Septic Shock

Etiology

The 2009 European Prevalence of Infection in Intensive Care (EPIC II study) determined that gram-negative bacterial infections far exceed other etiologies as the most common cause of sepsis syndromes, with a frequency of 62%, followed by gram-positive infections at 47%. An increase in the prevalence of the latter may be attributable to the performance of more invasive procedures and increased incidence of nosocomial infections.[2] Predominant micro-organisms isolated in patients include Staphylococcus aureus (20%), Pseudomonas (20%), and Escherichia coli (16%).[3] Predominant sites of infection include respiratory (42%), bloodstream (21%), and genitourinary (10%).[2] These data need to be assessed in the context of knowing that over a third of patients never grow positive cultures.[4]

The influence of bacterial strain and site of infection on mortality was illustrated in a large meta-analysis.[5] In this study, gram-negative infections were overall associated with higher mortality. However, gram-positive bacteremia with Acinetobacter or pneumonia with Staphylococcus carried a 40% mortality with Pseudomonal pneumonia carrying the highest mortality at 70%.

Sepsis syndromes caused by multidrug-resistant bacterial strains (methicillin-resistant Staphylococcus (MRSA), vancomycin-resistant enterococci (VRE)) are on the rise with a current incidence of up to 25%; viruses and parasites cause far fewer cases and are identified in 2% to 4% of cases.[6]

Risk factors that predispose to sepsis include:

• Diabetes
• Malignancy
• Chronic kidney and liver disease
• Use of corticosteroids
• Immunosuppressed state
• Burns
• Major surgery
• Trauma
• Presence of indwelling catheters
• Prolonged hospitalization
• Hemodialysis
• Extremes of age

Epidemiology

Annually, the rate of this debilitating condition is rising by almost 9%.[7] The incidence of sepsis and severe sepsis have risen over the past decade from approximately 600,000 to over 1,000,000 hospitalizations per year from 2000 through 2008.[8] Accompanying this trend has been a rise in healthcare expenditure, making sepsis the most expensive healthcare condition in 2009, accounting for 5% of total United States hospital costs.[9] The case fatality for patients with sepsis has been declining due to advances in sepsis management provided by the Surviving Sepsis Campaign. The United States Nationwide Inpatient Sample (NIS) from 2009 through 2012 showed a mortality rate declined from 16.5% to 13.8%.[10] However, severe sepsis continues to rank among the most common causes of death in hospitalized patients.[11] Moreover, up to 25% of patients with severe sepsis and 50% of patients with septic shock will suffer mortality.[2] However, overall mortality from sepsis syndromes can vary from 30% to 50% [12] depending on demographic factors such as age, race, sex, co-morbid conditions, and the presence of organ dysfunction.[13] For example, in-patient mortality was predicted most by the number and degree of organ injury, with the strongest predictors being respiratory, cardiovascular, hepatic, and neurologic failure.[10]

Pathophysiology

Sepsis is a clinical state that falls along a continuum of pathophysiologic states, starting with a systemic inflammatory response syndrome (SIRS) and ending in multiorgan dysfunction syndrome (MODS) before death. 

The earliest signs of inflammation are heralded by the following:

1. Fever (temperature higher than 38 C or hypothermia (temperature less than 36 C)
2. Tachycardia (heart rate more than 90 beats per minute), 
3. Tachypnea (respiratory rate more than 20 breaths per minute)
4. Leukocytosis (WBC greater than 12,000/cu mm) / leukopenia (white blood cells (WBC) less than 4,000/cu mm) with or without bandemia (more than 10%).

The presence of two of these four clinical signs is necessary for the diagnosis of systemic inflammatory response syndrome. After that, systemic inflammatory response syndrome with an infectious source suffices the clinical definition for sepsis.[14] 

With the development of hypotension, tissue demands are not adequately met by tissue oxygenation, and the patient is now defined to be in severe sepsis.[14] The decline in peripheral vascular perfusion and oxygenation leads to cellular and metabolic derangements, most notably a shift from aerobic respiration to anaerobic respiration with ensuing lactic acidosis. Tissue hypoperfusion may also be manifested by signs of end-organ damage, such as pre-renal azotemia or transaminitis. The difference in oxygen supply and demand can be monitored during resuscitation by trending the mixed venous oxygen saturation from a central line in the superior vena cava (SVC) when available.[1]

