Methods for Assessing Microbiologic Response
In experimental animals, antibacterial efficiency of nebulized antibiotics can be directly assessed by measuring antibiotic lung-tissue concentrations and assessing quantitative bacteriology of postmortem lung-tissue samples.32–35 Concentrations measured from homogenized lung-tissue samples are representative of antibiotic present in interstitial and cell compartments. Most bacteria do not penetrate into cells and remain in the interstitial space, where antibiotics exert their bactericidal activity by binding to bacterial cell membrane. Therefore, antibiotic concentrations measured from homogenized lung biopsies underestimate “effective” interstitial concentrations, secondary to a dilution factor caused by intracellular components.41
In patients with ventilator-associated pneumonia, it can only be assessed indirectly as the regression of clinical, biologic, and radiologic signs of lung infection, and the disappearance of pathogens initially found in cultures of distal pulmonary samples. In ventilated critically ill patients, a number of confounding factors may hamper the clinical evaluation. The bronchial deposition of nebulized antibiotics may render cultures of distal samples falsely negative although the lung parenchyma is still positive. Critically ill patients frequently have several sites of infection, and persisting fever and biologic signs of infection may be related to a persisting extrapulmonary infection.
Antibiotic concentrations representative of the alveolar space can be assessed from a bronchoalveolar lavage. To obtain the antibiotic concentration representative of the epithelial lining fluid, the concentration measured in the aspirated fluid is corrected by a dilution factor, which is derived from the ratio between urea concentrations simultaneously measured in plasma and the bronchoalveolar lavage.42 Such measurements may fail to provide true representations of antibiotic concentrations at the site of lung infection because of confounding factors such as dilution errors or cell lysis.42 Of particular interest is a bronchoscopic microsampling method for measuring the antibiotic concentration in bronchial epithelial lining fluid.43 It consists of introducing, through a bronchoscope positioned in the airways, an inner 1.9-mm polyester fiber rod probe, which immediately adsorbs fluid when placed on a bronchial wall for 10 seconds. Antibiotic concentrations are then measured from the probe without any dilution. To date, this attractive but expensive technique remains limited to research studies.
Microbiologic Response to Concentration-Dependent Antibiotics in Experimental Studies
Most experimental studies of nebulized antibiotics included aminoglycosides32,33,38,44–46 and polymyxins,35 families of antibiotics exerting a concentration-dependent antibacterial effect.32,33,35,38,44–46 With concentration-dependent antibiotics, a single daily nebulization, ensuring high lung-tissue concentrations, is enough to achieve a bactericidal effect at the site of infection for a period of 12 to 24 hours. The postantibiotic effect prevents the regrowth of bacterial strains despite the antibiotic lung-tissue concentrations falling below the minimal inhibitory concentration. After several consecutive daily nebulizations of aminoglycosides, there is no time-dependent tissue accumulation.47
In the late 1970s, experimental studies performed in nonventilated mice with Klebsiella pneumoniae bronchopneumonia had reported enhanced bacterial killing and higher survival rates when kanamycin was nebulized in comparison to intramuscular administration.44,45 In spontaneously breathing squirrel monkeys, these investigators demonstrated that nebulized kanamycin efficiently protected against K. pneumoniae bronchopneumonia induced by intratracheal bacterial instillation, whereas the intramuscular administration was not protective.45 Antibiotic clearance curves indicated that nebulized kanamycin remained longer in the lungs and at higher concentrations than intramuscular kanamycin. In the early 1990s, a study performed in spontaneously breathing guinea pigs demonstrated that a combination of aerosolized and intramuscular tobramycin achieved slightly higher survival and total eradication of P. aeruginosa compared to nebulized or intramuscular tobramycin alone.46 Ten years later, an experimental study looked at the antibacterial efficiency of nebulized amikacin in anesthetized piglets ventilated for a severe E. coli bronchopneumonia.33 Twenty-four hours after a massive bronchial inoculation, ventilated animals received equivalent doses of amikacin, either by ultrasonic nebulization or intravenously. Because of a 60% extrapulmonary deposition, 45 mg/kg were nebulized in a single dose and 15 mg/kg administered intravenously. The animals received a second dose after 24 additional hours of mechanical ventilation and were killed 1 hour later, and five subpleural specimens were excised from the upper, middle, and lower lobes. Amikacin lung-tissue peak concentrations were threefold to 30-fold higher after nebulization than after intravenous administration. As shown in Figure 64-6, after two nebulizations and 25 hours of treatment, 71% of lung segments were sterile, whereas cultures of lung segments were comparable in nontreated and intravenously treated animals. In 2010, the antibacterial efficiency of nebulized colistin was assessed in anesthetized piglets ventilated for severe P. aeruginosa bronchopneumonia.35 Twenty-four hours after a massive bronchial inoculation, ventilated animals received equivalent doses of colistin, either by nebulization or intravenously. Because of a 60% extrapulmonary deposition, 16 mg/kg were nebulized in two daily doses and 10 mg/kg administered intravenously in three doses. The animals were killed 1 hour after the third aerosol and the fourth intravenous administration, and five subpleural specimens were excised from the upper, middle, and lower lobes. Colistin lung-tissue concentrations were not detectable following intravenous administration and onefold to 12-fold higher than minimal inhibitory concentrations after nebulization. After three nebulizations and 25 hours of treatment, 67% of lung segments were sterile, whereas 88% of lung segments were positive in nontreated animals and 72% in intravenously treated animals.
