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* From the Department of Internal Medicine (Dr. Kollef), Pulmonary and Critical Care Division, the Department of Anesthesiology (Dr. Skubas), Division of Cardiothoracic Anesthesia, and the Department of Surgery (Dr. Sundt), Division of Cardiothoracic Surgery, Washington University School of Medicine, St. Louis, MO.
Correspondence to: Marin H. Kollef, MD, FCCP, Pulmonary and Critical Care Medicine Division, 660 S Euclid Ave, Campus Box 8052, St. Louis, MO 63110; e-mail: kollef{at}pulmonary.wustl.edu
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Abstract |
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Design: Prospective clinical trial.
Setting: Cardiothoracic ICU (CTICU) of Barnes-Jewish Hospital, St. Louis, a university-affiliated teaching hospital.
Patients: Three hundred forty-three patients undergoing cardiac surgery and requiring mechanical ventilation in the CTICU.
Interventions: Patients were assigned to receive either CASS, using a specially designed endotracheal tube (Hi-Lo Evac; Mallinckrodt Inc; Athlone, Ireland), or routine postoperative medical care without CASS.
Results: One hundred sixty patients were assigned to receive CASS, and 183 were assigned to receive routine postoperative medical care without CASS. The two groups were similar at the time of randomization with regard to demographic characteristics, surgical procedures performed, and severity of illness. Risk factors for the development of VAP were also similar during the study period for both treatment groups. VAP was seen in 8 patients (5.0%) receiving CASS and in 15 patients (8.2%) receiving routine postoperative medical care without CASS (relative risk, 0.61%; 95% confidence interval, 0.27 to 1.40; p = 0.238). Episodes of VAP occurred statistically later among patients receiving CASS ([mean ± SD] 5.6 ± 2.3 days) than among patients who did not receive CASS (2.9 ± 1.2 days); (p = 0.006). No statistically significant differences for hospital mortality, overall duration of mechanical ventilation, lengths of stay in the hospital or CTICU, or acquired organ system derangements were found between the two treatment groups. No complications related to CASS were observed in the intervention group.
Conclusions: Our findings suggest that CASS can be safely administered to patients undergoing cardiac surgery. The occurrence of VAP can be significantly delayed among patients undergoing cardiac surgery using this simple-to-apply technique.
Key Words: cardiac surgery • intensive care • mechanical ventilation • outcomes • ventilator-associated pneumonia
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Introduction |
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TOPAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Endotracheal tubes play a particularly important role in the occurrence of VAP. Endotracheal tubes facilitate bacterial colonization of the tracheobronchial tree because of associated mucosal injury with their insertion and manipulation.6 These tubes also predispose the patient to lower airway aspiration of contaminated secretions by the elimination of the cough reflex, pooling of contaminated secretions above the endotracheal tube cuff, formation of a contaminated biofilm surrounding the endotracheal tube, and development of nosocomial sinusitis.6 7 8 9 10 11 12 However, the need for endotracheal intubation, particularly among patients undergoing major surgical procedures, is usually unavoidable, resulting in a pool of patients who are at risk for developing VAP.
Recently, several clinical trials have demonstrated that a simple method for decreasing the occurrence of pooled secretions above the endotracheal tube cuff (continuous aspiration of subglottic secretions [CASS]) is associated with reductions in the occurrence of VAP.13 14 To better determine the optimal use of CASS in different patient populations, we performed a clinical trial examining the effectiveness of CASS in patients undergoing cardiac surgery. The main objectives of our investigation were to determine the incidence of VAP and other important clinical outcomes among patients receiving CASS and to compare these outcomes with patients receiving routine postoperative medical care without CASS.
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Materials and Methods |
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TOPAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Study Design
Patients were assigned to receive CASS using a specially
designed endotracheal tube (Hi-Lo Evac; Mallinckrodt Inc.; Athlone,
Ireland) or routine postoperative medical care without CASS. Group
assignment was determined using the patient’s birth year, with those
patients having odd birth years receiving CASS and those patients
having even birth years receiving routine postoperative medical care
without CASS.