When sepsis-induced hypotension remains refractory to initial management with fluid resuscitation, septic shock ensues.[14] Septic shock is distinguished from other shock states as a distributive type of shock. The action of a combination of inflammatory mediators (histamine, serotonin, super-radicals, lysosomal enzymes) elaborated in response to bacterial endotoxins leads to a marked increase in capillary permeability and a concomitant reduction in peripheral vascular resistance. This translates not only into a reduction in afterload but also in preload from a decline in venous return from third-spacing. The resulting reduction in stroke volume is accommodated initially by an elevation in heart rate, i.e., compensated septic shock. As a result, the patient is in a hyperdynamic state that is characteristic of septic shock.[1]

Clinically, patients have a dynamic precordium with tachycardia and bounding peripheral pulses. They are warm to the touch and have a reduction in capillary refill (flash cap refill). This is described as warm shock. As shock progresses, elevated catecholamine production leads to an increase in peripheral vascular resistance as the body attempts to shunt blood away from non-vital tissues (gastrointestinal (GI) tract, kidneys, muscle, and skin) to the vital tissues (brain and heart). This is described as cold shock. Understanding the pathophysiology and continuum of septic shock is imperative in initiating appropriate treatment measures.[1]

Functionally, septic shock is defined by persistent hypotension despite adequate fluid resuscitation from 60 ml/kg to 80 mL/kg of either crystalloid or colloid fluid. At this point, the initiation of appropriate vasoactive medications such as beta-adrenergic or alpha-adrenergic drugs is of utmost importance. The progression of organ dysfunction despite high-dose vasoactive administration defines the state of multiorgan dysfunction syndrome (MODS), which carries mortality as high as 75%. While the exact circumstances predicting poor prognosis and death have been difficult to determine, immunologic dissonance (exaggerated pro-inflammatory response) versus immunologic paralysis (exaggerated anti-inflammatory response) have been purported to play a role.[15]

History and Physical

Early Signs and Symptoms

Sepsis is defined as systemic inflammatory response syndrome plus an infectious source. Therefore, earlier on in the presentation of sepsis, patients present with the following vital sign changes:

• Fever, temperature higher than 38 C, or hypothermia, temperature lower than 36 C
• Tachycardia with a heart rate higher than 90 beats per minute in adult patients or less than two standard deviations for age in pediatric patients
• Tachypnea with respiratory rate greater than 20 breaths per minute in adult patients or more than two standard deviations for age in pediatric patients

Signs and Symptoms of Severe Sepsis Severe sepsis is defined as sepsis and end-organ dysfunction. At this stage, signs and symptoms may include: 

• Altered mental status
• Oliguria or anuria
• Hypoxia
• Cyanosis
• Ileus

Patients progressing to septic shock will experience signs and symptoms of severe sepsis with hypotension. Of note, at an early "compensated" stage of shock, blood pressure may be maintained, and other signs of distributive shock might be present, for example, warm extremities, flash capillary refill (less than one second), and bounding pulses, also known as warm shock. This stage of shock, if managed aggressively with fluid resuscitation and vasoactive support, can be reversed. With the progression of septic shock into the uncompensated stage, hypotension ensues, and patients may present with cool extremities, delayed capillary refill (more than three seconds), and thready pulses, also known as cold shock. After that, with continued tissue hypoperfusion, shock may be irreversible, progressive rapidly into multiorgan dysfunction syndrome and death.

Evaluation

Laboratory Findings

Findings in sepsis, severe sepsis, and septic shock are as follows [16]: 

• Hyperglycemia (glucose more than 120 mg/dL)
• Leukocytosis (WBC more than 12,000/mm³) or leukopenia (WBC less than 4000/mm³)
• Bandemia (more than 10%)
• C-reactive protein or procalcitonin more than 2 SD above normal
• Mixed venous saturation of more than 70%
• PaO2: FiO2 less than 300
• Pre-renal azotemia
• Coagulopathy, INR more than 1.5 or PTT more than 60 sec
• Thrombocytopenia (platelets less than 100,000/mL)
• Hyperbilirubinemia (total bilirubin more than 4 mg/dL)
• Lactic acidosis (more than 2 mmol/L)

Patients should be placed on continuous cardiopulmonary monitoring to allow close observation of vital signs. A thorough assessment of end-organ function and peripheral perfusion should be undertaken to determine where they may fall along the pathophysiologic continuum of sepsis. This should include a Glasgow Coma Scale (GCS) or mental status assessment, urine output measurement, or lactate/mixed venous saturation determination (with central lines). Regardless of where along the continuum patients are, all patients should have drawn a complete blood count with differential (CBC-d), source cultures (blood, urine, tracheal (if intubated), wound), and a urinalysis. Depending on the severity of presentation and age of the patient, a lumbar puncture may be indicated, for example, in patients with signs of encephalitis or meningitis or febrile pediatric patients under six weeks of age. The addition of C-reactive protein or procalcitonin, both acute-phase proteins, may be helpful in distinguishing viral from bacterial sepsis, with the latter showing steeper elevations in these proteins. A complete chemistry panel with liver function tests, a disseminated intravascular coagulation (DIC) panel, and an arterial blood gas are additional labs that may provide important information on the severity of sepsis syndrome in a patient.

