Distributive Shock

Distributive shock, also known as vasodilatory shock, refers to systemic vasodilation and decreased blood flow to vital organs such as the brain, heart, and kidneys. It can also cause fluid to leak from the capillaries into the surrounding tissues as a result.

Distributive shock is one of the four broad classifications of disorders that cause inadequate tissue perfusion. The other three types of shock include: cardiogenic shock (i.e., failure of the heart to pump), hypovolemic shock (i.e., intravascular volume loss), and obstructive shock (i.e., the physical obstruction of blood circulation and inadequate blood oxygenation). Like all forms of shock, distributive shock may eventually lead to multi-organ failure as a result of the imbalance between oxygen demand and supply. Distributive shock is life-threatening and requires immediate medical attention. 

The three main types of distributive shock are septic shock, anaphylactic shock, and neurogenic shock.

Septic shock refers to the condition where an infection causes the individual's blood pressure to drop to a dangerously low level. It is important to note that in half of the cases of septic shock, those with end-organ damage will present with cryptic shock, where blood pressure readings are normal but there is still inadequate tissue perfusion. 

The second leading cause of distributive shock is anaphylactic shock, an acute systemic reaction to environmental triggers, including allergic reactions to foods, such as nuts, or insect bites. Anaphylactic can occur at any age regardless of prior history and has proven to be fatal in 0.7 to 2 percent of cases. Mortality is exacerbated in those with asthma. 

The third type of distributive shock is neurogenic shock, which refers to a state of imbalance between sympathetic and parasympathetic regulation of the heart and vascular smooth muscle. This results in profound vasodilation, or capillary enlargement, with maintained blood volume but relative hypovolemia. Blood is then inadequately distributed in the body, leading to a loss of pressure and perfusion.

Of the three major forms of distributive shock, septic shock is the most common cause seen in the emergency room. Approximately one million people are admitted with severe sepsis every year and mortality rates can be up to 50%. It is the most common cause of death in critically ill patients worldwide and the most frequent non-cardiac diagnosis in the cardiac intensive care unit. This is in contrast to anaphylactic shock which has annual rates of 0.21 to 0.76 per million. Septic shock is the most severe form of sepsis, a life-threatening reaction that involves the release of massive amounts of chemicals that trigger inflammation in an attempt to resist infection. Sepsis affects the endothelial cells, which form the inner cellular lining of blood vessels, and exposes them to circulating signaling molecules and physical stresses.

Distributive shock has several causes depending on the specific subtype. In the case of septic shock, bacteria, specifically Staphylococcus aureus and group A streptococci, are the most common causes. Bacteria are also implicated in toxic shock syndrome (TSS), a form of septic shock associated with vaginal tampon use, recent surgery, and open wounds. Lastly, fungi, such as candida, and viruses can also cause sepsis. 

Anaphylactic shock is commonly triggered by food products (58% of cases) in children, whereas in adults it is triggered by insect venom (55% of cases, of which 70% are due to wasp stings and 20% are due to bee stings). Other risk factors include medications, such as diclofenac, acetylsalicylic acid, antibiotics, ACE inhibitors and beta-blockers. Physical exercise, stress, and acute infection may exacerbate the symptoms. 

Neurogenic shock can occur as a result of trauma to the spinal cord, cerebral ischemia, subarachnoid hemorrhage, or meningitis. When there is direct injury to the control centers for circulatory regulation, which can occur from direct compression of the brainstem; ischemia; or reduced blood flow to the brainstem due to blood clots (basilar artery thrombosis), an individual is at risk for neurogenic shock. Spinal cord injuries are the most common cause (15-20% of cases), followed by surgical intervention in the lumbar region. Examples of spinal cord injuries include sport injuries, gunshot wounds, and improper administration of anesthesia to the spinal cord. Additionally, drugs and medications that affect the autonomic nervous system (e.g., nitric oxide and alcohol), the system involved in regulating breathing and other automatic bodily functions, can lead to neurogenic shock. Individuals with paraplegia, who have sustained trauma above the thoracic spine may experience an interruption to autonomic signals from the brainstem to the spinal cord, which can lead to neurogenic shock. 

Other less common forms of distributive shock include adrenal insufficiency, capillary leak syndrome and systemic inflammatory response syndrome (SIRS). In the case of adrenal insufficiency, the adrenal glands, located just above the kidneys, are unable to produce certain hormones adequately, specifically, cortisol. As a result, there is decreased alpha-1 receptor expression on arterioles, causing subsequent arteriolar vasodilation. Adrenal insufficiency can also be seen in individuals who take steroids long term and suddenly stop taking their medication. Capillary leak syndrome, while rare, may also be a cause of distributive shock. Low blood albumin, a protein produced by the liver, results in decreased oncotic pressure and subsequent plasma loss from the capillaries into the interstitium. This results in a severe drop in blood pressure. Lastly, noninfectious causes, such as pancreatitis and burns, can cause distributive shock. SIRS is an exaggerated response of the body to trauma and results in a “cytokine storm,” or a massive release of inflammatory molecules, leading to reversible or irreversible end-organ dysfunction. 

