Particulate substrate retention in plug-flow and fully-mixed conditions during operation of aerobic granular sludge systems.

Particulate substrate (XB) is the major organic substrate fraction in most municipal wastewaters. However, the impact of XB on aerobic granular sludge (AGS) systems is not fully understood. This study evaluated the physical retention of XB in AGS sequencing batch reactor (SBR) during anaerobic plug-flow and then aerobic fully-mixed conditions. The influence of different sludge types and operational variables on the extent and mechanisms of XB retention in AGS SBR were evaluated. XB mass-balancing and magnetic resonance imaging (MRI) were applied. During the anaerobic plug-flow feeding, most XB was retained in the first few cm of the settled sludge bed within the interstitial voids, where XB settled and accumulated ultimately resulting in the formation of a filter-cake. Sedimentation and surface filtration were thus the dominant XB retention mechanisms during plug-flow conditions, indicating that contact and attachment of XB to the biomass was limited. XB retention was variable and influenced by the XB influent concentration, sludge bed composition and upflow feeding velocity (vww). XB retention increased with larger XB influent concentrations and lower vww, which demonstrated the importance of sedimentation on XB retention during plug-flow conditions. Hence, large fractions of influent XB likely re-suspended during aerobic fully-mixed conditions, where XB then preferentially and rapidly attached to the flocs. During fully-mixed conditions, increasing floc fractions, longer mixing times and larger XB concentrations increased XB retention. Elevated XB retention was observed after short mixing times < 60 min when flocs were present, and the contribution of flocs towards XB retention was even more pronounced for short mixing times < 5 min. Overall, our results suggest that flocs occupy an environmental niche that results from the availability of XB during aerobic fully-mixed conditions of AGS SBR. Therefore, a complete wash-out of flocs is not desirable in AGS systems treating municipal wastewater.

Organic substrate diffusibility governs microbial community composition, nutrient removal performance and kinetics of granulation of aerobic granular sludge.

Basic understanding of formation of aerobic granular sludge (AGS) has mainly been derived from lab-scale systems with simple influents containing only highly diffusible volatile fatty acids (VFA) as organic substrate. This study compares start-up of AGS systems fed by different synthetic and municipal wastewaters (WW), characterised by increasing complexity in terms of non-diffusible organic substrate. Four AGS reactors were started with the same inoculum activated sludge and operated for one year. The development of AGS, settling characteristics, nutrient and substrate removal performance as well as microbial community composition were monitored. Our results indicate that the higher the content of diffusible organic substrate in the WW, the faster the formation of AGS. The presence of non-diffusible organic substrate in the influent WW led to the formation of small granules and to the presence of 20-40% (% of total suspended solids) of flocs in the AGS. When AGS was fed with complex influent WW, the classical phosphorus and glycogen accumulating organisms (PAO, GAO) were outcompeted by their fermentative equivalents. Substrate and nutrient removal was observed in all reactors, despite the difference in physical and settling properties of the AGS, but the levels of P and N removal depended on the influent carbon composition. Mechanistically, our results indicate that increased levels of non-diffusible organic substrate in the influent lower the potential for microbial growth deep inside the granules. Additionally, non-diffusible organic substrates give a competitive advantage to the main opponents of AGS formation - ordinary heterotrophic organisms (OHO). Both of these mechanisms are suspected to limit AGS formation. The presented study has relevant implications for both practice and research. Start-up duration of AGS systems treating high complexity WW were one order of magnitude higher than a typical lab-scale system treating VFA-rich synthetic WW, and biomass as flocs persisted as a significant fraction. Finally, the complex synthetic influent WW - composed of VFA, soluble fermentable and particulate substrate - tested here seems to be a more adequate surrogate of real municipal WW for laboratory studies than 100%-VFA WW.

[A comparison of the effects of 50 % oxygen combined with CPAP to the non-ventilated lung vs. 100 % oxygen on oxygenation during one-lung ventilation]. / Oxygenierung während Ein-Lungen-Ventilation: Ein Vergleich von FiO.