Lung bacterial burden of Escherichia coli (expressed in colony-forming units per gram [cfu/g] of lung tissue) in lung segments of anesthetized piglets mechanically ventilated for severe E. coli bronchopneumonia. Postmortem tissue samples were collected 1 hour after the second aerosol or intravenous (IV) dose of amikacin, or 48 hours after the bacterial inoculation in the untreated control group. Each triangle refers to a single lung segment. The lung bacterial burden is significantly lower in the aerosol group as compared with the intravenous or control groups. (Used, with permission, from Goldstein et al.33)
Microbiologic Response to Time-Dependent Antibiotics in Experimental Studies
With time-dependent antibiotics, such as cephalosporins, tissue concentrations have to be maintained permanently above minimal inhibitory concentrations to provide a bactericidal effect. Therefore, aerosols have to be repeated several times a day.
An experimental study looked at the lung-tissue concentrations of ceftazidime administered to anesthetized piglets ventilated for a severe P. aeruginosa bronchopneumonia.31 Twenty-four hours after a massive bacterial inoculation, animals received equivalent doses of ceftazidime either by ultrasonic nebulization or intravenously. Because of a 30% extrapulmonary deposition, 50 mg/kg were nebulized in a single dose and 33 mg/kg administered intravenously. The animals were sacrificed 1 hour later, and five subpleural specimens were excised from the upper, middle, and lower lobes. Ceftazidime lung-tissue concentrations following nebulization were fivefold to 30-fold higher than after intravenous administration.31 In a second study performed in the same experimental model, animals received either nebulized or intravenous ceftazidime for 24 hours.34 Twenty-four hours after a massive P. aeruginosa bronchial inoculation, ventilated animals received equivalent doses of ceftazidime, either by nebulization or intravenously. Because of a 60% extrapulmonary deposition, 200 mg/kg were nebulized in eight daily aerosols of 25 mg/kg each, and 90 mg/kg was administered by continuous intravenous infusion. The animals were killed 3 hours after the eighth aerosol and 24 hours after initiation of intravenous administration, and five subpleural specimens were excised from the upper, middle, and lower lobes. Ceftazidime trough lung-tissue concentrations were fourfold higher after nebulization compared to continuous intravenous administration. After nine nebulizations and 25 hours of treatment, 80% of lung segments were sterile, whereas 90% of lung segments were positive in nontreated animals and 70% in intravenously treated animals.
