Page 846 - Hall et al (2015) Principles of Critical Care-McGraw-Hill

CHAPTER 65: Pneumonia 577

INTRODUCTION TABLE 65-1 Risk Factors for Oropharyngeal Colonization With Pathogens

Pneumonia is one of the most common precipitating causes for ICU Antibiotics
admission. It is a frequent cause of hemodynamic compromise and Malnutrition
septic shock. Pneumonia is also one of the most common causes for the
acute respiratory distress syndrome (ARDS). Viral Infection
Pneumonia on admission to the intensive care unit presents in three Uremia
different forms: traditional community-acquired pneumonia (CAP), Chemotherapeutic agents
hospital-acquired pneumonia (HAP), and the controversial entity of health
care–associated pneumonia (HCAP). HCAP is a community-onset pneu- Radiation
monia but with risk factors for pathogens more typical of HAP. In addition, Nasogastric tube
presence of any number of immunocompromised states within each of Chronic tracheostomy
these entities raises concern for a broader spectrum of potential etiologies,
especially opportunistic pathogens. While ventilator-associated pneumo- the antibiotic spectrum and the longer the duration of treatment, the
nia (VAP) is technically a subgroup of HAP, this type of pneumonia occurs
as a complication of critical illness, rather than the precipitating cause of more likely that pathogenic bacteria will colonize the oropharynx.
Colonization with bacteria that are not native to the oropharynx is
critical illness. As such, VAP is covered in a separate chapter (Chap. 59).
While each of these types of pneumonia has some common character- the principal factor discriminating between the two community-onset
istics, their differences warrant separate discussion. However, the basic pneumonias, CAP and HCAP.
pathophysiology of pneumonia in patients without an artificial airway is ■
very similar and will therefore be discussed in more general terms initially. ROLE OF ASPIRATION
For the majority of bacterial pneumonias, aspiration of oropharyngeal
secretions is the dominant mechanism by which bacteria gain entrance
PATHOGENESIS into the lower respiratory tract and alveoli. Usually, the volume of secre-
The lung represents the greatest amount of surface area in contact with the tions is relatively small and is termed microaspiration. Occasionally,
external environment in humans. The lung is therefore exposed routinely obvious large volume aspiration can occur, leading to severe pneumonia
to airborne infectious microorganisms. In addition, deposition of a liquid if the bolus is infectious or markedly predisposes to pneumonia even if
inoculum into the lower respiratory tract occurs on a frequent basis sec- noninfectious, such as with enteral feedings. Microaspiration is likely
ondary to microaspiration. As a result, the lungs and entire respiratory tract the predominant cause in CAP and VAP, while macroaspiration plays a
have effective and redundant host defense mechanisms in order to respond bigger role in HAP and HCAP.
to this infectious challenge. Despite this, lower respiratory tract infections The terms aspiration and aspiration pneumonia have been used for
remain the leading causes of infectious death, even in the modern world. multiple clinical entities, both infectious and noninfectious (Table 65-2).
1
Some have suggested that all humans remain immunodeficient and that As suggested above, most bacterial pneumonias result from an aspiration
2
the decrease in overall mortality from pneumonia and influenza in devel-
oped countries is likely due to salvage by antibiotic therapy. TABLE 65-2 Aspiration Syndromes
■ BACTERIAL MILIEU Syndrome Infectious Major Pathogen Clinical Scenario
Airways below the vocal cords and the alveolar spaces have gener- Anaerobic Yes Anaerobes Loss of consciousness in past—
ally been thought to be sterile. However, recent data using nonculture pleuropneumonia alcohol abuse, seizure disorder
molecular tools have suggested that the microbiome of the lower respira- Large-volume gastric aspiration Vomiting, esophageal motility
tory tract may not be free of bacteria. This is clearly true in patients with (Contents of bolus determine resultant syndrome) disorders
3
HAP and HCAP, as well as patients with chronic airway disease including
chronic bronchitis and bronchiectasis. Alterations in this microbiome Low pH No Acute lung injury/ARDS
are likely to predispose to subsequently culture-positive infections. Bland, enteral Yes Aspiration pneumonia if gastric pH
In contrast, bacteria are abundant in the upper respiratory tract, feedings not low
reaching concentrations as high as 10 to 10 colony-forming units No Aspiration pneumonitis if bilious or
10
12
(cfu)/mL. While most of these bacteria are generally considered non- moderately low pH
pathogenic, normal oropharyngeal and nasal colonization includes No Atelectasis, high risk of subsequent
potential pathogens such as Streptococcus pneumoniae, Staphylococcus
aureus, and Neisseria meningitides. pneumonia
In patients with HAP and HCAP, the bacterial milieu of the oro- Small bowel Yes Gram negatives, Small bowel obstruction, ileus
pharynx changes dramatically, with the emergence of colonization by contents anaerobes
gram-negative Enterobacteriaceae and methicillin-resistant S aureus Small volume aspiration
(MRSA). This gram-negative colonization can occur within the first Oropharyngeal Yes Anaerobes, normal Loss of consciousness or inability to
3 days of admission of moderately ill patients. Most of these gram- flora, any colonizer protect airway—stroke, sedation,
4,5
negative colonizers are endogenous flora from the patient’s own gastro- metabolic encephalopathy, etc
intestinal tract. Other pathogens can be introduced via poor infection
control practices, including poor hand washing by caregivers, or from Aspiration pneumonia
the environment. However, the oropharynx is remarkably refractory to Gastric No Usually associated with
colonization simply by exposure to pathogens. Pseudomonas aeruginosa gastroesophageal reflux
and Aspergillus species are fairly ubiquitous in the environment, even Acute—bronchospasm/asthma
in the hospital, but pneumonia from these pathogens essentially never or cough
occurs unless patients have been exposed to prolonged antibiotic therapy.
Risk factors for altered oropharyngeal flora that are particularly perti- Chronic—cough syndrome,
nent to patients likely to be admitted to the ICU are listed in Table 65-1. bronchiolitis obliterans, pulmonary
By far, the most important risk factor is use of antibiotics; the broader fibrosis

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