For purposes of this investigation, ventilator circuits were defined to include the gas delivery tubing, humidifier water reservoirs or hygroscopic condenser humidifiers, water traps, in-line suction catheters, and medication delivery devices (such as metered-dose inhaler chambers or adapters). Starting at the patient’s endotracheal tube, in-line suction catheters were attached followed by a medication delivery device (if used), a hygroscopic condenser humidifier, and the gas delivery tubing. Hygroscopic condenser humidifiers were used for the first 96 h of mechanical ventilation in all patients unless specifically contraindicated (because of excessive airway secretions). Patients with contraindications to the use of hygroscopic condenser humidifiers or patients receiving mechanical ventilation for > 96 h were placed on a heated wire humidification system. On the basis of published experience, the same ventilator circuit tubing and in-line suction catheter were used throughout each patient’s course of mechanical ventilation unless these became visibly soiled or experienced a mechanical failure.15 16 17 Heat and moisture exchangers were routinely changed every 24 h, or more frequently if visibly soiled. Respiratory therapists conducted rounds for all patients and ventilators at least every 2 h. During these rounds, the ventilator circuit was checked for condensate accumulation or air leaks, the in-line suction catheter was inspected, and the overall function of the ventilator was reviewed.
We used the following ventilators during the study period: Siemens Servo 900C (Siemens-Elema Ventilator Systems; Schaumberg, IL), Puritan-Bennett 700 Series (Puritan-Bennett Corporation; Carlsbad, CA), and Bird 8400 Series ventilators (Bird Products Corp; Palm Springs, CA). The following ventilator circuits and attachments were commercially obtained: hygroscopic condenser humidifiers (Ballard 1000; Ballard Medical Products; Draper, UT), gas delivery tubing (Hudson RCI Ventilator Set; Hudson RCI; Temecula, CA), heated water humidifier (MR730 Respiratory Humidifier; Fisher & Paykel Healthcare; New Zealand), heated wire gas delivery tubing (Isothermal Breathing Circuit; Baxter Healthcare Corp.; Deerfield, IL), in-line suction catheters (Trach Care; Ballard Medical Products), metered-dose inhaler chambers (Aerovent; Monaghan Medical Corp.; Plattsburg, NY), and water traps (Marquest Medical Products; Englewood, CO).
CASS
All study patients were intubated using the same type of
endotracheal tube (Hi-Lo Evac; Mallinckrodt Inc), which incorporates a
dorsal separate lumen ending into the subglottic area by creating a
large elliptical dorsal opening above the cuff for aspiration of
subglottic secretions.14
The size of each endotracheal
tube was selected by the attending anesthesiologist. Subglottic
suctioning was delivered using a standard wall suction unit, which
applied continuous low intermittent suction not exceeding 20 mm Hg. The
suctioning port of the Hi-Lo Evac tube was marked with a paper tag to
indicate that it was a suction port. All secretions were collected in
wall-mounted secretion collectors.
Data Collection
For all study patients, the following characteristics were
prospectively recorded by one of the investigators: age, sex, race,
premorbid lifestyle score,18
severity of illness based on
acute physiology and chronic health evaluation (APACHE)
II19
scores, the PaO2/fraction of
inspired oxygen (FIO2) ratio, and the
occurrence of a witnessed aspiration event. Specific processes of
medical care examined to assess risk factors for VAP were the
administration of antacids or histamine-2-receptor antagonists,
pharmacologic aerosol treatments during mechanical ventilation (such as
bronchodilators), supine positioning of the head of the bed, tracheal
reintubation, surgical tracheostomy, fiberoptic bronchoscopy, and the
administration of antibiotics during the same hospitalization but
before cardiac surgery. Additionally, surgical variables recorded for
purposes of this investigation included the surgical procedure
performed, whether perioperative antibiotic prophylaxis was
administered, the use of cardiopulmonary bypass, and the
cardiopulmonary bypass time.