OBJECTIVE: In a prospective, randomised, double-blind study the effects of FiO (2) of 0,5 with CPAP and a FiO (2) of 1.0 without CPAP on oxygenation and pulmonary shunt during one-lung ventilation (OLV) were examined. METHODS: In 20 patients undergoing thoracotomy (ASA II/III) two sequential ventilation methods were used during OLV: a) FiO (2) of 1.0 (OLV-100) and b) a FiO (2) of 0.5 in N (2)O combined with CPAP of 5 cm H (2)O to the non-ventilated lung (OLV-CPAP), whereby the sequence in 10 patients was OLV-CPAP followed by OLV-100; and the opposite in the remaining 10 patients. Operating conditions were graded by the surgeon. RESULTS: OLV-CPAP was associated with a better oxygenation and a lower shunt compared to OLV-100 (paO (2) : 198 +/- 40 mmHg vs 181 +/- 38 mmHg; p < 0.05 and Qs/Qt: 30 +/- 6 % vs. 34 +/- 8 %; p < 0.01, respectively). The different sequence of the ventilation methods did not cause significant differences in the results. The surgical conditions were not impaired by the CPAP of 5 cm H (2)O. CONCLUSION: The application of low FiO (2) with CPAP provided a better oxygenation and a lower pulmonary shunt during the OLV compared to high FiO (2) without CPAP.

Influence of airway-occluding instruments on airway pressure during jet ventilation for rigid bronchoscopy.

We measured changes in airway pressure (Paw) caused by microsurgical instruments introduced into a rigid bronchoscope during high frequency jet ventilation (HFJV). With approval of the institutional Ethics Committee, 10 adults undergoing elective tracheobronchial endoscopy and endosonography during general anaesthesia were investigated. Inflation of an endosonography probe balloon in the left main stem bronchus caused airway obstruction. Pressure measurements proximal and distal to the obstruction were compared after three degrees of obstruction (0%, 50% and 90%) and with two different driving pressure settings. Airway obstruction increased the mean (SD) peak inspiratory pressure (PIP) from 7.5 (2.6) to 9.5 (3.5) mm Hg for 2 atm (P = 0.0008) and from 9.7 (3.7) to 13.0 (5.1) mm Hg for 3 atm (P = 0.0001). Airway obstruction did not alter peripheral PIP (7.2 (4.1) to 7.1 (3.7) mm Hg for 2 atm and 8.8 (4.3) to 9.4 (5.2) mm for 3 atm), but resulted in an end-expiratory pressure (EEP) beyond the narrowing being significantly greater than in the unobstructed airway (2.5 (3.4) to 5.5 (3.7) mm Hg for 2 atm; P = 0.0005) and 3.2 (3.6) to 8.0 (4.3) mm for 3 atm; P < 0.0001). Severe airway narrowing increases inspiratory pressure proximal and expiratory pressure distal to the obstruction in relation to the applied driving pressure. Since the distal EEP never exceeded PIP, even near-total airway obstruction should not cause severe lung distension or barotrauma in subjects with normal lungs.

Carbon dioxide elimination during high-frequency jet ventilation for rigid bronchoscopy.

Oxygen saturation and carbon dioxide values should be monitored during high-frequency jet ventilation (HFJV). Modern transcutaneous PCO2 (PtcCO2) measurement allows the estimation of ventilation efficiency. We studied how tests of lung function could predict carbon dioxide elimination during HFJV. Lung function tests from 180 adult patients undergoing rigid bronchoscopy were analysed as factors affecting carbon dioxide elimination. The lung function test results showed a significant relationship with the efficiency of carbon dioxide elimination; the greatest impairment of carbon dioxide elimination was found in patients with combined abnormalities of lung function. Further factors associated with difficult carbon dioxide elimination were male gender and elevated body weight. Of the patients investigated, 72% had normal carbon dioxide elimination, whereas in 23% hypercapnia could be avoided only by increasing the driving pressure. The prevalence of abnormal preoperative lung function test results predicts (sensitivity 76%, positive predictive value 27%) impaired carbon dioxide elimination during jet ventilation and rigid bronchoscopy.

[High frequency jet ventilation during tracheal resection in children and infants]. / Hochfrequenz-Jet-Beatmung während Trachearesektion bei Kindern und Säuglingen.

