Fuzzy logic control of inspired isoflurane and oxygen concentrations using minimal flow anaesthesia

In order to evaluate the performance of feedback fuzzy logic control of inspired oxygen and isoflurane concentrations, we studied 30 patients undergoing discectomy for lumbar (n = 26) or cervical (n = 4) disc herniation. Patients were allocated random to one of two groups: a standard group (n = 15) with low flow anaesthesia (1.2-1.3 litre min-1) and manual control of gas concentrations; and a fuzzy group (n = 15) with minimal flow (0.5 litre min-1) and fuzzy logic feedback control of gas concentrations. Fuzzy logic control achieved and maintained very accurately the desired isoflurane concentration. Oxygen concentration was controlled more precisely than in the standard group. Delivery and costs of oxygen and nitrous oxide were significantly lower in the fuzzy group (P < 0.01). Accumulation of foreign gases was observed in one patient during low flow and in 11 patients during minimal flow anaesthesia. In conclusion, fuzzy logic control of inspired oxygen and isoflurane concentration during minimal flow anaesthesia was reliable and reduced anaesthetic gas delivery and costs.      

Arterial pressure control with isoflurane using fuzzy logic

Arterial pressure is still one of the most important measuresin estimating the required dose of inhaled anaesthetics. Itis measured easily and reacts rapidly which makes it suitableas a variable for feedback control of depth of anaesthesia.Fuzzy logic, a novel approach to feedback control, was usedto control arterial pressure in 10 patients during intra-abdominalsurgery by automatic adjustment of the concentration of isofluranein fresh gas. During anaesthesia, fuzzy control periods of 45-minduration were alternated randomly with human control periodsof equal duration. During the skin incision period (–3to +12min) 48.2% of all fuzzy control pressure values were within±10% of the desired mean arterial pressure compared with40.4% of the human control values (P < 0.05). The correspondingvalues for the remainder of the operation were 78.3% and 83.2%,respectively. Thus fuzzy outperformed human control at skinincision, but was slightly inferior during the rest of the operation.We conclude that fuzzy logic is a promising new technique forcontrol of isoflurane delivery during routine anaesthesia.     

CONTROLLED ANAESTHESIA: A CLINICAL EVALUATION OF AN APPROACH USING PATIENT CHARACTERISTICS IDENTIFIED DURING UPTAKE

The performance of a system to control the alveolar concentrationof halothane in patients undergoing halothane and nitrous oxideor halothane anaesthesia with controlled ventilation has beenevaluated. The method involved the identification and quantificationof the uptake characteristics of patients from their early responseto the anaesthetic and implements the vaporizer control necessaryto achieve and maintain a desired alveolar halothane concentration.Initial targets are based on the concept of MAC, but modificationsto the desired alveolar concentration may be effected readilyby the anaesthetist at any time during the procedure if evaluationof the normal clinical signs indicates inappropriate depth ofanaesthesia. The results obtained during anaesthesia for routinesurgery in 80 patients demonstrated that the system was accurate,stable, robust and able to adapt for variability between patientsin the uptake of halothane.     

Biologically variable ventilation prevents deterioration of gas exchange during prolonged anaesthesia

We have studied the time course of changes in gas exchange and respiratory mechanics using two different modes of ventilation during 7 h of isoflurane anaesthesia in pigs. One group received conventional control mode ventilation (CV). The other group received biologically variable ventilation (BVV) which simulates the breath-to-breath variation in ventilatory frequency (f) that characterizes normal spontaneous ventilation. After baseline measurements with CV, animals were allocated randomly to either CV or BVV (FIO2 1.0 with 1.5% end- tidal isoflurane). With BVV, there were 376 changes in f and tidal volume (VT) over 25.1 min. Ventilation was continued over the next 7 h and blood gases and respiratory mechanics were measured every 60 min. The modulation file used to control the ventilator for BVV used an inverse power law frequency distribution (I/fa with a = 2.3 +/- 0.3). After 7 h, at a similar delivered minute ventilation, significantly greater PaO2 (mean 72.3 (SD 4.0) vs 63.5 (6.5) kPa) and respiratory system compliance (1.08 (0.08) vs 0.92 (0.16) ml cm H2O-1 kg-1) and lower PaCO2 (6.5 (0.7) vs 8.7 (1.5) kPa) and shunt fraction (7.2 (2.7)% vs 12.3 (6.2)%) were seen with BVV, with no significant difference in peak airway pressure (16.3 (1.2) vs 15.3 (3.7) cm H2O). A deterioration in gas exchange and respiratory mechanics was seen with conventional control mode ventilation but not with BVV in this experimental model of prolonged anaesthesia.      

