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 stimulation of the sympathetic nervous system, which results in 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; timely intervention with appropriate therapy will prevent or decrease organ injury and/or death.
Shock is typically classified into three categories: hypovolemic, cardiogenic, and distributive. Hypovolemic shock develops when there is a blood volume deficit ≥15%; this may be from hemorrhage or other fluid losses (eg, as occurs with severe vomiting and diarrhea). 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; it can be caused by conditions such as anaphylaxis, corticosteroid deficiency (hypoadrenocorticism and critical illness–related corticosteroid insufficiency), and systemic inflammatory diseases that lead to systemic inflammatory response syndrome. 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. In veterinary patients, many stages and categories of shock will respond to fluid resuscitation alone; medications such as antiarrhythmics and inotropes may be necessary for primary cardiogenic shock, and vasopressor agents may be necessary for distributive shock. 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.