Vasoconstrictor & vasodilation medications

Vasoconstriction (constriction of blood vessels) and vasodilation (dilation of blood vessels) are normal body functions. Maintenance of a balance of the two counteracting processes and immediate reaction to acute needs are an important aspect of keeping a body alive. Flexible and rapid regulation of the two counteracting effects is necessary to provide tissues with nutrition and oxygen according to need, to enable regular and uninterrupted blood flow in general (homeostasis), as an emergency response to injury, and to maintain body temperature in warm-blooded animals including humans.

Balance of vasoconstriction and vasodilation is mediated by a number of different metabolic control cycles. The effect on blood vessels is achieved by action on the smooth muscles that form part of the walls of blood vessels which constrict or dilate, according to chemical signals from messenger molecules. The main neurotransmitters (messenger molecules) involved in the vasoconstriction process are adrenaline and noradrenaline, angiotensin and vasopressin. The central nervous system (brain and spinal cord), the sympathetic and parasympathetic, involuntary nervous systems are all part of the processes.

Vasodilators are substances that widen the blood vessels, either by direct action on the smooth muscles in the walls of the blood vessels, or by interacting with regulating (vasomotor) centres in the brain. Vasodilator drugs are powerful agents to reduce blood pressure. Vasodilators acting directly on the smooth muscles of the cell walls (hydralazine, minoxidil) have serious adverse effects, so their clinical use is limited to emergency interventions to treat otherwise resistant very high blood pressure, or high blood pressure in the presence of kidney failure. Many foods contain components with mild vasodilation effects, for example theobromine in chocolate or alcohol.

Vasoconstrictors, also known as vasopressors, are agents that cause constriction of blood vessels and raise blood pressure in this way. The raising of blood pressure can be achieved by direct action of an agent on the smooth muscles in the walls of the blood vessels or by interacting with various regulating mechanisms in the nervous system. Many foods contain components with mild vasoconstriction effects, for example caffeine or nicotine. Vasoconstrictor medications include agonists of vasopressin, agonists of the alpha-adrenoceptor (the receptor for adrenaline), adrenaline and noradrenaline, dopamine, dobutamine, phenylephrine.

In the context of maxillofacial surgery, vasoconstricting agents have various uses. Vasoconstriction is a means of stopping bleeding, for example a nose bleed or similar. Triptans are a particular type of medication for the treatment of migraines, one aspect of their specific working mechanism is their ability to trigger vasoconstriction. Vasodilating agents, including papaverine (an alkaloid isolated from opium), lidocaine and verapamil (a calcium channel blocker, otherwise used in the treatment of high blood pressure) are used in microvascular surgery to overcome spasms of the smooth muscles of the walls of blood vessels.

An important reason for maxillofacial surgery (and other disciplines) intervention is in the treatment of shock, the body’s emergency reaction to a variety of causes. Generally, the most common cause for shock is sepsis, other causes are severe blood loss in trauma, severe loss of other fluids (for example dehydration of large burn wounds), bacterial infections, severe allergic reactions (anaphylactic shock) and other out-of-control inflammatory conditions, nerve damage (in particular central nervous system injuries), effects of drugs and toxins, malfunction of the endocrine system (for example, hypothyroidism), extremely low blood sugar levels (hypoglycaemia). Haemorrhagic shock as a result of trauma and severe blood loss is the most common form of shock encountered in maxillofacial surgery; the common emergency interventions (application of pressure, surgical ligation, reduction and fixation of fractures) include the use of adrenaline as a locally applied vasoconstrictor (and often as part of a local analgesic) and/or the use of tranexamic acid to help stop excessive bleeding. This, in turn, will re-establish oxygen delivery to tissues by re-establishing and stabilising regular blood flow (and blood pressure).