A 65-YEAR-OLD man is admitted to the emergency department with a two-day history of melena associated with dizziness and light-headedness. The skin is pale, cold, and clammy. The radial pulse is thready and small at 130 beats per minute. The patient is dyspneic with a respiratory rate of 24/min, and the blood pressure (BP) is 110/60 mm Hg. As the examination progresses, the patient becomes agitated, and circumoral cyanosis appears. The mucous membranes become dry, the BP decreases to 80/40 mm Hg, and the pulse rate increases to 150 beats per minute. A nasogastric tube is inserted, and it elicits bright red blood.
This is a description of hypovolemic or hemorrhagic shock, probably secondary to upper gastrointestinal hemorrhage. Shock is defined as inadequate circulating blood volume with decreasing peripheral vascular perfusion ultimately causing cellular metabolic derangements. The volume loss from a bleeding gastric or duodenal ulcer stimulates pressure receptors in