The mechanism of distributive shock depends on the type of distributive shock. Septic shock occurs when bacteria produce toxins that damage the small blood vessels. As a reaction to the toxins, a dysregulated immune response is mounted that results in systemic cytokine release. Cytokines are small proteins that induce inflammation and result in vasodilation and fluid leak from capillaries. Cytokines can also cause cardiac dysfunction, known as septic cardiomyopathy, which can contribute further to the state of shock.

Anaphylactic shock is caused by an increased level of immunoglobulin E (IgE) antibodies forming due to exposure to an allergen. IgE attaches to the surface of mast cells and basophils. Once the individual is exposed to the same allergen a second time, histamine is released from the mast cells and basophils, leading to systemic vasodilation and capillary fluid leak, thereby resulting in distributive shock. 

During neurogenic shock, the body loses function of the sympathetic nervous system, which maintains bodily functions during physical activity (e.g., increasing the heart;s contractility, increasing blood pressure, and dilating airways to improve breathing). If the sympathetic nervous system is unable to function correctly, blood pressure drops and this can affect the brain, tissues, and spinal cord. Additionally, there may be decreased adrenergic stimulation of the blood vessels and heart, causing vasodilation with resultant hypotension and bradycardia.

The signs and diagnosis of distributive shock can be ascertained through careful and directed history-taking by a physician. Pertinent information can also be collected from the emergency department team, family members, or any witnesses. Additionally, any known allergies or a history of anaphylaxis should be noted, as this can point to an anaphylactic cause of distributive shock. Noting which medications the individual has taken, particularly steroid medications, anti-hypertensives, or illicit drug use, can also narrow down causes of shock. An immunocompromised status and recent hospitalizations should be noted as this information may point to sepsis. 

On a physical examination, the individual may be tachypneic (i.e., rapid breathing) and tachycardic (i.e., rapid heart rate) with hypotension, and they may feel dizzy upon standing. The individual may also have altered mental status, shortness of breath, cough, fever, chills, nausea, vomiting, diarrhea, abdominal pain, and dysuria (i.e., painful or difficult urination). Additionally, hypo or hyperthermia and low oxygen saturation can be present. A careful examination of the skin should be conducted to identify cutaneous sources of infection, such as cellulitis, ulcers, or abscesses. Urticaria, or hives, on the skin strongly suggests anaphylaxis. Finally, warm extremities with bounding pulses may point to vasodilation as the cause of early shock. 

Distributive shock is a life threatening condition and should be treated immediately. The individual is typically admitted to an intensive care unit in order to support the body’s functions while treatment of the underlying cause is initiated. In some cases, treatments may begin in the emergency department. General treatment protocols may include oxygen therapy, intravenous fluids, medications to increase blood flow, antibiotics, and potentially surgery. Oxygen may be given through a face mask, a nasal cannula, or an endotracheal tube inserted through the mouth. If there is severe shortness of breath, the individual may need to be mechanically ventilated to maintain proper blood oxygen levels. Administering intravenous fluids can help raise blood pressure and inotropic medications, such as dobutamine, can stimulate the heart to pump more blood to vital organs. The critical step in treating distributive shock is treatment of the underlying cause. In addition to replacing fluid, vasopressors, such as norepinephrine or vasopressin, can be given to increase constriction of the blood vessels, thereby increasing blood pressure. Mineralocorticoids to increase plasma volume are also a therapeutic option.

If the cause of distributive shock is due to bacterial infection, antibiotics should typically be administered as treatment. In patients with anaphylactic shock with symptoms of bronchospasm (i.e., airway contraction), beta-sympathomimetics (e.g., rimiterol, salbutamol, terbutaline) and glucocorticoids are indicated to relax bronchial smooth muscle. Histamine antagonists, such as diphenhydramine, suppress the histaminergic effects of the allergen on the body. Lastly, in severe cases of shock, the large decrease in blood pressure and blood flow can eventually result in death of organ tissue. If this occurs, surgery may be required to remove the dead tissue. 

Distributive shock is a life threatening event that results from lack of adequate blood flow and blood pressure throughout the body, and loss of blood volume through capillary leakage. There are three subcategories of distributive shock: septic shock, anaphylactic shock, and neurogenic shock. Sepsis and septic shock are the most common forms of distributive shock and are typically caused by bacterial infection. Anaphylactic shock is a severe reaction to an allergen or environmental trigger, and neurogenic shock occurs due to trauma to the spinal cord, resulting in loss of regulation of the sympathetic nervous system. Regardless of the cause, distributive shock typically presents with tachypnea, tachycardia, and hypotension. Other symptoms include an altered mental status, shortness of breath, cough and fever. Diagnosis involves taking a thorough history, and treatment is centered around treating the causes. Overall, the goal is to restore blood flow and blood pressure, which can be completed via administration of fluids and medications that allow the heart to pump stronger and the blood vessels to constrict. More specific treatment involves giving antibiotics to patients with septic shock and beta-sympathomimetics for cases of anaphylactic shock. Altogether, distributive shock is a life threatening event and treatment should be administered rapidly to avoid multi-organ failure.