Perioperative intravenous contrast administration and the incidence of acute kidney injury after major gastrointestinal surgery: prospective, multicentre cohort study

BACKGROUND

This study aimed to determine the impact of preoperative exposure to intravenous contrast for CT and the risk of developing postoperative acute kidney injury (AKI) in patients undergoing major gastrointestinal surgery.

METHODS

This prospective, multicentre cohort study included adults undergoing gastrointestinal resection, stoma reversal or liver resection. Both elective and emergency procedures were included. Preoperative exposure to intravenous contrast was defined as exposure to contrast administered for the purposes of CT up to 7 days before surgery. The primary endpoint was the rate of AKI within 7 days. Propensity score-matched models were adjusted for patient, disease and operative variables. In a sensitivity analysis, a propensity score-matched model explored the association between preoperative exposure to contrast and AKI in the first 48 h after surgery.

RESULTS

A total of 5378 patients were included across 173 centres. Overall, 1249 patients (23·2 per cent) received intravenous contrast. The overall rate of AKI within 7 days of surgery was 13·4 per cent (718 of 5378). In the propensity score-matched model, preoperative exposure to contrast was not associated with AKI within 7 days (odds ratio (OR) 0·95, 95 per cent c.i. 0·73 to 1·21; P = 0·669). The sensitivity analysis showed no association between preoperative contrast administration and AKI within 48 h after operation (OR 1·09, 0·84 to 1·41; P = 0·498).

CONCLUSION

There was no association between preoperative intravenous contrast administered for CT up to 7 days before surgery and postoperative AKI. Risk of contrast-induced nephropathy should not be used as a reason to avoid contrast-enhanced CT.

Purinergic modulation of glutamate transmission: An expanding role in stress-linked neuropathology

Chronic stress has been extensively linked to disturbances in glutamatergic signalling. Emerging from this field of research is a considerable number of studies identifying the ability of purines at the pre-, post-, and peri-synaptic levels to tune glutamatergic neurotransmission. While the evidence describing purinergic control of glutamate has continued to grow, there has been relatively little attention given to how chronic stress modulates purinergic functions. The available research on this topic has demonstrated that chronic stress can not only disturb purinergic receptors involved in the regulation of glutamate neurotransmission, but also perturb glial-dependent purinergic signalling. This review will provide a detailed examining of the complex literature relating to glutamatergic-purinergic interactions with a focus on both neuronal and glial contributions. Once these detailed interactions have been described and contextualised, we will integrate recent findings from the field of stress research.

UX007 for the treatment of long chain-fatty acid oxidation disorders: Safety and efficacy in children and adults following 24weeks of treatment

BACKGROUND

Long-chain fatty acid oxidation disorders (LC-FAOD) lead to accumulation of high concentrations of potentially toxic fatty acid intermediates. Newborn screening and early intervention have reduced mortality, but most patients continue to experience frequent hospitalizations and significant morbidity despite treatment. The deficient energy state can cause serious liver, muscle, and heart disease, and may be associated with an increased risk of sudden death. Triheptanoin is a medium odd-chain fatty acid. Anaplerotic metabolites of triheptanoin have the potential to replace deficient tricarboxylic acid (TCA) cycle intermediates, resulting in net glucose production as a novel energy source for the treatment of LC-FAOD.

STUDY DESIGN

A single-arm, open-label, multicenter Phase 2 safety and efficacy study evaluated patients with severe LC-FAOD evidenced by ongoing related musculoskeletal, cardiac, and/or hepatic events despite treatment. After a four-week run-in on current regimen, investigational triheptanoin (UX007) was titrated to a target dose of 25-35% of total daily caloric intake. Patients were evaluated on several age/condition-eligible endpoints, including submaximal exercise tests to assess muscle function/endurance (12-minute walk test; 12MWT) and exercise tolerance (cycle ergometry), and health related quality of life (HR-QoL). Results through 24weeks of treatment are presented; total study duration is 78weeks.

