In obstructive shock the heart is strong, the blood volume is adequate, and the vessels have normal tone. The problem is mechanical: something physically prevents blood from filling the heart or from leaving it. Output falls because flow is blocked, not because the pump or the tank has failed. That distinction matters more here than in any other shock state, because it inverts the usual reflex. Fluid is the first response to most hypotension, and in obstructive shock it is frequently ineffective and can be harmful. A heart that cannot fill because something outside it is squeezing it, or cannot empty because the outflow is blocked, does not improve when you add volume to a circulation that already cannot get in. Three causes account for nearly all cases: fluid or blood accumulating in the sac around the heart, air trapped under pressure in the chest, and a large clot obstructing the pulmonary circulation. Each is reversible, and each is reversed by removing the obstruction rather than by supporting the circulation. The nurse's task is to recognize a hypotension that does...