At least two sets of blood cultures are recommended prior to administration of antibiotics. However, less than 40% of blood cultures are positive.

A chest x-ray may reveal signs of pneumonia or ARDS. Plain X-rays of the extremities may reveal the presence of gas in tissues if the patient has necrotizing fasciitis. Ultrasound may be used to evaluate the gallbladder. CT scan is used to assess the abdomen for abscess, bowel perforation, or ischemia.

Treatment / Management

The below guidelines are derived from the Surviving Sepsis Campaign Guidelines [17][18]

Source Control

1. Broad-spectrum antibiotics within one hour of diagnosis for all patients. Initial empiric anti-infective therapy should have activity against all likely pathogens and adequate penetration of source tissue.  
2. Removal of infected/necrotic tissue if it is the source of septic shock, ie, patients with cellulitis, abscess, infected devices, or purulent wounds.

Management of Shock [19](A1)

1. Measures are most effective if achieved within the first six hours of diagnosis.
2. Restore central venous pressure (CVP) to 8 mmHg to 12 mm Hg
3. Restore mean arterial pressure (MAP) greater than 65 mm Hg
4. Restore superior vena cava saturation to 70% or mixed venous saturation to 65%
5. Fluid resuscitation with crystalloid (NS or albumin) and colloid (blood products) up to 80 ml/kg
6. Mechanical ventilation to reduce metabolic demand
7. First-line vasoactive agents (epinephrine in cold shock versus norepinephrine in warm shock) when fluid-refractoryNote: dopamine as a first-line agent has fallen out of favor given its inhibitory effect on the HPA axis, namely prolactin and growth hormone, which can confer immunologic dysfunction [20]
(A1)

Enhancing Host Response

1. Corticosteroids indicated in vasoactive-refractory shock and or in patients with low (unstimulated) basal cortisol levels less than 150 μg/L) [21][22]
2. Addition of vasopressin indicated in vasoactive-refractory shock
(B3)

While central lines are not required for the resuscitation of patients with septic shock, they provide an accurate means of monitoring CVP and mixed venous oxygen. Remember that CVP and MVO2 are most accurate from a central line that lies within the right atrium; lower extremity central lines do not provide the most accurate data for monitoring these indices of resuscitation. Regarding the need for central venous access for the administration of vasoactive agents, a recent study showed that both dopamine, norepinephrine, and phenylephrine at high doses could be safely administered via peripheral venous access.[23] 

Early goal-directed therapy (EGDT) has not been shown to confer a survival benefit in more recent studies.[24] All studies comparing EGDT to standard practice have shown an increase in the administration of crystalloid and packed red blood cells in the first six hours and the placement of central lines. Furthermore, survival was influenced most by the maintenance of blood pressure independent of the type of fluid or vasoactive used and not CVP or MVO2.[25] That said, the Surviving Sepsis Campaign guidelines continue to support EGDT as the standard of practice for the management of severe sepsis and septic shock.(A1)

The placement of an arterial line becomes important in the management of vasoactive-refractory shock for close monitoring of blood pressure and tissue oxygenation status via regular blood gasses with key attention to lactate levels and pO₂.

Patients with sepsis have a high metabolism, and thus, prolonged starvation should be avoided. Early nutrition can help protect gut mucosa and prevent the translocation of organisms from the GI tract into systemic circulation.

Differential Diagnosis

• ARDS
• DIC
• Distributive shock
• Hemorrhagic shock
• Adrenal crisis
• Cardiogenic shock
• Toxic shock syndrome
• Drug toxicity

Prognosis

Septic shock is a serious illness, and despite all the advances in medicine, it still carries high mortality, which can exceed 40%. Mortality does depend on many factors, including the type of organism, antibiotic sensitivity, number of organs affected, and patient age. The more factors that match SIRS, the higher the mortality. Data suggest that tachypnea and altered mental status are excellent predictors of poor outcomes. Finally, prolonged use of inotropes to maintain blood pressure is also associated with adverse outcomes. Even those who survive are left with significant functional and cognitive deficits.

Complications

• ARDS
• Acute/chronic renal injury
• DIC
• Mesenteric ischemia
• Acute liver failure
• Myocardial dysfunction
• Multiple organ failure