Effets ventilatoires d’une hypothermic modérée (36° C–28° C) chez le chien anesthésie à l’halothane

Changes in systemic haemodynamic variables (mean arterial pressure, MAP; heart rate, HR; cardiac output, Qc), in oxygen consumption, VO2, and in ventilation (minute ventilation, V; respiratory frequency, f; tidal volume, VT; and arterial blood gases) with particular attention to respiratory times (duration of inspiration, TI; duration of expiration, TE; duration of the breathing cycle, TTOT), to respiratory timing (TI/TTOT) and respiratory drive (VT/TI) were studied during moderate progressive hypothermia (36 degrees C to 28 degrees C) during stable halothane anaesthesia (MAC = 1.5) in six dogs. MAP, HR and Qc decreased; V and f decreased, the decrease in f being correlated with that in temperature (r = 0.66; P < 0.01). Tidal volume did not change. The PaO2 and pHa decreased while PaCO2 increased slightly. The decrease in ventilation was related to changes in respiratory times (TI and TE) which increased (TE more than TI) and in respiratory drive (VT/TI which decreased due to the increase in TI). The relation between VT/TI and TI/TTOT changes was not constant during cooling. Changes in respiratory times and drive could be due to the effect of cold on medullar respiratory control.     

Pressions exercées sur la trachée par les ballonnets des sondes d'intubation en présence de protoxyde d'azote

ObjectiveTo compare the time course of endotracheal tube cuff pressures in presence of nitrous oxide (N₂O), obtained in a tracheal model with those measured during clinical anaesthesia.Study designExperimental and clinical prospective study.MaterialTwelve brands of low-pressure tracheal tubes.MethodsThe pressure changes in the cuffs were measured over a three-hour-period in presence of a N₂O (50 vol%)/O₂ (50 vol%) mixture and mechanical ventilation, the tube being inserted either in a tracheal model or in the trachea of patients during general anaesthesia.ResultsThe results obtained in vitro were correlated with those measured in the patients. Therefore the tracheal model is a helpful guide for the choice of endotracheal tubes.     

Closed-loop control of propofol anaesthesia

We describe the use of a closed-loop system to control depth of propofol anaesthesia automatically. We used the auditory evoked potential index (AEPindex) as the input signal of this system to validate it as a true measure of depth of anaesthesia. Auditory evoked potentials were acquired and processed in real time to provide the AEPindex. The AEPindex was used in a proportional integral (PI) controller to determine the target blood concentration of propofol required to induce and maintain general anaesthesia automatically. We studied 100 spontaneously breathing patients. The mean AEPindex before induction of anaesthesia was 73.5 (SD 17.6), during surgical anaesthesia 37.8 (4.5) and at recovery of consciousness 89.7 (17.9). Twenty-two patients required assisted ventilation before incision. After incision, ventilation was assisted in four of these 22 patients for more than 5 min. There was no incidence of intraoperative awareness and all patients were prepared to have the same anaesthetic in future. Movement interfering with surgery was minimal. Cardiovascular stability and overall control of anaesthesia were satisfactory.      

Early treatment with arteriovenous extracorporeal lung assist and high-frequency oscillatory ventilation in a case of severe acute respiratory distress syndrome

BACKGROUND: Lung protective ventilation can reduce mortality in acute respiratory distress syndrome (ARDS). However, many patients with severe ARDS remain hypoxemic and more aggressive ventilation is necessary to maintain sufficient gas exchange. Pumpless arteriovenous extracorporeal lung assist (av-ECLA) has been shown to remove up to 95% of the systemic CO(2) production, thereby allowing ventilator settings and modes prioritizing oxygenation and lung protection. High-frequency oscillatory ventilation (HFOV) is an alternative form of ventilation that may improve oxygenation while limiting the risk of further lung injury by using extremely small tidal volumes (VT). METHODS: We discuss the management of a patient suffering from severe ARDS as a result of severe bilateral lung contusions and pulmonary aspiration. RESULTS: Severe ARDS developed within 4 h after intensive care unit admission. Conventional mechanical ventilation (CV) with high-airway pressures and low VT failed to improve gas exchange. Av-ECLA was initiated to achieve a less aggressive ventilation strategy. VT was reduced to 2-3 ml/kg, but oxygenation did not improve and airway pressures remained high. HFOV (8-10 Hz) was started using a recruitment strategy and oxygenation improved within 2 h. After 5 days, the patient was switched back to CV uneventfully and av-ECLA was removed after 8 days. CONCLUSION: The combination of two innovative treatment modalities resulted in rapid stabilization and improvement of gas exchange during severe ARDS refractory to conventional lung protective ventilation. During av-ECLA, extremely high oscillatory frequencies were used minimizing the risk of baro- and volutrauma.     

