Intensive care
Nosocomial infections in the intensive care unit

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Abstract

Nosocomial infection in the intensive care unit (ICU) is associated with increased mortality, morbidity and length of stay. It is defined as infection that begins 48 hours after admission to hospital. The commonest types are ventilator-associated pneumonia (VAP), central line-associated bloodstream infection (CLABSI), urinary catheter-related infection and surgical site infection. The common pathogens include Staphylococcus aureus, Pseudomonas aeruginosa, Candida, Escherichia coli and Klebsiella species. Antimicrobial resistance is increasing and has emerged from selective pressure from antibiotic use and transmission via health workers. Prevention of infection is fundamental and can be achieved through good antimicrobial use and infection control, including hand hygiene. Grouped, easy-to-follow best practice activities called ‘care bundles’ have been developed to prevent VAP and CLABSI. Microbiological cultures are central to rapid and accurate diagnosis, which improves outcomes and reduces resistance. The principles of treatment include early antimicrobial therapy (after appropriate specimens are taken) targeted to the local microbes, then de-escalation according to culture and susceptibility results. This article summarizes the pathogenesis, risk factors, microbiology, diagnosis, prevention and treatment of VAP, CLABSI and nosocomial urinary tract infection in the adult ICU.

Introduction

Nosocomial infection (defined as onset more than 48 hours after hospital admission) in the intensive care unit (ICU) is associated with increased mortality, morbidity and length of stay. Prevalence rates of infection acquired in ICUs vary from 9 to 37% when assessed in Europe and the USA. Case fatality is high, with crude mortality rates up to 50% for bacteraemia. Timely diagnosis, appropriate management and prevention are essential to improve patient outcomes and reduce antimicrobial resistance. The commonest types are ventilator-associated pneumonia (VAP), central line-associated bloodstream infection (CLABSI), urinary catheter-related infection and surgical site infection. Other types of nosocomial infection are also important, such as those in immunocompromised hosts and neonates, but beyond the scope of this article.

Colonization of critically ill patients with nosocomial organisms usually occurs after 48–72 hours of admission; the most important pathogens are displayed in Table 1. The spectrum of nosocomial microorganisms comprises different ones from those originating from the community, with higher rates of resistant organisms. Antimicrobial resistance emerges in ICU because of:

  • evolution of resistance in existing bacteria, through selective pressure from antibiotic use

  • transmission (usually nosocomial) especially through frequent contact with healthcare workers or via procedures.

Many studies have previously shown increasing incidence of resistant bacteria in ICUs. These include: methicillin-resistant Staphylococcus aureus (MRSA), vancomycin-resistant Enterococcus (VRE) and multi-resistant Gram-negatives. All these pathogens are associated with poor outcomes. There is a longer time to receipt of effective therapy; and the newer agents used for treatment often have inferior efficacy, poor pharmacokinetics and increased toxicity (e.g. vancomycin, linezolid, amikacin, colistin). More recently, studies describe success in controlling some types of resistant organisms (most notably reductions in MRSA, particularly attributed to better hand hygiene practices), but with little impact on Gram-negative and fungal resistance.

The emergence of resistant organisms tends to add to the total burden of infections, rather than substituting for the more sensitive organisms previously present. For example, as MRSA becomes endemic in a unit, the total number of staphylococcal infections tends to increase; in contrast in units where MRSA is reduced, the total number of staphylococcal infections tends to come down.

Rapid and accurate diagnosis of nosocomial infection both improves patient outcomes and decreases selection pressure for resistance. It ‘streamlines’ patients onto the most effective treatments allowing rapid cessation of unnecessary antibiotics and minimizing unnecessary side effects. Correct timing is vital with all microbiologic tests. Tests rapidly lose sensitivity once new antibiotics are introduced.

The single most useful microbiologic test in ICU is a correctly performed blood culture. Our unit's simple protocol is shown in Box 1.

Nosocomial infections are reduced by good antibiotic use (to maximize cure rates and reduce of selection pressure for resistance) and strict infection control.

Establishing active ongoing liaison between the ICU, infectious diseases (or clinical microbiology) department and pharmacy is essential. This multidisciplinary approach is important to develop local guidelines (preferably guided by local microbiology data), provide day-to-day advice, oversee control measures for broad spectrum antibiotics, and monitor usage and report back to the ICU staff in a useful manner. Elements of good antibiotic use in the ICU are given in Box 2. The role of combination antibiotics in preventing resistance is controversial. However, it is common practice to use combination empiric therapy for sepsis for reasons of coverage, de-escalating to narrower cover once culture results are known or the patient improves. ‘Cycling’ antibiotic use is poorly studied, and cannot be recommended.

Infection control minimizes cross-transmission and prevents colonizing bacteria from causing infection. One of the key elements in infection prevention over the past decade has been hand hygiene. It is estimated that over 30% of healthcare-associated infections are preventable by hand hygiene. Hand hygiene prevents cross-transmission of pathogens by hands of healthcare workers between patients. Multiple studies have shown a reduction in healthcare-associated infection rates, specifically reductions in MRSA and even elimination in some centres. The WHO has recommended ‘Five Moments for Hand Hygiene’ in healthcare settings both resource rich and poor.1 Alcohol-based hand-rubs should be used before touching a patient, before a procedure, after body fluid exposure, after touching a patient and after touching patient surroundings. Other aspects of infection control include surveillance for, and isolation of, patients with multi-resistant organisms.

Recent years have seen the widespread promotion of infection control ‘care bundles’. These are groupings of ‘best practices’ that when applied together appear to result in greater improvement in outcomes. This is based on the philosophy that the ‘total may be greater than the sum of the parts’. They have the virtues of being simple, logical (mostly) and easily evaluable (the elements are dichotomous, so compliance can be measured as a simple ‘yes/no’). Not all possible proven therapies are included in the bundle, as factors such as ease of implementation, adherence and cost are considered. Examples of care bundles are given in Box 3.

Section snippets

Ventilator-associated pneumonia (VAP)

Hospital-acquired pneumonia (HAP) (i.e. pneumonia that begins 48 hours or more after admission) is the leading cause of hospital-acquired infection leading to mortality.2 Ventilator-associated pneumonia (VAP) is a subset of HAP, occurring 48 hours or more after endotracheal intubation. Mechanical ventilation increases the risk of pneumonia by 6–20 times. It is associated with mortality rates of up to 50%.3, 4 Some authors also describe ventilator-associated tracheobronchitis (VAT) as a

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