Cardiac function; intravascular volume; and vascular tone, integrity, and patency are critical to normal circulation. An abnormality in one or more of these components of circulation leads to compensatory changes to maintain perfusion. The hemodynamic and cellular changes that develop as a result of these abnormalities are called shock. As shock progresses, oxygen and substrate delivery to the tissues becomes insufficient to meet energy requirements for cellular maintenance and repair. If shock progresses and cellular energy demands cannot be met, the ensuing organ failure leads to death. Early recognition of the type and stage of shock is vital to establishing a successful fluid therapy plan.

Shock is typically classified into 3 categories: hypovolemic, cardiogenic, and distributive. Hypovolemic shock develops when there is a blood volume deficit ≥15%. Cardiogenic shock results when the heart fails as a pump; common causes include pulmonary emboli, cardiac tamponade, valvular insufficiency, cardiomyopathy, and cardiac arrhythmias. Distributive shock is caused by maldistribution of blood flow away from the central circulation as a result of peripheral vasodilation, and can be caused by systemic inflammatory diseases such as sepsis and anaphylaxis. The different types of shock may have different hemodynamic profiles during the early and middle stages. Frequently, more than one type of shock is present, with hypovolemia likely to play a role in each form. Rapid and aggressive fluid resuscitation yields the best outcome, with hemostasis used as required. The ability to create an effective fluid resuscitation plan depends on an understanding of the different body fluid compartments and the dynamics of fluid movement and distribution between fluid compartments.

Last full review/revision March 2012 by Rebecca Kirby, DVM, DACVIM, DACVECC; Andrew Linklater, DVM, DACVECC