Hypotension is a decrease in systemic blood pressure below accepted low values. While there is not an accepted standard hypotensive value, pressures less than 90/60 are recognized as hypotensive. Hypotension is a relatively benign condition that is under-recognized mainly because it is typically asymptomatic. It only becomes a concern once pumping pressure is not sufficient to perfuse key organs with oxygenated blood. This leads to symptoms impacting the quality of life of a patient.  Hypotension is classified based on the biometric parameters of the blood pressure measurement. It may be absolute with changes in systolic blood pressure to less than 90 mm Hg or mean arterial pressure of less than 65 mm Hg. It may be relative to a decrease in diastolic blood pressure to less than 40 mm Hg. It may be orthostatic with a decrease in systolic pressure or 20 mm Hg or greater or a decrease in diastolic pressure of 10 mm Hg or greater on positional change from lying to standing. It may be profound which is defined as being medication-dependent. In acute conditions, the hypotensive shock is a possible and life-threatening condition. Blood pressure is defined as:

• Blood Pressure = Cardiac output x Total peripheral vascular resistance

• The mean arterial pressure is an average blood pressure over the course of one cardiac cycle. It is calculated as:

• Mean arterial pressure = 2/3 diastolic pressure + 1/3 systolic pressure

Blood pressure is modulated via 2 primary mechanisms: cardiac output and total peripheral vascular resistance. Therefore, any disease of pathology that impacts one or more of these parameters will induce hypotension.[1][2][3][4][5]

The heart functions as a pump system to generate a pressure gradient for the distribution of blood throughout the body. This pumping potential is referred to as the cardiac output. Cardiac output is mathematically determined via an equation where:

• Cardiac output = Stroke Volume x Heart Rate

Disease states that reduce stroke volume or heart rate will decrease the total cardiac output of the heart, functionally decreasing the ability to generate blood pressure. Various medications are also capable of inducing hypotension via augmenting these biologic parameters. Most notorious for decreasing heart rate are the beta-blockers and calcium channel blocker classes of medicines. Diuretic medications are also possible sources that decrease cardiac stroke volume. Disease states include arrhythmias, valvular regurgitation, valvular stenosis, diastolic or systolic heart failure, large volume losses of blood, and cardiac tamponade.

Total peripheral vascular resistance is the resistance to blood flow through the terminal arterioles of the various organ sites in the body.  This mathematically equals:

• Systemic Vascular Resistance = 80 x (Mean Arterial Pressure - Mean Venous Pressure) / Cardiac Output

or

• Systemic Vascular Resistance = (8 X L x n) / (3.14 x radius of a vessel^ 4) 

Where L equals the length of the vessel and n equals the viscosity of blood. Functionally, vessel length is not subject to change in the body and viscosity does not rapidly adjust and can be accepted as standard value in most cases. Therefore, the only modifiable physiological value is the radius of the vessel. A decrease in arteriolar caliber increases the resistance to blood flow, thus increasing blood pressure. Conversely, increasing the diameter of terminal arterioles will decrease resistance to blood flow, thus decreasing blood pressure. Total peripheral vascular resistance is primarily controlled via autonomic neuronal responses to modulate fluctuations in blood pressure. The natural state for arteriolar smooth muscle tone is to be relaxed with dilated arterioles. Therefore, the absence or blunting of autonomic input by medications or disease states will lead to hypotension. In orthostatic hypotension, a combination of the blunting of the autonomic nervous system and mild hypovolemia from dehydration is the culprit. When lying flat, there is even distribution of fluid throughout the body. However, on standing heart rate fails to increase appropriately and peripheral resistance fails to increase appropriately leading to a rapid, transient decrease in blood pressure that improves with postural changes. This is classically symptomatic with dizziness and syncope.

Both cardiac output and total peripheral vascular resistance function as feedback compensation mechanisms for the other in healthy individuals. When cardiac output decreases, peripheral resistance should increase via constriction of terminal arterioles to decrease vessel caliber to maintain blood pressure. When peripheral resistance decreases, cardiac output will increase via increased heart rate to maintain blood pressure.

Acute disease processes that are life-threatening are possible and are classified depending on etiology as distributive shock, cardiogenic shock, hypovolemic shock, obstructive shock, or a combined-type hypotensive shock. 

Distributive shock occurs as a failure of the ability to maintain total peripheral resistance with maintained cardiac function attempting to compensate. This classically presents with warm extremities and skin, edema, increased mucous secretions, and tachycardia. This is classically associated with anaphylactic allergic reactions and septic shock.

Cardiogenic shock is a failure to achieve sufficient cardiac output with maintained total peripheral resistance. Classically these patients present with cool, dry extremities and skin with bradycardia.

Hypovolemic shock is a loss of total blood volume such that blood pressure is not maintained. Both cardiac output and total peripheral vascular resistance are maintained. This is possible via trauma with massive loss of blood or overuse of diuretic medications with fluid volume loss via urine. Cortisol deficiency as seen in Addison disease leads to a loss of fluid via urine and a relative cortisol deficiency. Sheehan syndrome is postpartum pituitary necrosis leading to a loss of many pituitary hormones as a result of postpartum shock or hemorrhage.

Obstructive shock occurs with the obstruction, constriction, or compression of the cardiovascular system such that blood flow does not efficiently occur or there is a decrease in stroke volume of the heart. This leads to a relative drop in blood pressure systemically. Obstruction may occur secondary to pulmonary embolism, tension pneumothorax, cardiac tamponade, constrictive pericarditis, or some other restrictive cardiomyopathy. These will classically present with signs of congestive failure including distended jugular veins, peripheral edema, pulmonary crackles, quiet heart sounds, or pulsus paradoxus.

Hypotensive shock is also possible via any combination of the above pathologies occurring simultaneously. One example is Waterhouse-Friderichsen syndrome, which is an adrenal-gland failure to produce mineralocorticoids, glucocorticoids, and sex steroids due to frank hemorrhage into the adrenal glands secondary to bacterial infection by Neisseria. This leads to a host of hypovolemic and distributive shock symptoms.

The precise epidemiology of hypotension is highly variable and depends on the exact etiology. In general, elderly patients are more prone to non-traumatic, symptomatic hypotensive episodes. Also, more physically active and healthy patients will have lesser resting asymptomatic blood pressures.

Blood pressure is continuously regulated via the autonomic nervous system as a balance of the sympathetic nervous system and the parasympathetic nervous system. The sympathetic nervous system acts to raise blood pressure by increasing heart rate and constricting arterioles. The parasympathetic nervous system lowers blood pressure by decreasing heart rate and relaxing arterioles to increase vessel diameter.