A clinical study coordinator made daily rounds during the work week on all study patients in the CTICU. Patients entered into the study were prospectively followed for the occurrence of VAP until they were successfully weaned from mechanical ventilation, were discharged from the hospital, or died. All patients suspected of having VAP were prospectively and independently reviewed by another investigator (MHK) who was blinded to the patients’ treatment group assignments. The diagnosis of VAP was strictly based on the predetermined criteria described below. In addition to the occurrence of VAP, we assessed secondary outcomes, including lobar atelectasis, the lengths of hospitalization and ICU stay, the duration of mechanical ventilation, the number of acquired organ system derangements using the organ system failure index,20 and hospital mortality.
Definitions
All definitions were selected prospectively as part of the
original study design. The premorbid lifestyle score was used as
previously defined.18
Zero indicated that the patient was
employed without restriction; 1 indicated that the patient was
independent, fully ambulatory, not employed, or employed with
restriction; 2 indicated that the patient had restricted activities,
could live alone and get out of the house to do basic necessities, or
had severely limited exercise ability; 3 indicated that the patient was
housebound, could not get out of the house unassisted, could not live
alone, or could not do heavy chores; and 4 indicated that the patient
was bed- or chair-bound. We calculated APACHE II scores on the basis of
clinical data available from the first 24-h period of intensive
care.19
The organ system failure index was modified from that used by Rubin and
coworkers.20
One point was given for acquired dysfunction
of each organ system: renal dysfunction, a twofold increase in baseline
creatinine level or an absolute increase in baseline creatinine level
of 176.8 µmol/L (2.0 mg/dL); hepatic dysfunction, an increase in
total bilirubin level to > 34.2 µmol/L (2.0 mg/dL); pulmonary
dysfunction, (1) a requirement for mechanical ventilation beyond
24 h after surgery for a diagnosis of pneumonia, COPD, asthma, or
pulmonary edema (cardiogenic or noncardiogenic), (2) a
PaO2 of < 60 mm Hg while receiving
an FIO2 of 
0.50, or (3) the use
of at least 10 cm H2O of positive end-expiratory
pressure; hematologic dysfunction, the presence of disseminated
intravascular coagulation, a leukocyte count of < 1,000
cells/mm3 (1.0 x 109/L),
or a platelet count of
< 75 x 103/mm3
(75 x 109/L); neurologic dysfunction, a new
focal deficit (such as hemiparesis after cerebral infarction) or a new
generalized process (eg, seizures or coma); GI dysfunction,
GI hemorrhage requiring transfusion, new ileus, or diarrhea lasting
> 24 h and unrelated to previous bowel surgery; and cardiac
dysfunction, acute myocardial infarction, cardiac arrest, or the new
onset of congestive heart failure.
The diagnostic criteria for VAP were modified from criteria established
by the American College of Chest Physicians because routine
bronchoscopic or nonbronchoscopic sampling of lower airway secretions
was not required.21
VAP was considered to be present when
a new or progressive radiographic infiltrate developed in conjunction
with either radiographic evidence of pulmonary abscess formation
(ie, cavitation within preexisting pulmonary infiltrates),
histologic evidence of pneumonia in lung tissue, or a positive blood or
pleural fluid culture or with any two of the following: fever,
leukocytosis, and purulent tracheal aspirate. Blood and pleural fluid
cultures could not be related to another source, and both had to be
obtained within 48 h before or after the clinical suspicion of
VAP. Microorganisms recovered from blood or pleural fluid cultures also
had to be identical to the organisms recovered from cultures of
respiratory secretions. A new infiltrate was prospectively determined
to be present if it developed after the start of mechanical ventilation
or within 48 h of extubation. Persistence of the infiltrate was
established if it was radiographically visible for at least 72 h.
Fever was defined as an increase in the core temperature of 1°C
and a core temperature of > 38.3°C. Leukocytosis was defined as a
25% increase in circulating leukocytes from baseline and a leukocyte
count of
> 10 x 103/mm3
(10 x 109/L). Tracheal aspirates were
considered purulent if a Gram’s stain showed > 25 neutrophils per
high-power field.
