Hypotension

Hypotension Classification and external resources
ICD-10	I95
ICD-9	458
DiseasesDB	6539
MedlinePlus	007278
MeSH	D007022

In physiology and medicine, hypotension refers to a condition of abnormally low blood pressure, in contrast to hypertension, which is high blood pressure. Hypotension is best understood as a physiologic state, rather than a disease. Since blood pressure varies according to factors such as exercise and stress, and people can be healthy and yet have blood pressure well below the average, the term hypotension generally is reserved for a minimal condition where blood pressure is so low that insufficient blood reaches the brain, causing dizziness and fainting (Turkington 2004).

Hypotension often is associated with shock, though not necessarily indicative of it. Hypotension can be life-threatening.

Normal physiology

Blood pressure refers to the force exerted by circulating blood on the walls of blood vessels, and constitutes one of the principal vital signs. The pressure of the circulating blood decreases as blood moves through arteries, arterioles, capillaries, and veins; the term blood pressure generally refers to arterial pressure, i.e., the pressure in the larger arteries, arteries being the blood vessels which take blood away from the heart. The systolic arterial pressure is defined as the peak pressure in the arteries, which occurs near the beginning of the cardiac cycle; the diastolic arterial pressure is the lowest pressure (at the resting phase of the cardiac cycle). The average pressure throughout the cardiac cycle is reported as mean arterial pressure; the pulse pressure reflects the difference between the maximum and minimum pressures measured.

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.

Blood pressure in the arteries rises and falls according to the demands of daily life, rising in cases of exercise and stress and falling during sleep. Healthy people, with completely normal hearts and blood vessels, may have blood pressure well below the average for their age, such as athletes who are in superior physical shape. For such reasons, just having a below average blood pressure is not typically classified as hyptotension. Rather, the term is used when blood pressure is so low that not enough blood can reach the brain, resulting in fainting or dizziness (Turkington 2004).

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. Varieties of meditation and/or other mental-physiological disciplines can create temporary hypotension effects, as well, and should not be considered unusual.

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, angiotensin II receptor inhibitors or ACE inhibitors. Many anesthetic agents and techniques, including spinal anesthesia and most inhalational agents, produce significant vasodilation.

Syndromes

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 blood pressure-lowering medications, many psychiatric medications, in particular antidepressants, can have this side effect. Simple blood pressure and heart rate measurements while lying, seated, and standing (with a two-minute delay in between each position change) can confirm the presence of orthostatic hypotension. Orthostatic hypotension is indicated if there is a drop in 20 mmHg of systolic pressure (and a 10 mmHg drop in diastolic pressure in some facilities) and a 20 bpm increase in heart rate.

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, but rarer form, is postprandial hypotension, which occurs 30–75 minutes after eating substantial meals. When a great deal of blood is diverted to the intestines to facilitate digestion and absorption, the body must increase cardiac output and peripheral vasoconstriction in order to maintain enough blood pressure to perfuse vital organs, such as the brain. It is believed that postprandial hypotension is caused by the autonomic nervous system not compensating appropriately, because of ageing or a specific disorder.

Indicators

For most individuals, a healthy blood pressure ranges from 90/60 mmHg to 120/80 mmHg. A small drop in blood pressure, even as little as 20 mmHg, can result in transient hypotension.^{[citation needed]}

Evaluating neurocardiogenic syncope is done with a tilt table test.

Symptoms

The cardinal symptom of hypotension is lightheadedness or dizziness. If the blood pressure is sufficiently low, fainting and often seizures will occur.

Low blood pressure is sometimes associated with certain symptoms, many of which are related to causes rather than effects of hypotension:

• Chest pain
• Shortness of breath
• Irregular heartbeat
• Fever higher than 101 °F (38.3 °C)
• Headache
• Stiff neck
• Severe upper back pain
• Cough with phlegm
• Prolonged diarrhea or vomiting
• Dysphagia
• Dysuria
• Foul-smelling urine
• Adverse effect of medications
• Acute, life-threatening allergic reaction
• Seizures
• Loss of consciousness
• Profound fatigue

Treatment

The treatment for hypotension depends on its cause. Asymptomatic hypotension in healthy people usually does not require treatment. Severe hypotension needs to be aggressively treated because reduced blood flow to critical organs including the brain, heart and kidneys may cause organ failure and can ultimately lead to death. Treatment options include systemic vasoconstrictors and other drugs. Adding sodium to a diet can also relieve symptoms of mild hypotension. In mild cases, where the patient is still responsive, laying the person in dorsal decubitus (laying over the back) position and lifting the legs will increase venous return, thus making more blood available to critical organs at the chest and head.

References

ISBN links support NWE through referral fees

• Turkington, C. A. 2004. Hypotension. Pages 1752-1753 in J. L. Longe, The Gale Encyclopedia of Medicine, 2nd edition, volume 3. Detroit, MI: Gale Group/Thomson Learning. ISBN 0787654922 (volume); ISBN 0787654892 (set).

• Hypotension details
• hypotension WebMD
• hypotension Medterms

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