Microbiologic Considerations in the Intensive Care Patient

Objectives

This chapter will:

1.
Discuss general concepts of common infections in the intensive care unit.
2.
Review risk factors and strategies to decrease microbial resistance including antimicrobial stewardship.
3.
Discuss particular characteristics pertinent to critically ill patients such as immunosuppression.

Epidemiology of Infections in the Intensive Care Unit

Serious infections leading to hemodynamic instability are a common cause for admission to the intensive care unit (ICU). Despite increased awareness, early recognition, and treatment with antibiotics, community-acquired and hospital-acquired infections continue to contribute substantially to the morbidity and mortality of critically ill patients. The incidence, type, and site of infection in critically ill patients depend on numerous factors, including prehospital comorbidities such as diabetes mellitus, chronic lung disease, and immunocompromised states, and in-hospital interventions and devices such as endotracheal intubation, central venous catheters, and urinary catheters. Common infection sites include pneumonia, bloodstream infections, intravascular catheter-related infections, intraabdominal infections, urosepsis, and surgical wound infections.

Originally published in 1995, the European Prevalence of Infection in Intensive Care (EPIC) study was updated in 2009 and provides one of the most comprehensive assessments of infection in the ICU. This study was a multicenter, single-day point prevalence study conducted in 1265 ICUs in 75 countries, which included 13,796 ICU patients. Infections were defined based on International Sepsis Forum definitions. This study showed that more than half of the patients (51%) admitted to the ICU were infected, and these patients had more comorbid conditions and higher Simplified Acute Physiology (SAP) II and Sequential Organ Failure Assessment (SOFA) scores on admission compared with those without an active infection ( Table 81.1 ). The most common sites of infection were lung (63.5%), abdomen (19.6), bloodstream (15.1%), renal/urinary tract (14.3%), skin (6.6), catheter insertion site (4.7%), and central nervous system (2.9%). Gram-negative bacteria were the most frequent isolates from cultures (62.2%); Pseudomonas species were the most prevalent. Infected patients also had higher ICU and in-hospital mortality when compared with noninfected patients and longer ICU and hospital length of stay. A posthoc analysis of this study published in 2009 identified that high nurse:patient ratio was associated independently with a lower risk of in-hospital death.

TABLE 81.1

Common Comorbid Illnesses That Affect the Risk of Infection

COMORBID ILLNESS	OR	p VALUE
Chronic obstructive pulmonary disease (COPD)	1.21 (1.07–1.38)	<.01
Cancer	1.33 (1.15–1.53)	<.001
Heart failure	1.45 (1.25–1.70)	<.001
Diabetes mellitus	0.98 (0.83–1.15)	0.79
Chronic renal failure	1.02 (0.87–1.20)	0.81
Immunosuppression	1.83 (1.47–2.28)	<.001
Cirrhosis	2.14 (1.68–2.74)	<.001
Hematologic cancer	1.05 (0.76–1.45)	0.75
HIV	0.90 (0.53–1.52)	0.69

HIV, Human immunodeficiency virus; OR, odds ratio.

Systemic Manifestation of Infection in the Intensive Care Unit

The diagnosis of infection in critically ill patients requires a detailed history and physical examination supplemented by laboratory investigations and radiologic imaging. Because of the numerous medications and interventions used only in the intensive care unit, critically ill patients often present unique obstacles that clinicians caring for them must overcome when diagnosing infection. For example, although fever can be a sign of infection, especially when accompanied by an increased WBC and abnormal cultures, the list of noninfectious causes of fever in the ICU are numerous ( Box 81.1 ). Furthermore, because of factors such as old age, the use of immunosuppressive medications, and extracorporeal circuits, the absence of fever does exclude infection as a cause of deterioration of an unstable patient.

Box 81.1

Marik et al. Fever in the ICU. Chest. 2000; 117:855–869.

Noninfectious Causes of Fever in the Intensive Care Unit

Alcohol/drug withdrawal
Postoperative fever
Posttransfusion fever
Drug fever
Cerebral infarction/hemorrhage
Adrenal insufficiency
Myocardial infarction
Transplant rejection
Deep vein thrombosis
Pulmonary emboli
Gout/pseudogout
Hematoma
Pancreatitis
Acalculous cholecystitis
Ischemic bowel
Aspiration pneumonitis
ARDS
Subarachnoid hemorrhage
Fat emboli
Cirrhosis
GI bleed
IV contrast reaction
Neoplastic fevers
Decubitus ulcers

ARDS, Acute respiratory distress syndrome; GI, gastrointestinal; IV, intravascular.

Recently the definition of sepsis used in clinical practice was updated to reflect advances in the pathobiology of sepsis and septic shock. Sepsis now is defined as life-threatening organ dysfunction caused by a dysregulated host response to infection. Organ dysfunction is represented clinically as an increase in the SOFA score by two or more points. In comparison, septic shock is a subset of sepsis in which there is profound circulatory, cellular, and metabolic abnormalities.

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