other direction, taking away receptors on the nerve cells until the pre-medication blood pressure is achieved.
References
- "Basic and Clinical Pharmacology"; Bertram Katzung et al; 11th Ed 2009
- "Harrison's Principles of Internal Medicine"; Anthony S. Fauci et al; 17th Ed 2008