Clinical Response in Human Studies
The experimental studies reported above clearly suggest that aminoglycosides, polymyxins, and cephalosporins have a higher bactericidal efficiency when administered by nebulization as compared to the parenteral route. Beneficial effects of nebulization and endotracheal administration of antibiotics have been repeatedly reported in spontaneously breathing patients with cystic fibrosis.48 Several human studies have also demonstrated that the endotracheal administration or nebulization of aminoglycosides and polymyxins may prevent bronchial infection and ventilator-associated pneumonia in ventilated critically ill patients.49–52 The risk of encouraging resistive strains, however, has limited this prophylactic approach.53 Two studies performed in ventilated patients treated by intravenous antibiotics demonstrated that the addition of endotracheal tobramycin is useful for eradicating the pathogens that cause gram-negative pneumonia.54,55 In 1992, a comparative pharmacokinetic study, performed in ventilated patients with nosocomial pneumonia, reported high antibiotic bronchial concentrations following the nebulization or the endotracheal administration of 1 g of ceftazidime.56 In addition, the minimal inhibitory concentrations for 90% of the most important pathogens responsible for nosocomial infections were exceeded by concentrations in bronchial secretions for up to 12 hours after intravenous infusion and for up to 24 hours after endotracheal and aerosol administration.56
More than 40 years ago, aerosols of colistin were successfully administered to spontaneously breathing patients with pulmonary suppuration.57 In the early 1970s, two spontaneously breathing patients with P. aeruginosa pneumonia were treated with inhaled polymyxin B, but aerosols had to be stopped because of airway obstruction.58 Over the last decade, several investigator groups have reported microbiologic response following nebulization of antibiotics to critically ill patients with ventilator-associated pneumonia.59–70 Most of these studies were retrospective and concerned patients treated by nebulized colistin for ventilator-associated pneumonia caused by multidrug-resistant pathogens.59,60,62,63,66–69 Three studies were prospective64,65,70 and two were randomized.65,70 Most investigators reported “beneficial” effects, either resulting from a combination of nebulized and intravenous antibiotics59,60,65,65 or from nebulized antibiotics alone.61–63,67–70 Except for a prospective randomized study published in 2011,70 ventilator settings were not optimized in these studies and the rationale for choosing the aerosol dose was not given. Clinical response was considered beneficial when aerosol antibiotics were efficient in treating patients whose lungs were infected with multidrug-resistant microorganisms.60,62,64,65,67,69,70
In a matched case-control study, aerosolized plus intravenous colistin was not more efficient than intravenous colistin alone for treating ventilator-associated pneumonia caused by multidrug-resistant pathogens.68 In a double-blind, randomized, placebo-controlled study performed in forty-five critically ill patients with ventilator-associated tracheobronchitis, aerosolized gentamicin or vancomycin administered over the course of 14 days and combined with systemic antibiotics were associated with several benefits65: better resolution of ventilator-associated pneumonia, less bacterial resistance, reduced used of systemic antibiotics, and facilitation of weaning from mechanical ventilation. Several limitations, however, preclude drawing firm conclusions from the study: The small number of patients included in the aerosol group (n = 19), the small doses used in the aerosol group (gentamicin = 180 mg/day and vancomycin = 180 mg/day), the lack of optimization of ventilator settings during nebulization, and the heterogeneity of clinical and bacterial criteria for determining efficiency of antimicrobial therapy. In a randomized, controlled, phase II trial performed in forty critically ill patients with ventilator-associated pneumonia caused by P. aeruginosa, aerosolized ceftazidime and amikacin were compared to intravenous ceftazidime and amikacin.70 In both arms, antimicrobial therapy was administered over the course of 7 days. Resolution of ventilator-associated pneumonia was assessed by objective criteria based on eradication of P. aeruginosa from bronchoalveolar lavage, resolution of clinical signs of sepsis, and lung reaeration on computed tomography. During aerosol administration, ventilator settings were optimized to minimize extrapulmonary deposition and intravenous and nebulized doses were determined so as to deliver comparable amounts of ceftazidime and amikacin in the trachea and pulmonary artery. Aerosol and intravenous antibiotics had a similar efficiency for treating ventilator-associated pneumonia caused by P. aeruginosa sensitive to ceftazidime and amikacin. Unlike systemic antibiotics, aerosolized antibiotics were efficient against intermediate strains and they reduced the emergence of resistive P. aeruginosa. Because of the small number of patients included, no conclusion could be drawn about the potential benefit of aerosol antibiotics on emergence of resistive strains, duration of mechanical ventilation, length of stay in the intensive care unit, and mortality. Multicenter randomized trials are required to determine the clinical impact and the indications of nebulized antibiotics, and more specifically their efficiency in ventilator-associated pneumonia caused by multidrug-resistant microorganisms. Apart from potential benefit in patients with ventilator-associated pneumonia, nebulized antibiotics may also prevent microbial biofilm formation on the endotracheal tube internal surface,74 thereby suppressing a reservoir of infecting microorganisms for the deep lung.7