RESULTS

Twenty-nine patients (0.8 to 58years) were enrolled; most qualified based on severe musculoskeletal disease. Twenty-five patients (86%) completed the 24-week treatment period. At Week 18, eligible patients (n=8) demonstrated a 28% increase (LS mean=+181.9 meters; p=0.087) from baseline (673.4meters) in 12MWT distance. At Week 24, eligible patients (n=7) showed a 60% increase in watts generated (LS mean=+409.3W; p=0.149) over baseline (744.6W) for the exercise tolerance test. Improvements in exercise tests were supported by significant improvements from baseline in the adult (n=5) self-reported SF-12v2 physical component summary score (LS mean=+8.9; p<0.001). No difference from baseline was seen in pediatric parent-reported (n=5) scores (SF-10) at Week 24. Eighteen patients (62%) had treatment-related adverse events, predominantly gastrointestinal (55%), mild-to-moderate in severity, similar to that seen with prior treatment with medium chain triglyceride (MCT) oil. One patient experienced a treatment-related serious adverse event of gastroenteritis. One patient discontinued from study due to diarrhea of moderate severity; the majority of patients (25/29; 86%) elected to continue treatment in the extension period.

CONCLUSIONS

In patients with severe LC-FAOD, UX007 interim study results demonstrated improved exercise endurance and tolerance, and were associated with positive changes in self-reported HR-QoL.

Astrocyte and microglial control of glutamatergic signalling: a primer on understanding the disruptive role of chronic stress

It is now well established that chronic stress can induce significant structural remodelling of astrocytes and microglia. Until recently, however, the full significance of these morphological disturbances has remained unclear. Clues to the significance of astroglial re-organisation following stress are beginning to emerge from a compelling literature describing how astrocytes contribute to glutamatergic neurotransmission. The present review briefly summarises these two fields of research, identifies points of overlap and, in doing so, pin-points future research directions for stress neurobiology. Ultimately, understanding how chronic stress can disrupt the interactions of astrocytes and microglia with neurones has the potential in the future to improve the development of therapeutics designed to treat stress-related illnesses such as depression.

Early detection of impending necrotizing enterocolitis with urinary intestinal fatty acid-binding protein

BACKGROUND

Necrotizing enterocolitis (NEC) is diagnosed after the development of feeding intolerance and characteristic physical and imaging findings. Earlier detection of a subclinical prodrome might allow for the institution of measures that could prevent or attenuate the severity of the disease.

OBJECTIVES

We sought to determine whether urinary intestinal fatty acid-binding protein (iFABPu) might be elevated prior to the first clinical manifestations of NEC.

METHODS

Urine was collected daily from 62 infants of a gestational age of 24-28 weeks. Based on clinical, imaging and operative findings, subjects were determined to have Bell stage 2 or 3 NEC. In all the subjects with NEC and in 21 age-matched controls, iFABPu was determined using an ELISA, and was expressed in terms of its ratio to urinary creatinine (Cr), i.e. iFABPu/Cru. Receiver operating characteristic (ROC) curves were constructed to define the predictive value of iFABPu/Cru for impending NEC in the days prior to the first clinical manifestations.

RESULTS

Five subjects developed NEC (stage 2: n = 3 and stage 3: n = 2). The day before the first clinical manifestation of NEC, a ROC curve showed that an iFABPu/Cru >10.2 pg/nmol predicted impending NEC with a sensitivity of 100% and a specificity of 95.6%. iFABPu/Cru did not predict NEC 2 days prior to the first sign of disease.

CONCLUSIONS

An elevated iFABPu was a sensitive and specific predictor of impending NEC 1 day prior to the first clinical manifestations. iFABPu screening might identify infants at a high risk and allow for the institution of measures that could ameliorate or prevent NEC.

Randomized, blinded trial of weekend vs daily prednisone in Duchenne muscular dystrophy

OBJECTIVE

To perform a double-blind, randomized study comparing efficacy and safety of daily and weekend prednisone in boys with Duchenne muscular dystrophy (DMD).

METHODS

A total of 64 boys with DMD who were between 4 and 10 years of age were randomized at 1 of 12 centers of the Cooperative International Neuromuscular Research Group. Efficacy and safety of 2 prednisone schedules (daily 0.75 mg/kg/day and weekend 10 mg/kg/wk) were evaluated over 12 months.

RESULTS

Equivalence was met for weekend and daily dosing of prednisone for the primary outcomes of quantitative muscle testing (QMT) arm score and QMT leg score. Secondary strength scores for QMT elbow flexors also showed equivalence between the 2 treatment groups. Overall side effect profiles of height and weight, bone density, cataract formation, blood pressure, and behavior, analyzed at 12 months, did not differ between weekend and daily dosing of prednisone.

