Subspecialists in infectious diseases and critical-care medicine now generally agree regarding the terms that should be used to classify serious infections, despite less experience with their application in pediatrics (and neonatology). Currently accepted definitions include the following:

Unlike the situation in adult medicine, SIRS and sepsis as defined here are common “problem statements” in pediatrics. Because pediatric patients compensate well for shock states with tachycardia and vasoconstriction, septic shock and MODS by these definitions become relatively uncommon (but ominous) clinical entities.

Typically, sepsis and the various septic syndromes are caused by bacterial infections of an advanced or rapidly progressive nature. Contrary to popular belief, most patients with sepsis do not have documented bacteremia, which accounts for some of the recent changes in definitions. However, the probability of having positive blood cultures increases as one progresses down the classification list to septic shock and MODS. Even with negative blood cultures, bacterial etiology often can be established by positive Gram stains and cultures at focal sites of infection, characteristic alterations in hematologic parameters, tests for the presence of bacterial exotoxins, or clinical responses to empiric antimicrobial therapy.

The common (and some of the unusual) bacterial etiologies of sepsis in previously normal children are presented in Box 135.1, according to the presence or absence of a focal source and according to the presence of accidental or surgical alterations in integumentary and mucosal barriers. A working knowledge of these organisms and the clinical settings in which they are most likely to present provides a rational basis for the selection of empiric antibiotic regimens. (The causes of neonatal infections are discussed comprehensively in Chapters 71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88. A complete description of the causes of community-acquired and nosocomial sepsis in compromised hosts can be found in Chapter 88.) Sepsis of a critical nature, even in a previously normal child, should prompt consideration of an important defect in host defense. For example, meningococcemia should suggest an abnormality in the terminal complement pathway.

The encapsulated organisms—Streptococcus pneumoniae, Neisseria meningitidis, and Haemophilus influenzae type b—have been the most common causes of sepsis (and bacteremia) of occult origin. These organisms occur most frequently in children aged 3 months to 5 years and commonly are preceded by viral upper respiratory illnesses. Peaks in incidence correspond to the nadir in transplacentally acquired maternal IgG antibodies. Use of infant vaccination has reduced dramatically the incidence of H. influenzae and S. pneumoniae sepsis in recent years in the United States.