The Human Body

Classification of Shock by Causes

Though the final clinical events are similar in all types of shock, there are several general causes.

1. Hypovolemic Shock: Hypovolemia is most often due to hemorrhage and is characterized by a reduction in blood volume as a precipitating event; effective hypovolemia may also be due to “third spacing” of fluid, as in peritonitis or burns, which results in lowering of blood volume.
2. Cardiogenic Shock: This type of shock is due to major myocardial infaraction an dresults from an abrupt drop in cardiac output.
3. Vascular shock: Types of shock resulting from lowered peripheral resistance include the following:-

• Gram-negative shock is caused by the vasodilating properties of endotoxin, decreasing peripheral resistance, the initial effect of which is to increase the cardiac output(“warm shock”). Though cardiac output is increased initially, the associated sustained increase in venous pooling ultimately results in a lower venous return, a consequent drop in cardiac output, and more typical appearance of “cool shock” with vasoconstriction and pallor as noted above.
• Anaphylactic shock, though most notable for its effect on the airways, is characterized by a profoundly low peripheral resistance; refractory hypotension can be a cause of death in this situation, which is best known for its occurrence after penicillin injection or bee sting.
• Toxic shock is caused by a staphylococcal exotoxin usually resulting from vaginal organism growth associated with tampon use. Fever, rash, hypotension, and occasional central nervous system and hepatorenal dysfunction are observed.

Clinical manifestations

The clinical picture depends on whether hypovolemia, reduced cardiac output, or lowered vascular resistance was the primary event.

Treatment

Management varies with the cause of shock, but volume resuscitation is common to all. Military antishock trousers (MAST) markedly redistribute intravascular fluid into the upper circulation, and this is potentially valuable in all forms except cardiogenic shock. The fluid of choice to replace volume is a matter of dispute, but there is probably no important difference between crystalloid (eg, normal saline) and colloid (eg, albumin); in hemorrhage, it is just important to replace volume as it is to transfuse erythrocytes. In all types of shock, dopamine and dobutamine are the pressor agents of choice; any pressor therapy should be reserved for situations where blood volume has been shown to be adequate but ineffective. In gram-negative shock, there is new information favoring the use of the narcotic antagonist naloxone because of the proposed mediator role of endogenous opiates in causing the hypotension of the shock state. The role of corticosteroids remains controversial, though they are in widespread use in this type of Shock.

To monitor all patients with all types of shock, central venous pressure determination is widely used and has a good positive correlation with left ventricular filling pressure, except in patients with preexisting cardiac disease; thus, in suspected cardiogenic shock or in shock states in patients with underlying cardiovascular disease it is essential to monitor therapy by using the Swan-Ganz catheter to measure the pulmonary artery wedge pressure. In addition, an indwelling arterial line is placed to monitor mean blood pressure, and cardiac output can be calculated easily using the thermodilution technique.