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In physiology and medicine, hypotension refers to an abnormally low blood pressure. This is best understood as a physiologic state, rather than a disease. It is often associated with shock, though not necessarily indicative of it. Hypotension is not to be confused with hypertension, which is high blood pressure, the opposite of hypotension.
Blood pressure is continuously regulated by the autonomic nervous system, using an elaborate network of receptors, nerves, and hormones to balance the effects of the sympathetic nervous system, which tends to raise blood pressure, and the parasympathetic nervous system, which lowers it. The vast and rapid compensation abilities of the autonomic nervous system allow normal individuals to maintain an acceptable blood pressure over a wide range of activities and in many disease states.
Mechanisms and Causes
Reduced blood volume, called hypovolemia, is the most common mechanism producing hypotension. This can result from hemorrhage, or blood loss; insufficient fluid intake, as in starvation; or excessive fluid losses from diarrhea or vomiting. Hypovolemia is often induced by excessive use of diuretics. (Other medications can produce hypotension by different mechanisms.)
Decreased cardiac output despite normal blood volume, due to severe congestive heart failure, large myocardial infarction, or bradycardia, often produces hypotension and can rapidly progress to cardiogenic shock. Arrhythmias often result in hypotension by this mechanism. Beta blockers can cause hypotension both by slowing the heart rate and by decreasing the pumping ability of the heart muscle.
Excessive vasodilation, or insufficient constriction of the resistance blood vessels (mostly arterioles), causes hypotension. This can be due to decreased sympathetic nervous system output or to increased parasympathetic activity occurring as a consequence of injury to the brain or spinal cord or of dysautonomia, an intrinsic abnormality in autonomic system functioning. Excessive vasodilation can also result from sepsis, acidosis, or medications, such as nitrate preparations, calcium channel blockers, or ACE inhibitors. Many anesthetic agents and techniques, including spinal anesthesia and most inhalational agents, produce significant vasodilation.
Orthostatic hypotension, also called "postural hypotension", is a common form of low blood pressure. It occurs after a change in body position, typically when a person stands up from either a seated or lying position. It is usually transient and represents a delay in the normal compensatory ability of the autonomic nervous system. It is commonly seen in hypovolemia and as a result of various medications. In addition to the classes of blood pressure-lowering medications listed above, many psychiatric medications, in particular antidepressants, can have this side effect. Simple blood pressure and heart rate measurements while lying, seated, and standing can confirm the presence of orthostatic hypotension.
Neurocardiogenic Syncope is a form of dysautonomia characterized by an inappropriate drop in blood pressure while in the upright position. Neurocardiogenic syncope is related to vasovagal syncope in that both occur as a result of increased activity of the vagus nerve, the mainstay of the parasympathetic nervous system.
Another — albeit rare — form, is Postprandial hypotension, which occurs 30–75 minutes after eating. Because digestion sends a great deal of blood to the intestines, the body must increase heart rate and vasoconstriction to compensate for local blood volume changes. It is believed that this form of hypotension is caused by a disordered or aging autonomic nervous system not compensating appropriately while digestion takes place.
For most individuals, a healthy blood pressure lies from 90/50 mmHg to 135/90 mmHg. A small drop in blood pressure, even as little as 20 mmHg, can result in transient hypotension.
Evaluating Neurocardiogenic Syncope is done with a tilt table test.
Low blood pressure is often accompanied by: (Most of these are related to causes rather than effects of hypotension.)
- Chest pain
- Shortness of breath
- Irregular heartbeat
- Fever higher than 101 °F (38.3 °C)
- Stiff neck
- Severe upper back pain
- Cough with phlegm
- Prolonged diarrhea or vomiting
- Inability to eat or drink
- Burning with urination
- Foul-smelling urine
- Adverse effect of medications
- Acute, life-threatening allergic reaction
- Dizziness, or light-headedness, particularly when suddenly standing up from sitting down
- Loss of consciousness
- Profound fatigue
- systemic vasoconstrictors
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