Use of Elastin Fibres Detected in Non-Directed Low Volume Bronchial Lavage in Ventilated ICU Patients

By Boots, R J; Phillips, G E; George, N; Faoagali, J L

SUMMARY

Elastin fibres in sputum have been described as a more sensitive marker of pulmonary necrosis than plain chest X-rays. This study aimed to determine the prevalence of elastin fibres using non- directed non-protected minibronchoalveolar lavage (BM-BAL) in mechanically ventilated patients in the intensive care unit. Patients admitted to the general intensive care unit of a tertiary referral hospital requiring more than 48 hours of mechanical ventilation had surveillance BM-BAL performed on admission and were then examined weekly using potassium hydroxide wet preparations for the presence of elastin fibres. All positive and a random selection of 16 negative preparations from patients with acute respiratory distress syndrome or pneumonia were fixed and examined using Weigert’s staining method for elastin. Of 412 patients enrolled, 130 (32%) had pneumonia on admission, 50 (12%) developed 58 episodes of ventilator-associated pneumonia and acute respiratory distress syndrome was diagnosed in 86 patients (21%). No chest X-ray showed cavitating infiltrates. Of 985 specimens examined, only seven had elastin fibres. Elastin fibres are uncommonly found using BM-BAL in general screening, acute respiratory distress syndrome or pneumonia in the intensive care unit, the incidence too low to be a useful indicator of pulmonary necrosis.

Key Words: elastin fibres, mechanical ventilation, pneumonia, broncho-alveolar lavage, cohort study

Pulmonary inflammatory disorders and infection causing lung injury are common in the intensive care unit (ICU). Methods to quantify the clinical severity of pulmonary injury include physiological disturbance using various oxygenation indices1, scoring systems for illness severity which are non-specific such as APACHE IP or disease specific such as communityacquired pneumonia (CAP)3. However, these have not been well correlated to pathological disease severity. Cavitation within pulmonary infiltrates represents necrosis of lung tissue and is associated with the finding of elastin fibres in sputum4. This study aimed to examine the elastin fibre prevalence in small volume, non-bronchoscopic, non-directed bronchoalveolar lavage (BM-BAL) fluid and define their relationship to acute respiratory distress syndrome (ARDS), CAP, ventilator- associated pneumonia (VAP) and patient outcome in ventilated patients in the ICU.

METHODS

All patients admitted to a 12-bed Adult General ICU in a tertiary referral hospital and potentially requiring mechanical ventilation via an endotracheal tube or tracheostomy for more than 48 hours were included. The ICU cared for both medical and surgical patients and is a regional centre for burns care. Neither cardiac surgical nor neurosurgical patients were managed in the General ICU.

Within 12 hours of admission, a BM-BAL was performed and then weekly thereafter with clinical staff and investigators blinded to the results. Patients had their BM-BAL deferred if they required more than 70% oxygen, were on more than 10 cm H2O of PEEP or clinically had severe bronchospasm. Patients were pre-oxygenated and additional sedation was administered as required based upon the opinion of the bedside nurse. A sterile suction catheter (Argyle catheter, Sherwood Medical, Tullamore, Ireland) was inserted, using sterile gloved hands, through a bronchoscopy adaptor (Bodaii suctionsafe(TM) Sonrek Medical Inc, Hingham MA, U.S.A.) into the endotracheal or tracheostomy tube until it wedged in the lower airway. Sterile non-pyrogenic saline 20-30 ml was instilled as required for a return volume of more than 3 ml. A suction specimen was collected into a sterile specimen collection jar (40cc Specimen Trap Sherwood Medical, St Louis MO, U.S.A.). All patients were monitored with continuous electrocardiograph, blood pressure and oxygen saturations.

BM-BAL specimens were processed for the presence of elastin fibres as described by Shales4. A 0.25 ml aliquot was cytospun at 8000g for 10 minutes. A drop of 40% potassium hydroxide was added to the preparation and a cover-slip positioned over the sample. Refractile fibres with split ends were screened for using standard light microscopy with a magnification of 100 x and confirmation was made at 400x. Clumps of fibres were considered positive. Airdried cytospin preparations were stored. All positive wet preparation samples and a random selection of 16 samples from patients with ARDS or pneumonia were stained for elastin using Weigert’s method5. The slides were examined by a cytologist experienced in finding elastin fibres in BM-BALs.

For admission pneumonia, final clinician diagnosis was used. VAP was diagnosed in the setting of a clinical pneumonia syndrome and a Clinical Pulmonary Infection Score (CPIS) >66. Chest X-ray infiltrates were quantitatively scored by a thoracic physician following training to an intra-rater reliability of Kappa >0.8 on four consecutive occasions using the modified method of McLeod7,8. Specific record was made of cavitation. Patients were reviewed daily until 48 hours post weaning from mechanical ventilation. All patients diagnosed with pneumonia needed a clinical course consistent with this diagnosis.

