Formally speaking, “Shock is defined as a state of cellular and tissue hypoxia due to reduced oxygen delivery and/or increased oxygen consumption or inadequate oxygen utilization” (Gaieski). Shock is classified into four different categories: distributive, hypovolemic, cardiogenic, and obstructive. There can also be shock with a combination of the aforementioned categories.
Distributive shock is characterized by vasodilation and a low systemic vascular resistance would be expected. Distributive shock can be further subdivided into distinct etiologies. Septic shock, shock from bacteremia or fungemia, is the leading cause of distributive shock. Systemic Inflammatory Response Syndrome (SIRS) can also cause distributive shock. Unlike sepsis, SIRS does not have to be the result of an infectious etiology. SIRS can arise from trauma, pancreatitis, air/fat embolism, after return of spontaneous circulation after resuscitation from cardiac arrest, and post cardiac bypass. There are other causes for SIRS as well. Next, neurogenic shock can cause distributive shock; this would be associated with central neurologic injury (brain or spinal cord). Anaphylaxis and anaphylactoid reactions are yet another cause of distributive shock, and these would most likely be associated with medication or latex reactions in the perioperative period. Shock from drugs and toxins can also be distributive in nature. Cyanide toxicity (sodium nitroprusside administration can lead to cyanide toxicity) and even a blood-product transfusion reaction would fall under this sub-category. Finally, distributive shock can be caused by endocrine etiologies: mineralocorticoid deficiency (Addisonian crisis) and myxedema.
Cardiogenic shock can also result from multiple etiologies. Whereas the systemic vascular resistance is affected in distributive shock, cardiogenic shock is caused by a low cardiac output. The heart muscle can fail (cardiomyopathy), an arrhythmia can cause a significant decrease in cardiac output, and even the heart can mechanically/anatomically fail with severe valvular abnormality or tumors/abscesses affecting normal anatomic function among other abnormalities.
Like cardiogenic shock, hypovolemic shock reduces cardiac output. Hypovolemic shock is further divided into two subcategories: hemorrhagic and non-hemorrhagic. Hemorrhagic is simply losing blood resulting in hypovolemia. Non-hemorrhagic is losing fluid while the patient is not bleeding. This can be as obvious as high-volume GI losses through emesis or diarrhea or more occult such as third-spacing, ascites, or increased insensible losses through a large wound intraoperatively or a burn covering a large surface area.
Finally, there are obstructive sources for shock. These obstruct cardiac output (thus another category with low cardiac output), but are extracardiac in origin. Some sources come from the pulmonary system, such as pulmonary hypertension or a hemodynamically significant pulmonary embolism leading to right ventricular failure. Other obstructive sources exist, such as pericardial tamponade, tension pneumothorax, and others.
Again, the above categories of shock are not mutually exclusive, they can also be seen in combination. Immediate treatment of shock is critical in order to adequately oxygenate and perfuse major organs to prevent major organ failure and death.
Defined by: Josh Morris, MD