Prophylactic antibiotics were defined as antibiotic administration in the perioperative period aimed at reducing the incidence of nosocomial infections. Cefazolin was the routine agent administered before surgery in the preoperative area and for 24 to 48 h postoperatively. Vancomycin was also used in some high-risk patients (eg, heart transplant, valve surgery, and aortic root repair) at the surgeon’s discretion.
All chest radiographs were interpreted by a radiologist blinded to treatment group assignments. Lobar atelectasis was defined as complete opacification of a lobe with evidence of volume loss manifested by displacement of a fissure, hilum, hemidiaphragm, or narrowing of the intercostal spaces.
Statistical Analysis
We estimated sample size to provide 80% power to detect a 7.5%
difference (17.5% as compared with 10.0%) in the rate of occurrence
of VAP between the two study groups. We used an 
-error of 0.05
(two-tailed). On the basis of these assumptions, 178 patients were
needed in each of the two study groups. All comparisons were unpaired,
and all tests of significance were two-tailed. Continuous variables
were compared using the Student’s t test for normally
distributed variables and the Wilcoxon rank-sum test for non-normally
distributed variables. The 
2 or Fisher’s
Exact Test was used to compare categorical variables. The primary data
analysis compared the incidence of VAP between patients assigned to
receive CASS and patients assigned to receive routine postoperative
medical care without CASS. We used Kaplan-Meier survival analysis to
compare the time of occurrence of VAP for the patients developing this
complication in each study group. The log-rank test was used for
statistical comparisons. The mortality rate in each group was treated
as a censoring event.
Multiple logistic regression analysis was used to determine the independent risk factors for VAP. A stepwise approach was used to enter new terms into the logistic regression models, for which 0.05 was set as the limit for the acceptance or removal of new terms. In addition to patient demographics, risk factors for VAP, and surgical variables (Tables 1 and 2 ), the use of CASS was entered into the logistic regression model to determine whether it was independently associated with the development of VAP. Results of the logistic regression analysis are reported as adjusted odds ratios (ORs) with 95% confidence intervals (CIs).
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Results |
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TOPAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Two patients (25.0%) with VAP receiving CASS and three patients (20.0%) with VAP not receiving CASS had no growth of pathogens from their sputum cultures. The isolated pathogens for the remaining patients with VAP are shown in Table 3 . Patients receiving CASS were less likely to be infected with Staphylococcus aureus or Hemophilus influenzae than patients not receiving CASS, but this difference was not statistically significant. All eight patients receiving CASS who developed VAP also received antibiotic prophylaxis (cefazolin). Fourteen of the patients (93.3%) not receiving CASS received antibiotic prophylaxis (cefazolin, 12 patients; vancomycin, 1 patient; cefazolin and vancomycin, 1 patient).
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Discussion |
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TOPAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Our findings agree with those reported by Valles et al14 and Mahul et al.13 These investigators used a similar endotracheal tube; however, Mahul et al13 did not use continuous aspiration and used intermittent aspiration instead. Valles et al,14 examining a general ICU population, found that the overall incidence of VAP was statistically lower among patients receiving CASS than among control patients (19.9 episodes/1,000 ventilator days vs 39.6 episodes/1,000 ventilator days; RR, 1.98; 95% CI, 1.03 to 3.82). Additionally, these authors found that episodes of VAP occurred significantly later in patients receiving CASS, which was similar to our observation. Finally, the distribution of pathogens causing VAP in the study by Valles et al14 was similar to ours, with fewer cases of VAP caused by S aureus or H influenzae among patients receiving CASS.