Between 1986 and 1996, 16 infants and children less than 11 years of age (m = 11, f = 5) underwent resections for acquired or congenital tracheobronchial stenoses. During this period, various techniques of total intravenous anaesthesia (TIVA) were employed (midazolam, fentanyl, pancuronium; propofol, fentanyl, pancuronium). During the phase of dividing the airways, high-frequency-jet ventilation (HFJV) into the trachea or the main bronchi by 8-12Fr catheter(s) was applied for 10-75 min with driving pressures between 0.3-1.8 bar, frequencies between 100-200/min, I:E ratio between 1:4-1:1, and FjetO2 1.0. Catheter position was controlled visually, gas exchange was monitored by pulse oximetry and blood gas analysis. There were two incidents of transient hypoxaemia (paO2 less than 60 mmHg), and 4 cases of hypercapnia (paCO2 more than 45 mmHg). No complications due to the HFJV-catheter technique, such as barotrauma or aspiration were seen. All children were kept postoperatively on a ventilator due to swelling of the airway anastomosis. In 5 children ventilator treatment exceeded 7 days, 3 children were discharged tracheostomised. These observations serve to confirm that HFJV is capable of maintaining gas exchange during tracheal resection in infants and children, if the following prerequisites are met: 1. Tracheobronchial pathology suitable for poststenotic placement of jet catheter. 2. No respiratory impairment by parenchymal pathology. 3. Monitoring by continuous visual control of respiratory mechanics, pulse oximetry and blood gas analysis. Cardiopulmonary bypass should be applied if airway pathology precludes safe placement of jet catheters, or in the presence of parenchymal respiratory failure.

[Muscle relaxants for kidney transplantation. A comparison between vecuronium and atracurium]. / Muskelrelaxanzien zur Nierentransplantation. Ein Vergleich zwischen Vecuronium und Atracurium.

A total of 26 patients with end-stage renal failure were assigned randomly to receive either atracurium (A) or vecuronium (V) for intra-operative relaxation during kidney transplantation. Following the induction of balanced anesthesia an initial bolus dose of A 0,5 mg/kg body weight (BW) or V 0,1 mg/kg BW was administered. Relaxation was maintained by repeated doses of A 0,1 mg/kg BW or V 0,02 mg/kg BW whenever the twitch height (T1) recovered to 20% of the control value. Neuromuscular function was monitored throughout using the train-of-four twitch technique. There were no significant differences in time of onset (A: 219 +/- 87 s, V: 206 +/- 70 s), initial clinical duration (T1 = 0% to T1 = 20%; A: 39 +/- 12 min, V 39 +/- 8 min) or recovery index (A: 19 +/- 6 min, V: 22 +/- 7 min). The clinical duration (T1 = 20% to T1 = 20%) was significantly different for A, with 27 +/- 9 min than for V, with 32 +/- 7 min (P less than 0.05, Wilcoxon-Mann-Whitney U-Test; all values means +/- standard deviation). Reversal of relaxants was mandatory according to electromyographic and clinical criteria in only one patient in the A group but in 5 in the V group (P less than 0.05, Fisher's exact test). We conclude that A might be more suitable in patients with renal insufficiency and that monitoring of neuromuscular transmission is highly valuable in this risk group.

Clonidine as a sedative adjunct in intensive care.

A 63-year-old man underwent distal oesophagectomy and proximal gastrectomy. Postoperatively, controlled ventilation was necessary for 53 days because of anastomotic leakage. Multiple sedative regimens proved to be inadequate. By contrast, a fentanyl-midazolam combination with continuous supplementation of clonidine 0.014 micrograms kg-1 min-1 (1.44 mg 70 kg-1 24 h-1) was very effective in terms of sedation and pain relief. During combined fentanyl-midazolam and clonidine infusion, cardiovascular depression gradually developed over several days necessitating the institution of a dobutamine infusion (dose: 8-12 micrograms kg-1 min-1). Four attempts of abrupt clonidine withdrawal were followed by sympathetic overshoot reactions consisting of tachycardia, hypertension, agitation, and sweating. Discontinuation of clonidine was finally possible after a 12-day weaning period.