Demographic data collected included patient age, gender, admission diagnosis, principal specialty, APACHE II Score2, organ system failures’, chronic obstructive pulmonary disease10, antibiotics on admission and during admission, smoking history (presently smoking, ceased <3 months, ceased >3 months, pack years smoking history), ARDS”, patient mortality, intensive care length of stay, hospital length of stay and ventilation days.

Univariate analysis used paired Student’s t-test and Kruskal- Wallis tests for continuous variables and chisquare and Fisher’s exact tests for categorical variables where appropriate. Analysis used the statistical package Stata 7 (Stata Corporation. College Station, Texas, U.S.A.). Assuming that elastin fibres would be found in 50% of admissions with severe pulmonary injury and only 10% of other patients, 19 cases of severe pulmonary inflammation would be required12. Assuming a VAP rate of 10%, 190 patients needed to be recruited.

The institutional ethics committee approved the study protocol and deferred consent obtained from the patient or next of kin.

RESULTS

Over two years, 412 patients were enrolled. The demographic characteristics of the study group are summarised in Table 1. There were 130 patients (32%) with pneumonia on admission and 50 patients (12%) who developed 58 episodes of VAP. No radiological evidence of cavitation was seen on the chest X-rays. ARDS was diagnosed in 86 patients (21%).

Of 985 BM-BALs, only seven had elastin fibres (Table 2). All elastin fibres found in the potassium hydroxide preparation were also seen in the elastin stains. Of the volumes instilled, the average return was 53%. Four percent of specimens were described as pauci-cellular, suggesting that the catheter was not adequately wedged as none of the pauci-cellular specimens came from neutropenic patients. Eightyfour percent of BM-BALs had macrophages present in the specimens with 18% and 62% contaminated with squamous and bronchial epithelial cells respectively.

The characteristics of the patients with elastin fibres are summarised in Table 3. For the only patient with elastin fibres and CAP, the causative organism was not determined. Of the two patients with chronic obstructive airways disease who had elastin fibres isolated, one also developed VAP. Of the three ARDS patients with elastin fibres, one also had VAP. The median time from a diagnosis of pneumonia or ARDS to the isolation of elastin fibres was six days (range 1-19) with three isolates on the first day of ICU presentation (two chronic obstructive airways and one CAP) and between 6-19 days from the diagnosis of VAP or ARDS. The pathogens found in specimens isolating elastin fibres were MRSA, Staphylococcus aureus and Pseudomonas aeruginosa. The isolates of elastin fibres were too infrequent from the BM-BAL specimens for any significant relationships to be found. Of interest, all patients isolating elastin fibres had respiratory failure, the PEEP used was higher, the P^sub a^O^sub 2^/FiO^sub 2^ ratio lower and they had a greater density of infiltrates on their chest X-rays.

DISCUSSION

This study found that the prevalence of elastin fibres using BM- BAL was extremely low in patients with pulmonary injury in ICU and of limited clinical utility in the diagnosis of lung necrosis. The finding of elastin fibres in sputum of patients with necrotising pulmonary processes was initially described by Schroeder van der Kols13. In the absence of chest X-rays, this was a useful method to determine cavitation in tuberculosis. With the advent of improved microbiologic culture techniques and greater availability of chest X- rays, the need for the technique declined. However, cavitation is difficult to appreciate on the portable chest X-rays performed in the ICU and a significant amount of lung volume is not readily apparent on such films. As such, elastin fibres as a marker of necrosis of intra-alveolar septa, blood vessels and respiratory bronchioles resulting from pulmonary infection or inflammation might allow the diagnosis of lung necrosis in the absence of CT scans of the chest or overt cavitation.

Our study found a prevalence of 2% in venti\lated ICU patients, being uncommon in both ARDS and pneumonia patients. This differs from the findings of Shlaes where elastin fibre prevalence was 9% for non-cavitating pneumonias and 85% of cavitating pneumonia4,14. Shlaes’ study included a similar number of patients to our study recruiting 80 hospital patients with pneumonia diagnosed using clinical criteria. Recruitment, unlike in our study, was not prospective and used presumptive pneumonia diagnostic criteria rather than an objective score. ARDS was not specifically mentioned. As such, more severely ill patients or diagnoses other than pneumonia may have been included. The lack of computerised tomograms of the chest in the present study and Shlaes’ potentially underestimates the frequency of pulmonary cavitation.

Shlaes found enteric gram-negative bacilli, Staphylococcus aureus, tuberculosis and aspergillus were associated with sputum elastin fibres4. These organisms are common ICU isolates. No elastin fibres were found in pneumococcal or other communityacquired pneumonias. A prevalence of 16% cavitation seems high in an unselected hospital population with pneumonia. The severity of the pneumonia was not described.

