**8.1 Hypovolemic shock**

Hypovolemic shock occurs due to loss of intravascular volume. This is the most common type of shock. Loss of intravascular volume can be in the form of loss of blood or loss of fluids from the body other than blood. Causes of blood loss can include trauma, gastrointestinal bleeding, postpartum hemorrhage, esophageal varices and ruptured abdominal aortic aneurysm. Causes of non-blood fluid losses can include diarrhea, vomiting, reduced intake, third degree burns and diabetic ketoacidosis. Hypovolemic shock can be further classified according to the amount of volume loss. The classes of hypovolemic shock are given in **Table 4**.

### **8.2 Cardiogenic shock**

Cardiogenic shock occurs due to inability of the heart to pump blood adequately to the peripheral circulation as a result of impaired contractility. This leads to end-organ


**Table 4.** *Classes of hypovolemic shock.* hypoxia and shock. Causes of cardiogenic shock can be myocardial infarction, myocarditis (secondary to Coxsackie B virus), dilated cardiomyopathy, and congenital heart disease, valvular dysfunction like aortic valve stenosis or mitral valve stenosis and arrhythmias. Both tachy-arrhythmias and brady-arrhythmias can lead to cardiogenic shock. Tachy-arrhythmias cause the heart to beat abnormally fast which impairs the filling ability of the heart, hence decreasing the preload and subsequently decreasing the cardiac output. Brady-arrhythmias decrease the heart rate and since CO = SV x HR, this also causes the cardiac output to decrease.

#### **8.3 Distributive shock**

Distributive shock occurs due to inappropriately distributed blood volume. Under normal physiology, vascular tone is under control of the autonomic nervous system. Sympathetic stimulation causes vascular smooth muscle to contract and vasoconstrict, while the parasympathetic nervous system causes vascular smooth muscle to relax and vasodilate. Distributive shock occurs when the sympathetic nervous system is unable to maintain the tone of the vascular system, allowing abnormal vasodilation of blood vessels. This allows pooling of blood and decreases preload. This also leads to increased vascular permeability and third-space fluid loss. This in turn causes intravascular hypovolemia and decreased end-organ perfusion. Distributive shock can have different etiologies like septic shock, anaphylactic shock and neurogenic shock.

Septic shock is the most common cause of distributive shock [5]. It can be defined as "sepsis-induced hypotension (systolic blood pressure <90 mm Hg or a reduction of 40 mm Hg from baseline) despite adequate fluid resuscitation along with the presence of perfusion abnormalities that may include, but are not limited to, lactic acidosis, oliguria, or an acute alteration in mental status." Septic shock results from an overwhelming systemic inflammatory response which leads to vasodilation and subsequent hypotension. Most common causes of septic shock are gram negative bacteria like *Escherichia coli*, Proteus species, *Klebsiella pneumoniae* which release endotoxins which are responsible for activation of the immune system.

Anaphylactic shock occurs due to type 1 hypersensitivity reaction to any foreign antigen. Antigens bind to IgE molecules on pre-sensitized mast cells and cause mast cell degranulation and release of inflammatory mediators like histamine. Histamine causes vasodilation and increased capillary permeability. This causes severe hypovolemia and cardiovascular collapse leading to shock.

Neurogenic shock results from the inability of the sympathetic nervous system to maintain the tone of blood vessels. In most cases, this is a result of trauma to the brain or spinal cord above the level of T6 [6]. The trauma leads to a loss of background sympathetic stimulation to the vascular smooth muscles. This causes vasodilation resulting in a sudden decrease in blood pressure (secondary to a decrease in peripheral vascular resistance).

#### **8.4 Obstructive shock**

Obstructive shock occurs when there is a barrier to the flow of blood or a barrier which impairs proper filling of the heart. There are several conditions which can cause obstructive shock. These include cardiac tamponade, tension pneumothorax and pulmonary embolism.

Cardiac tamponade is the result of fluid in the pericardial space which impairs the filling ability of the heart during diastole. This reduces the preload and subsequently

decreases cardiac output. This is similar in presentation to constrictive pericarditis in which the pericardium shrinks and hardens.

Tension pneumothorax is the presence of air in the pleural cavity under positive pressure. The elevated intrathoracic pressure leads to decreased venous return to the heart as it compresses the inferior vena cava, thus leading to reduced cardiac output.

Pulmonary embolism is an embolus (usually dislodged from the proximal deep veins of the lower limb) lodged in the vasculature of the lungs. This obstructs blood flowing to the lungs and the blood coming from the lungs to the heart. There is a decrease in end-diastolic volume which leads to a decreased stroke volume and hence decreased cardiac output and oxygen delivery to the peripheral tissues resulting in shock.
