Hypotension - Introduction
Definition of Hypotension and its Causes
Hypotension is a physiologic state in which the arterial blood pressure is abnormally low. For an adult, hypotension exists when the systolic pressure is less than 90 mmHg and the diastolic pressure is less than 60 mmHg. Because arterial pressure is determined by cardiac output, venous pressure and systemic vascular resistance (Click here for more details), a reduction in any of these variables can lead to hypotension. Hypotension may result from:
- Reduced cardiac output
- Blood volume redistribution
- Reduced systemic vascular resistance
- Vascular obstruction (e.g., pulmonary embolism)
Cardiac output can be reduced by arrhythmias, structural disease or hypovolemia. Bradycardia caused by a low sinus rate or resulting from atrioventricular block reduces cardiac output. Tachycardia, by reducing ventricular filling time, can cause a large reduction in stroke volume and therefore cardiac output. Ventricular fibrillation causes cardiac output to fall to zero, and therefore leads to profound hypotension. Cardiac structural disease such as cardiomyopathies can impair either systolic function (inotropic state) or diastolic function (ventricular filling) and thereby reduce cardiac output and arterial pressure. Valve disease, pericardial disease, and congenital defects, can impair ventricular filling or net forward flow, thereby reducing cardiac output. Ischemic heart disease caused by atherosclerosis or thromboembolism impairs ventricular function. Primary pulmonary hypertension can lead to right ventricular failure and impaired left ventricular filling and output. Hypovolemia (e.g., hemorrhage) and orthostatic volume shifts reduce cardiac output by decreasing central venous pressure and ventricular filling (preload), which impairs the pumping ability of the heart (Frank-Starling mechanism). Excessive water loss (dehydration) caused by profuse sweating, restricted water intake, or use of diuretic drugs can also lead to a hypovolemic state.
Hypotension may also be of vascular origin, and is generally caused by excessive systemic vasodilation (decreased systemic vascular resistance), which may result from sepsis, anaphylaxis, autonomic dysfunction (e.g., diabetic neuropathy), neurogenic shock (e.g., spinal cord injury) or drugs (e.g., antihypertensive vasodilators). A second vascular origin of hypotension is vascular obstruction such as pulmonary embolism that diminishes venous return to the left ventricle, thereby decreasing its output.
To understand more fully the physiology and pathophysiology of hypotension, see the Blood Pressure Regulation Tutorial.