El-Ebiary’s study found that elastin fibres had a sensitivity of 32% for all pneumonias rising to 43% for gram negatives and 44% for Pseudomonas aeruginosa in ventilated ICU patients15. The specificities were 72%, 85% and 81% respectively. Pneumonia was diagnosed on clinical criteria. For YAP, elastin fibres had a sensitivity of 52% and 100% specificity for radiological cavitation preceding radiological evidence by 1.8 1.3 days16.

We did not confirm the high incidence of elastin fibres in airway secretions when collected using the BM-BAL technique. This may be due to our weekly sampling. However, specimens were collected on admission to the ICU or within seven days of the diagnosis of pneumonia. Shedding of elastin fibres would be expected to continue for some time in the setting of necrosis as reflected by the mean time to finding elastin fibres in ARDS and VAP being six days. Our sampling intervals were similar to the regular sampling performed in other studies15. The BM-BAL technique may be insensitive for elastin fibre detection. Blinded BM-BALs have previously been described as adequate for the diagnosis of VAP”. However, the low volume of fluid in the lavage reflects airway secretion sampling due to the lack of a discard volume and BM-BAL allows the opportunity to collect specimens when airway secretions are minimal. The specimen was also concentrated by the cytospin method following the aliquot taken from a thoroughly mixed specimen. It would be expected that tracheal aspirates would similarly potentially dilute the specimen as elastin is derived from the alveolar tissue. During our study, there were several episodes of severe pneumonia and ARDS where it would be expected that elastin fibres should be present. Most of the BM-BAL samples had alveolar macrophages present suggesting that the specimen was adequate. No direct comparison of tracheal aspirates and BM-BAL was performed for the presence of elastin fibres. The methodology to find elastin fibres in the BM-BALs was considered robust as it conformed to previously described methods and all positive KOH preparations were also positive on using elastin stains on fixed specimens. An experienced cytopathologist reviewed all slides. No elastin fibres were found in patients without pulmonary injury of some form either in KOH preparations or the randomly selected severe pulmonary injury patients using elastin stains of fixed specimens.

Previous studies used a clinical method of pneumonia diagnosis and as such we used the CPIS for consistency rather than using quantitative culture techniques. However, despite pneumonia diagnosis similarities, the use of a validated score added greater rigour to the diagnosis.

The time period over which the ICU studies find elastin fibres associated with cavitating pneumonia is not specified and so estimating the incidence of necrotising pulmonary disease is not possible. The lack of finding elastin fibres on repeated specimens is intriguing. This may represent a periodicity of finding of elastin fibres related to pulmonary necrosis or a narrow time period for their presence.

Although elastin fibres were found and confirmed in elastin stains, their incidence in our population was too small to be of use as a prognostic indicator. BM-BALs have not previously been used to detect elastin fibres. No cases of pulmonary cavitation were seen on the chest X-rays. Although the lung injury score18 was not used as lung compliance was not formally measured, the lung injury of some patients was severe and as such elastin fibres would have been expected. Ventilation patterns however have changed to strategies of lower tidal volume and limited shear injury potentially decreasing pulmonary injury and thus the chances of finding elastin fibres.

CONCLUSIONS

In our prospective study, the prevalence of elastin fibres and therefore necrosis in ventilated patients is low using BM-BALs and of no value in the routine evaluation of lung injury. CT due to its more complete assessment of anatomical location and cavitation size is more likely to be of clinical utility than subclinical lung necrosis. Further comparisons with tracheal aspirate preparations and CT of the chest is required to better determine the method’s sensitivity, specificity and relevance.

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R. J. BOOTS*, G. E. PHILLIPS[dagger], N. GEORGE[double dagger], J. L. FAOAGALI

Departments of Intensive Care Medicine, Pathology and Microbiology, Royal Brisbane and Women’s Hospital, Brisbane, Queensland, Australia

* M.B., B.S., Ph.D., M.Med.Sci., M.H.A.I.S., F.J.F.I.C.M., F.R.A.C.P., Associate Professor and Deputy Director, Department of Intensive Care Medicine, Royal Brisbane and Women’s Hospital; Burns, Trauma and Critical Care Research Centre, Royal Brisbane and Women’s Hospital and The University of Queensland.

[dagger] M.B., B.S., F.R.C.P.A., Senior Staff Specialist Anatomical Pathologist, Department of Pathology.

[double dagger] M.Sc., Senior Scientist, Department of Microbiology.

M.B., Ch.B., F.R.C.P.A., Professor and Director of Microbiology.

Reprints will not be available from the authors.

Accepted for publication on November 8, 2006.

Copyright Australian Society of Anaesthetists Apr 2007

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