CONCLUSIONS

Weekend dosing of prednisone is equally beneficial to the standard daily dosing of prednisone. Analysis of side effect profiles demonstrated overall tolerability of both dosing regimens.

CLASSIFICATION OF EVIDENCE

This study provides Class I evidence that weekend prednisone dosing is as safe and effective as daily prednisone in preserving muscle strength and preventing body mass index increases in boys with DMD over a 12-month period.

Increased Oxygen Consumption Following Activation of Brain: Theoretical Footnotes Using Spectroscopic Data from Barrel Cortex

Optical imaging spectroscopy (OIS) and laser Doppler flowmetry (LDF) data sequences from anesthetized rats were used to determine the relationship between changes in oxy-and deoxygenated hemoglobin concentration and changes in blood volume and flow in the presence and absence of stimulation. The data from Jones et al. (accompanying paper) were used to explore the differences between two theoretical models of flow activation coupling. The essential difference between the two models is the extension of the model of Buxton and Frank by Hyder et al. (1998, J. Appl. Physiol. 85: 554--564) to incorporate change in capillary diffusivity coupled to flow. In both models activation-increased flow changes increase oxygen transport from the capillary; however, in Hyder et al.'s model the diffusivity of the capillary itself is increased. Hyder et al. proposed a parameter (Omega), a scaling "constant" linking increased blood flow and oxygen "diffusivity" in the capillary bed. Thus, in Buxton and Frank's theory, Omega = 0; i.e., there are no changes in diffusivity. In Hyder et al.'s theory, 0 < Omega < 1, and changes in diffusivity are assumed to be linearly related to flow changes. We elaborate the theoretical position of both models to show that, in principle, the different predictions from the two theories can be evaluated using optical imaging spectroscopy data. We find that both theoretical positions have limitations when applied to data from brief stimulation and when applied to data from mild hypercapnia. In summary, the analysis showed that although Hyder et al.'s proposal that diffusivity increased during activation did occur; it was shown to arise from an implementation of Buxton and Frank's theory under episodes of brief stimulation. The results also showed that the scaling parameter Omega is not a constant as the Hyder et al. model entails but in fact varies over the time course of the flow changes. Data from experiments in which mild hypercapnia was administered also indicated changes in the diffusivity of the capillary bed, but in this case the changes were negative; i.e., oxygen transport from the capillary decreased relative to baseline under hypercapnia. Neither of the models could account for the differences between the hypercapnia and activation data when matched for equivalent flow changes. A modification to the models to allow non-null tissue oxygen concentrations that can be moderated by changes due to increased metabolic demand following increased neural activity is proposed. This modification would allow modulation of oxygen transport from the capillary bed (e.g., changes in diffusivity) by tissue oxygen tension and would allow a degree of decoupling of flow and oxygen delivery, which can encompass both the data from stimulation and from hypercapnia.

Concurrent Optical Imaging Spectroscopy and Laser-Doppler Flowmetry: The Relationship between Blood Flow, Oxygenation, and Volume in Rodent Barrel Cortex

Functional magnetic resonance imaging (fMRI) is based on the coupling between neural activity and changes in the concentration of the endogenous paramagnetic contrast agent deoxygenated hemoglobin. Changes in the blood oxygen level-dependent (BOLD) signal result from a complex interplay of blood volume, flow, and oxygen consumption. Optical imaging spectroscopy (OIS) has been used to measure changes in blood volume and saturation in response to increased neural activity, while laser Doppler Flowmetry (LDF) can be used to measure flow changes and is now commonplace in neurovascular research. Here, we use concurrent OIS and LDF to examine the hemodynamic response in rodent barrel cortex using electrical stimulation of the whisker pad at varying intensities. Spectroscopic analysis showed that stimulation produced a biphasic early increase in deoxygenated hemoglobin (Hbr), followed by a decrease below baseline, reaching minima at approximately 3.7 s. There was no evidence for a corresponding early decrease in oxygenated hemoglobin (HbO(2)), which simply increased after stimulation, reaching maximum at approximately 3.2 s. The time courses of changes in blood volume (CBV) and blood flow (CBF) were similar. Both increased within a second of stimulation onset and peaked at approximately 2.7 s, after which CBV returned to baseline at a slower rate than CBF. The changes in Hbr, Hbt, and CBF were used to estimate changes in oxygen consumption (CMRO(2)), which increased within a second of stimulation and peaked approximately 2.2 s after stimulus onset. Analysis of the relative magnitudes of CBV and CBF indicates that the fractional changes of CBV could be simply scaled to match those of CBF. We found the relationship to be well approximated by CBV = CBF(0.29). A similar relationship was found using the response to elevated fraction of inspired carbon dioxide (FICO(2)).

