Study design
This study was designed as a single-centre, prospective, randomised single-blind trial. It was conducted in a medical ICU of a tertiary care university teaching hospital during 25 months, from April 2013 to April 2015. The study protocol was approved by the ethics committee of the institutional review board of La Rabta Hospital.
Patients and randomisation
All critically ill patients older than 18 years, with mechanical ventilation during more than 48 h, and who have presented a VAP, were eligible for study entry. Age <18 years, pregnancy and septic shock were considered as exclusion criteria. Patients who did not meet any exclusion criteria were randomly assigned into an intervention group and a control group (AS vs IV). Block randomisation was conducted by a random selection of computer-generated algorithm, the allocation sequence was followed by an independent statistician, and communicated to the investigator.
In the following cases: suspension of colistin (multisensitive strain imposing de-escalation, or a colistin-resistant strain), occurrence of a major side effect of inhaled route (severe bronchospasm or alveolar haemorrhage), decline in creatinine clearance below 10 ml/min in 48 h, occurrence of bacteraemia and/or septic shock, the patient should leave the trial protocol.
Study intervention
Colistin used was colimycine® powder (Sanofi Winthrop Industrie), i.e. colistimethate sodium—CMS. A flacon of 1 million units (MU) of colimycine® = 80 mg of CMS = 33.3 mg of colistin base activity.
Included randomised patients were treated with an empirical anti-infective therapy combining imipenem and colistin, depending on our local bacterial ecology. According to the randomisation, patients were divided into two groups: intervention and control groups. The intervention group (AS group) received 4 million units (MU) of AS colistin by nebulisation for 30 min three times per day in addition to IV imipenem 1 g three times per day. Nebulisation was made via an ultrasonic vibrating plate nebuliser (Aeroneb Pro® Aerogen Nektar Corporation, Galway, Ireland). This technique required specific settings in order to limit turbulence inspiratory flow. The specific settings were: a volume controlled mode with a tidal volume <8 ml/kg, respiratory rate at 12 cycles/min, I/E: 1/1 and an end inspiratory break >20 %.
The control group (IV group) received IV colistin as a loading dose of 9 MU during 60 min followed by 4.5 MU two times per day combined to IV impenem 1 g three times per day.
According to anti-infective susceptibility results, a targeted therapy was started. If the strain was sensitive to other anti-infective drugs, colistin was combined with β-lactam or aminoglycoside or tygecyclin. If the isolated pathogen was a colistin-only susceptible, colistin was administered in monotherapy.
The treatment duration was maintained at least 14 days. After extubation, AS colistin dose was calculated according to a 40 % extra pulmonary deposition, as shown in experimental studies [11]. Thus, the prescribed dose was modified to 7 MU in the nebuliser chamber, equivalent to a delivered dose of 4.2 MU in the respiratory tract.
In case of renal insufficiency, the relay doses of IV colistin were modified according to the creatinine clearance as follows: 4.5 MU per day if 10 ml/min < clearance < 30 ml/min, and 4.5 MU per 48 h if clearance is less than 10 ml/min. The loading dose of 9 MU was maintained.
Definitions and data collection
At study entry, demographic data, co-morbidities and the admission diagnosis were collected.
An episode of VAP was defined as a Clinical Pulmonary Infection Score (CPIS) of more than six [12, 13]. The investigators calculated the CPIS when the patient presented features suggesting a VAP such as fever, leukocytosis, purulent secretions, hypoxemia or radiological infiltrate. In case of the CPIS was higher than 6 points, the diagnosis of VAP was suspected and a tracheal aspirate was performed before colistin administration.
Bacteriological samples were performed on tracheal aspirate. A positive tracheal sample was defined as more than or equal to 106 colony-forming units (CFU)/ml. Sensitivity to colistin was determined by the E test, and the isolated strain was considered sensitive when the minimum inhibitory concentration (MIC) was less than 2 mg/l. Microbiological eradication was defined as a negative culture, i.e. no pathogen isolated in tracheal aspiration. The oxygenation was assessed by the PaO2/FiO2 ratio, and its improvement was considered when the ratio exceeded 300.
The acute renal failure (ARF) was defined as an increase of plasma creatinine more than 1.5 times its base value.
Outcome assessment
Patients were followed during 28 days. A physical examination (including temperature measurement and examination of tracheal secretions aspect) and a biological exploration including blood cells count, arterial blood gas and renal function were conducted every day. Chest X-ray and tracheal aspirate culture were performed every 3 days.
Therapeutic efficacy was assessed by the cure of VAP at the end of colistin therapy (day 14). The cure of VAP was defined as the resolution of clinical and biological signs of infection, i.e. a CPIS less than 6 and bacteriological eradication. Secondary outcomes were incidence of ARF, mechanical ventilation length, ICU length of stay and 28-day mortality.
Post-therapy assessment
A clinical, biological and bacterial re-assessment was performed at day 7 of the anti-infective cure, by the CPIS calculation and renal function analysis. The tracheal aspirate was replaced by cyto-bacteriological sputum in case of ventilator weaning.
Statistical analysis
We estimated that a sample size of 149 patients (AS, n = 73 and IV, n = 76) would provide a power of 80 % to demonstrate the non-inferiority of AS regimen compared with parenteral colistin with a lower toxicity in the intervention arm at a two-sided alpha error of 5 %. All statistical analyses were based on intention-to-treat principle.
Quantitative data were reported as mean ± SD and compared by the Student’s t test or the Mann–Whitney U test, as appropriate. Qualitative data were expressed as percentages and compared by the Chi-square or Fisher’s exact tests, as appropriate. The risk association measurement was expressed as odds ratio and performed by stratified analysis. Survival analysis was processed by the Kaplan–Meier survival curves and compared by the log-rank test. All tests were two-sided, and a p value <0.05 was considered to indicate statistical significance. The IBM SPSS Statistics 20 software was used for statistical analysis.