The main effect of CASS in our study was to delay the onset of VAP among cardiac surgery patients developing this complication. Whether this is a worthwhile goal can be questioned because none of the secondary outcomes examined were influenced by the use of CASS. This is also similar to the findings of the previous investigations, which found that the administration of CASS had no influence on hospital mortality or lengths of stay in the hospital or ICU. The overall usefulness of CASS has recently been examined by Cook et al22 who used an evidence-based approach. They concluded that individual clinicians need to decide whether the administration of CASS will significantly contribute to their VAP-prevention strategy. This will, in large part, be based on the preexisting strategies that the hospital has for VAP prevention, the additional costs associated with the use of CASS, and the clinical benefits realized with the application of CASS.
Our study has several limitations. First, we used birth years to assign patients to treatment groups and not a blinded randomization scheme. However, the treatment groups were comparable for severity of illness, demographics, and surgical procedures (Tables 1 , 2) , which should minimize this limitation. Second, we used a clinical diagnosis of VAP that did not rely on quantitative cultures of lower respiratory secretions obtained bronchoscopically. However, a recent study suggests that the use of clinical criteria to establish the diagnosis of VAP is acceptable because of its greater diagnostic sensitivity, compared with bronchoscopically obtained cultures, and its good correlation with hospital mortality.23 Other authors have also suggested that diagnositc criteria for VAP that are not dependent on bronchoscopically obtained specimens are acceptable.24 25 Third, the study was performed within a single ICU, and the results may therefore not be applicable to other institutions.
Another limitation of this investigation is the relatively small sample
size examined. This has heightened importance because the occurrence of
VAP was relatively uncommon among our cardiac surgery patients,
although its detrimental impact on their outcomes is well
described.26
The small sample size used in our study
may predispose to both types I and II statistical errors. Based on the
rates of VAP observed in this investigation, a sample size of 1,006
patients would be needed for the observed difference in VAP rates to be
statistically significant between the two study groups (assumed power,
0.80; , 0.05). Additionally, three patients developed VAP during the
first 24 h of intensive care (one receiving CASS, two without
CASS). These patients may have actually had aspiration pneumonia and
not VAP, although none of these patients had a witnessed aspiration
event. The clinical impact of CASS seemed to differ for this patient
population compared with the patients examined by Mahul et
al13
and Valles et al.14
This may be related
to the differences in the patient populations examined, differences in
the technique of applying CASS, or use of other infection control
techniques at the different hospitals. Nevertheless, our study suggests
that 32 patients undergoing cardiac surgery require the application of
CASS to prevent one episode of VAP, as opposed to approximately 8
patients in the study by Valles et al.14
The prevention of VAP is important because of its impact on patient outcomes, associated costs, and need for additional antibiotic therapy, which may further predispose critically ill patients to super-infection with antibiotic-resistant pathogens.27 A variety of prevention measures are currently available.7 27 The benefits derived from implementing a VAP prevention program can be demonstrated in terms of both improved clinical outcomes and reduced medical care costs.28 29 30 31 Such efforts require the presence of a dedicated team approach to the prevention of nosocomial infections to be most successful. Additionally, avoiding or reducing the need for endotracheal intubation may be the most important prevention strategy for this specific nosocomial infection, as suggested by several recent investigations.32 33
In summary, we have demonstrated that the occurrence of VAP can be significantly delayed with the application of CASS among cardiac surgery patients developing this nosocomial infection. Future clinical trials are needed to confirm these results and to determine the overall impact of CASS as part of a more systematic approach to the prevention of VAP. Until such data become available, individual clinicians must decide how best to use their available resources to prevent the occurrence of VAP.
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Acknowledgements |
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Footnotes |
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Abbreviations: APACHE = acute physiology and chronic health evaluation; CASS = continuous aspiration of subglottic secretions; CI = confidence interval; CTICU = cardiothoracic ICU; FIO2 = fraction of inspired oxygen; OR = odds ratio; RR = relative risk; VAP = ventilator-associated pneumonia
Supported in part by grants from the Barnes-Jewish-Christian Innovations in Healthcare Program and a research grant from Mallinckrodt Medical, Inc.
Received for publication March 2, 1999. Accepted for publication May 18, 1999.
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