Dizziness, lightheadedness and fainting may occur if blood pressure falls dangerously low, states the American Heart Association. The person may exhibit fast, shallow breathing; have cold, pale and clammy skin; and experience blurred vision, nausea, fatigue or unusual thirst. He may be unable to concentrate and may feel depressed.
There is no specific threshhold at which doctors consider blood pressure too low, as long as the patient has no adverse symptoms, explains the American Heart Association. A sudden decline in blood pressure may occur due to uncontrolled bleeding, allergic reaction or infection. Other issues that may cause low blood pressure include pregnancy, endocrine disorders, certain medications and nutritional deficiencies.