Low Blood Pressure (Hypotension)
Shock represents inadequate circulatory perfusion to meet metabolic demands. A significant sign is hypotension.
Types of shock are hypovolemic (i.e., hemorrhage, dehydration), cardiogenic (myocardial infarction, cardiac tamponade), septic, neurogenic and anaphylactic.
The systolic pressure is usually low (below 70 mmHg; however, a hypertensive patient may be in shock at a pressure of 120/80). The body’s automatic responses are activated.
The prototype is seen in hypovolemic shock:
1. The baroreceptor mechanism is stimulated.
2. Arterioles in skin, muscles, kidneys and the GI tract containing alpha receptors constrict. Blood is shunted to vital organs such as the heart and brain. The result is cool, clammy skin, decreased urinary output and fluid retention.
3. Oxygen is low, carbon dioxide is high and chemoreceptors are stimulated, assisting in elevated blood pressure.
4. The sympathetic nervous system is stimulated and norepinephrine is released from sympathetic nerve endings.
5. Stimulation of the sympathetic system causes release of epinephrine and norepinephrine from the adrenal medulla, resulting in further vasoconstriction and tachycardia.
6. Hypotension causes vasopressin (antidiuretic hormone, ADH) release from the posterior pituitary gland. In addition to its antidiuretic effect on the kidney, ADH is a vasoconstrictor.
7. The kidney secretes the enzyme renin, which acts in the lungs form angiotensin II, a potent vasoconstrictor. Baroreceptor and chemoreceptor control mechanisms occur in seconds. Hormonal and kidney mechanisms require several minutes.