Signal source separation in the analysis of neural activity in brain

This research examines the spatial and temporal characteristics of the responses to stimulation of the barrel cortex in anesthetized rats using optical imaging with particular emphasis on methods of analysis which reduce the effects of low-frequency oscillations on localization of the activated cortical region. Image sequences were captured using a light source with a narrow bandwidth of wavelength (590 +/- 2 nm). On each trial image data were collected at 15 Hz and stored over a 12-s period starting 8 s before stimulation onset. Stimulation was for 1 s using an oscillating whisker vibrator (approximately 1-mm deflection at 5 Hz). For each subject a total of 30 experimental trials were collected and averaged. There was an interstimulus interval of 26 s. The trial-averaged data were analyzed using two related signal source separation algorithms. Both algorithms use a weak model of the expected temporal response as a filter to exclude contributions from low-frequency baseline oscillations which we call the V-signal. We found that both algorithms successfully separated most of the effects of the V-signal from the response to the stimulation. The performance of the algorithms compared favorably with the performance of related algorithms without weak constraints and the "ratio of means" strategy used by C. H. Chen-Bee et al. (1996b, J. Neurosci. Methods 68:28-37; C. H. Chen-Bee et al., 2000, J. Neurosci. Methods 97:157-173).

Clinical evaluator reliability for quantitative and manual muscle testing measures of strength in children

Measurements of muscle strength in clinical trials of Duchenne muscular dystrophy have relied heavily on manual muscle testing (MMT). The high level of intra- and interrater variability of MMT compromises clinical study results. We compared the reliability of 12 clinical evaluators in performing MMT and quantitative muscle testing (QMT) on 12 children with muscular dystrophy. QMT was reliable, with an interclass correlation coefficient (ICC) of >0.9 for biceps and grip strength, and >0.8 for quadriceps strength. Training of both subjects and evaluators was easily accomplished. MMT was not as reliable, and required repeated training of evaluators to bring all groups to an ICC >0.75 for shoulder abduction, elbow and hip flexion, knee extension, and ankle dorsiflexion. We conclude that QMT shows greater reliability and is easier to implement than MMT. Consequently, QMT will be a superior measure of strength for use in pediatric, neuromuscular, multicenter clinical trials.

Spectroscopic analysis of neural activity in brain: increased oxygen consumption following activation of barrel cortex

This research investigates the hemodynamic response to stimulation of the barrel cortex in anaesthetized rats using optical imaging and spectroscopy (Bonhoeffer and Grinvald, 1996; Malonek and Grinvald, 1996; Mayhew et al., 1999). A slit spectrograph was used to collect spectral image data sequences. These were analyzed using an algorithm that corrects for the wavelength dependency in the optical path lengths produced by the light scattering properties of tissue. The analysis produced the changes in the oxy- and deoxygenation of hemoglobin following stimulation. Two methods of stimulation were used. One method mechanically vibrated a single whisker, the other electrically stimulated the whisker pad. The electrical stimulation intensity varied from 0.4 to 1.6 mA. The hemodynamic responses to stimulation increased as a function of intensity. At 0.4 mA they were commensurate with those from the mechanical stimulation; however, the responses at the higher levels were greater by a factor of approximately 10. For both methods of data collection, the results of the spectroscopic analysis showed an early increase in deoxygenated hemoglobin (Hbr) with no evidence for a corresponding decrease in oxygenated hemoglobin (HbO(2)). Evidence for increased oxygen consumption (CMRO(2)) was obtained by converting the fractional changes in blood volume (Hbt) into estimates of changes in blood flow (Grubb et al., 1974) and using the resulting time course to scale the fractional changes in Hbr. The results show an early increase CMRO(2) peaking approximately 2 s after stimulation onset. Using these methods, we find evidence for increased oxygen consumption following increased neural activity even at low levels of stimulation intensity.

