Ventilator-Associated Pneumonia

The clinical approach to pneumonia can be characterized by one word: problematic. Fundamental problems include a limited ability to detect parenchymal lung infections, and the lack of a standardized method for identifying responsible pathogen(s).

This chapter presents the current state of affairs regarding pneumonias that appear after 72 hours of mechanical ventilation (i.e., ventilator-associated pneumonias), and includes recommendations from clinical practice guidelines (1,2,3) and recent reviews of this condition (4,5).

I. General Information

The following statements summarize some of the relevant observations about ventilator-associated pneumonia (VAP).

Pneumonia is the most common nosocomial infection in ICU patients (6), and more than 90% of these pneumonias occur during mechanical ventilation (2). However, the prevalence of VAP is overstated, because post-mortem studies have shown that over half of the cases of VAP are false-positive diagnoses (7).
Unlike community-acquired pneumonias, where the predominant pathogens are pneumococci, atypical organisms, and viruses, three-quarters of the responsible pathogens in VAP are gram-negative aerobic bacilli and Staphylococcus aureus (see Table 16.1) (8).
The mortality rate associated with VAP varies widely, from 0% to 65% (3,9), and there are claims that VAP is not a life-threatening illness (9). However, VAP-associated mortality rates must be viewed with caution because of the tendency for overdiagnosis of VAP (as mentioned earlier) (7).

Table 16.1 Pathogenic Isolates in Ventilator-Associated Pneumonia

II. Preventive Measures

Aspiration of pathogenic organisms from the oropharynx is believed to be the inciting event in most cases of VAP. The
pathogens that most often colonize the oropharynx in ICU patients are gram-negative aerobic bacilli (see Chapter 3, Figure 3.2), and this explains the predominance of these pathogens in VAP.

A. Oral Decontamination

The realization that VAP begins with pathogenic colonization of the oropharynx resulted in the introduction of measures to decontaminate the oropharynx as a preventive measure for VAP.
The methods of oral decontamination (i.e., with chlorhexidine or topical antibiotics) are described in Chapter 3, Section II, and the benefits of oral decontamination in reducing tracheal colonization and VAP are shown in Figure 3.3.
Routine oral care with chlorhexidine (as a mouth rinse or gel, used 2–3 times daily) has become a standard practice in ventilator-dependent patients.

B. Routine Airway Care

The inner surface of artificial airways (endotracheal and tracheostomy tubes) becomes colonized with pathogenic organisms, and passing a suction catheter through the tubes can dislodge these organisms and introduce pathogens into the lower airways (10). Because of this risk, endotracheal suctioning is not recommended as a routine procedure, and should be used only when necessary to clear secretions from the airways (11).

C. Clearing Subglottic Secretions

Contrary to popular belief, inflation of the cuff on tracheal tubes to create a seal does not prevent aspiration of mouth secretions into the lower airways. Aspiration of saliva and liquid tube feedings has been documented in over 50%
of patients with tracheostomies, and the aspiration is clinically silent in most cases (12).
Concern about aspiration around inflated cuffs prompted the introduction of specialized endotracheal tubes equipped with a suction port just above the cuff (Mallinckrodt TaperGuard Evac Tube). The suction port is connected to a source of continuous suction (usually not exceeding −20 cm H₂O) to clear the secretions that accumulate in the subglottic region, as illustrated in Figure 16.1.
Clinical studies have shown a significant reduction in the incidence of VAP when subglottic secretions are cleared using these specialized endotracheal tubes (13).

III. Clinical Features

A. Diagnostic Accuracy

The traditional clinical criteria for the diagnosis of VAP include: (a) fever or hypothermia, (b) leukocytosis or leukopenia, (c) an increase in volume of respiratory secretions or a change in character of the secretions, and (d) a new or progressive infiltrate on the chest x-ray (4).

In cases of VAP diagnosed using traditional clinical criteria, the incidence of pneumonia on postmortem exam is only 30% to 40% (7).
The accuracy of clinical criteria for the diagnosis of VAP is demonstrated in Table 16.2. This table shows the results of two studies that used autopsy evidence of pneumonia to evaluate the premortem diagnosis of VAP based on clinical findings (14,15). In both studies, the clinical findings were just as likely to occur in the presence or absence of pneumonia. These studies demonstrate that the diagnosis of VAP is not possible using clinical criteria alone.

FIGURE 16.1 Endotracheal tube with a suction port placed just above the cuff to clear secretions that accumulate in the subglottic region.

Table 16.2 Predictive Value of Clinical Criteria for Identifying Ventilator-Associated Pneumonia

Study	Clinical Criteria	Likelihood Ratio for Pneumonia on Autopsy^†
Fagon et al. (14)	Radiographic infiltrate + purulent sputum + fever or leukocytosis	1.03
Timset et al. (15)	Radiographic infiltrate + 2 of the following: fever, leukocytosis, or purulent sputum	0.96
^†The likelihood ratio is the likelihood that patients with pneumonia will have the clinical findings compared to the likelihood that patients without pneumonia will have the same clinical findings. A likelihood ratio of 1 indicates that a pneumonia is just as likely to be present or absent based on the clinical findings.

B. Chest Radiography

The performance of portable chest x-rays in detecting pulmonary consolidation is shown in Table 16.3 (16). Note that the poor diagnostic accuracy (49%) is primarily due to a low sensitivity for detecting pulmonary infiltrates. This is demonstrated in Figure 16.2, which shows a portable chest x-ray and CT scan of the lungs in an ICU patient with fever. Note that the chest x-rays shows no apparent infiltrates, while the CT image shows a fine pattern of consolidation in the posterior region of both lungs.

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