Pseudo-random procedures for rapid presentation rates using optical imaging and spectroscopy

Optical imaging of rat barrel cortex has provided detailed spatio-temporal maps of functional cortical architecture. We describe an event-related procedure (ERP) for optical imaging based on selective signal averaging as reported by Burock et al., using an anti-correlative pseudo-random event sequence. The sequence used 1 s vibrissal stimulation at 5 Hz, with an interevent interval of 2 s. This rapid presentation rate allows for greater statistical power per unit time, and allows for direct comparison of event-related studies with other imaging modalities. The spatio-temporal characteristics of single wavelength and spectrographic results were found to be comparable with those obtained by standard techniques, although a general lessening of haemodynamic response function (HRF) was noted. We also describe a method of locating barrel activity by spectral analysis of summed event data. Using this technique, the power spectrum of remitted light from the barrel region was found to peak within +/- 0.12 Hz of the inter-event interval frequency.

Spectroscopic analysis of changes in remitted illumination: the response to increased neural activity in brain

Imaging of neural activation has been used to produce maps of functional architecture and metabolic activity. There is some uncertainty associated with the sources underlying the intrinsic signals. It has been reported that following increased neural activity there was little increased oxygen consumption ( approximately 5%), although glucose consumption increased by approximately 50%. The research we describe uses a modification of the Beer-Lambert Law called path-length scaling analysis (PLSA) to analyze the spectra of the hemodynamic and metabolic responses to vibrissal stimulation in rat somatosensory cortex. The results of the PLSA algorithm were compared with those obtained using a linear spectrographic analysis method (we refer to this as LMCA). There are differences in the results of the analysis depending on which of the two algorithms (PLSA or LMCA) is used. Using the LMCA algorithm, we obtain results showing an increase in the volume of Hbr at approximately 2 s, following onset of stimulation but no complementary decrease in oxygenated haemoglobin (HbO(2)). These results are similar to a previous report. In contrast, after using the PLSA algorithm, the time series of the chromophore changes shows no evidence for an increase in the volume of deoxygenated haemoglobin (Hbr). However, after further analysis of the time series from the PLSA using general linear models (GLM) to remove contributions from low frequency baseline oscillations, both the HbO(2) and Hbr times series of the response to stimulation were found to be biphasic with an early decrease in saturation peaking approximately 1 s after onset of stimulation followed by a larger increase in saturation peaking at approximately 3 s. Finally, following the PLSA-then-GLM analysis procedure, we do not find convincing evidence for an increase in cytochrome oxidation following stimulation, though we demonstrate the PLSA algorithm to be capable of disassociating changes in cytochrome oxidation state from changes in hemoglobin oxygenation.

Experience with vascular control before excision of giant, highly vascular sacrococcygeal teratomas in neonates

Sacrococcygeal teratomas (SCT) are the most common neoplasms in newborns with a reported occurrence of 1 in 35,000 live births. Highly vascularized tumors in which the ratio of tumor weight to patient weight approaches 1:1 are frequently associated with hyperdynamic states, prenatal hydrops, placentamegaly, postnatal high-output cardiac failure, and carry a high perinatal mortality rate. Operative management of giant, highly-vascular sacrococcygeal teratomas in neonates can be complicated by life-threatening hemorrhage. Laparotomy, control of the aorta, and the arterial blood supply to these tumors before resection has been advocated as a safer alternative. The authors report their experience with three infants successfully treated using this approach. Hemodynamic stability was maintained during the operations. All tumors were resected successfully. The patients are alive without evidence of recurrence, 8.5 months to 18 months (average, 14.3 months) after the operation. Alpha-fetoprotein levels dropped to normal range after the removal of SCT and have remained normal in follow-up. Vascular control before excision of giant, highly-vascular SCT in neonates is safe, decreases intraoperative blood loss, and postoperative morbidity.

An evaluation of linear model analysis techniques for processing images of microcirculation activity

Sequences of images of the cortical surface can be processed to reveal information about the cortical microcirculation, regional cerebral blood flow (rCBF), and changes induced by neuronal activity. This study examined the use of different analysis methodologies on intrinsic optical images taken from rat sensory motor cortex and testes. Generalized linear model (GLM) analysis was used and compared with standard signal processing methods including principal component analysis. The GLM method has been used by Friston et al. (1994, Hum. Brain Map., 1: 214-220) in the analysis of functional magnetic resonance imagery to identify regions of focal activity. We investigated the use of this method to analyze video image data of the modulation of rCBF from rat cortex. The results revealed spatiotemporal variations in rCBF in response to stimulation within local regions of cortex. The advantage of the GLM method is that it augments ordinary signal processing methods with an estimate of statistical reliability. The use of different wavelengths of illumination reveals spatial structures with different temporal relationships. In image time series data collected under green and red illumination a phase difference was found in the low frequency approximately 0.1 Hz vasomotion oscillation. This phase difference occurred in data from both cortex and testes. A possible explanation of these differences is that the spectral absorption characteristics of the tissue reflect changes in the volume proportions of the different hemoglobin derivatives in interacting with the modulation of the volume of blood. It is suggested that the combination of these effects produces the phase differences we detect.

Alcohol risk assessment and intervention for family physicians. Project of the College of Family Physicians of Canada

At-risk and problem drinkers (excluding those with severe dependency) are estimated to be 20% of the Canadian population. With minimal training family physicians can effectively manage patients with alcohol problems. The Alcohol Risk Assessment and Intervention Project of the College of Family Physicians of Canada has developed materials and training for family physicians to use in helping their patients reduce the risks of alcohol-related harm.

The interpretation of stereo-disparity information: the computation of surface orientation and depth

Two methods for interpreting disparity information are described. Neither requires extraretinal information to scale for distance: one method uses horizontal disparities to solve for the viewing distance, the other uses the vertical disparities. Method 1 requires the assumption that the disparities derive from a locally planar surface. Then from the horizontal disparities measured at four retinal locations the viewing distance and the equation of local surface 'patch' can be obtained. Method 2 does not need this assumption. The vertical disparities are first used to obtain the values of the gaze and viewing distance. These are then used to interpret the horizontal disparity information. An algorithm implementing the methods has been tested and is found to be subject to a perceptual phenomenon known as the 'induced effect'.

Carrier detection in Duchenne muscular dystrophy

Serum creatine kinase, myoglobin, and percentage lymphocyte capping was determined in ten patients with Duchenne muscular dystrophy, 12 carriers (nine definite and three probable), 16 other female relatives, and eight normal controls. There was no detectable difference in lymphocyte capping ability between any of these clinical groups. Significant myoglobinaemia was present in all the affected males, but the difference in levels between carriers and controls suggested that this test has no advantage over creatine kinase estimations in carrier detection.

Argon detector: alternative detection system for gas-liquid chromatographic analysis of short-chain organic acids

Two detection systems for gas-liquid chromatography were compared for the identification of metabolic end products (short-chain organic acids) from anaerobic bacteria. Argon and flame ionization detectors were connected in series with inert argon as the carrier gas for analysis of 35 stock strains and 148 clinical isolates. There was an excellent correlation between the argon and flame ionization chromatograms, and both were readily comparable to the thermal conductivity detector tracings published in the Virginia Polytechnic Institute manual. The linear response of the argon detector was examined by analyzing twofold serial dilutions of the short-chain organic acids and comparing the results to the very linear flame ionization detector. The argon detector was found to react with sufficient linearity within the organic acid concentration range normally obtained from anaerobic bacterial broth cultures. The argon detector, therefore, appears to be a viable alternative to the flame ionization and thermal conductivity detector systems in the identification of anaerobic organisms.

Should fiberoptic bronchoscopy aspirates be cultured?

The reliability of fiberoptic bronchoscopy as a method to study the bacteriology of the lower respiratory tract was tested. The procedure used was suction aspiration through the inner channel after topical anesthesia with lidocaine. To detect contamination by oropharyngeal bacteria, the aspirates were cultured in patients with no evidence of active infection, comparison was made with results of transtracheal aspiration cultures, and the aspirate was tested for the presence of an oral dye marker. Results with all 3 methods of analysis indicated contamination with oropharyngeal bacteria that were presumably introduced during instrumentation through the upper airways. An additional factor studied was the effect of topical anesthetics. Analysis of aspirates showed that as much as 96 per cent of the specimen was anesthetic solution. Lidocaine also proved toxic to lower respiratory tract pathogens, although there were significant differences between bacterial species. It was concluded that fiberoptic bronchoscopy as performed in this study does not reliably reflect the bacteriology of the lower respiratory tract.