三因素三水平呼吸参数在腹腔镜手术全麻中的选用

目的探讨腹腔镜手术CO2气腹(高腹压12mmHg)状态下全麻控制呼吸中,较低气道压力同时又能维持良好通气的呼吸参数的最佳组合。方法取麻醉机上最基本的呼吸频率(f)、潮气量(VT)、吸呼比(I∶E)三参数作为三因素A、B、C,f=15、12、9(次/min),VT=8、10、12(ml/kg),I∶E=1∶2.5、1∶2.0、1∶1.5为相应的三水平;以L9(34)K=3重复正交实验设计及分析用表统计、检验、分析各因素及水平变化影响气道峰压(peak inspiratory pressure,PIP)、气道平均压(Pmean)、呼气末二氧化碳分压(PETCO2)的统计学意义。结果9组试验27例腹腔镜手术全麻中,生命体征平稳,SpO2100%。PEEP监测值整个手术麻醉过程中始终为1hPa。三因素三水平呼吸参数的不同组合:①潮气量(B因素三水平)的不同对PIP的影响无统计学意义(P=0.074);②呼吸频率(A因素三水平)的改变对PETCO2有显著影响(P=0.002);③吸呼比(C因素三水平)的不同对气道平均压(Pmean)有较显著的统计学的意义(P=0.017)。结论多呼吸参数的最佳组合方式是A2B2C2:f...     

Ventilation with negative airway pressure induces a cytokine response in isolated mouse lung

We tested the hypothesis that, under relatively low tidal volume (VT) mechanical ventilation, continuing lung decruitment induced by negative end-expiratory pressure (NEEP) would increase the lung cytokine response, potentially contributing to lung injury. Mouse lungs were excised and randomly assigned to one of 3 different ventilatory strategies: 1) the zero end-expiratory pressure group served as a control, 2) the NEEP7 group received a NEEP of -7.5 cm H(2)O, and 3) the NEEP15 group received a NEEP of -15 cm H(2)O. In all 3 groups, a VT of 7 mL/kg was used. After 2 h of ventilation, lung lavage fluid was collected for measurements of tumor necrosis factor-alpha, monocyte chemoattractant protein-1, and lactate dehydrogenase. Increases in plateau pressure before and after mechanical ventilation were significantly greater in the NEEP15 group compared with the zero end-expiratory pressure group or NEEP7 group. Lung compliance was decreased in the NEEP15 compared with the other two groups. Concentrations of tumor necrosis factor-alpha, monocyte chemoattractant protein-1, and lactate dehydrogenase in lung lavage were larger in the NEEP15 group than in the other groups. Atelectatic lung during repeated collapse and reopening of lung units accentuates the lung cytokine response that may contribute to lung injury even during relatively low VT mechanical ventilation. IMPLICATIONS: Repeated closing and reopening of lung units induced by negative end-expiratory pressure resulted in lung inflammation and cell injury even under mechanical ventilation using a normal tidal volume. This finding may have clinical relevance in certain patients who are prone to atelectasis during mechanical ventilation.     

Diaphragmatic movement using ultrasound during spontaneous and mechanical ventilation: effect of tidal volume.

Using ultrasound (US) the effect of various tidal volumes on the movement of ventral, dome and dorsal parts of the right hemidiaphragm was studied, both during spontaneous and mechanical ventilation. Six healthy non-medicated volunteers who were in the supine position breathed spontaneously shallowly (tidal volume (VT) being 400 ml) (SB), and deeply (VT 1000 ml) (SB-deep). In addition, they were mechanically ventilated with intermittent positive pressure ventilation at three different VT's: 500 ml (IPPV-500), 1000 ml (IPPV-1000) and 1700 ml (IPPV-1700). The maximal movement was recorded in the ventral part in 2 volunteers during SB, in 3 during SB-deep, and in 3 and 5 subjects during IPPV-500 and IPPV-1700, respectively. The movement in dome was 100% during SB (all others standardized to this), 303 +/- 107% during SB-deep, 82 +/- 30% during IPPV-500, 165 +/- 70% during IPPV-1000 and 266 +/- 153% during IPPV-1700. An increased tidal volume is associated with an increase in the diaphragmatic movement studied by US. However, a larger VT is needed during mechanical ventilation to achieve the same amount of change as occurred with deep spontaneous breathing.     

Respiratory effects of nitrous oxide during halothane or enflurane anaesthesia in children

The respiratory effects of nitrous oxide (N2O) were studied during halothane and enflurane anaesthesia in 12 children (mean age 46.4 +/- 29.3 months, mean weight 15.3 +/- 4.2 kg) during surgery under continuous extradural anaesthesia. Four equipotent anaesthetic states were studied in random order: 1) halothane 1 MAC in oxygen, 2) halothane 0.5 MAC + 50% N2O, 3) enflurane 1 MAC in oxygen, 4) enflurane 0.5 MAC +50% N2O. End-tidal fractions of CO2 (PetCO2) and halothane and enflurane were measured using infrared analysers. The respiratory variables (tidal volume VT, minute ventilation VE, respiratory frequency F, inspiratory time Ti, mean inspiratory flow VI, effective inspiratory time Ti/Ttot) were measured using a pneumotachograph. Significant changes were observed between the four states for VE, VI, F and PetCO2, whereas the values of VT, Ti and Ti/Tot did not differ significantly. The respiratory depressant effect of 1 MAC of either halothane alone or of the mixture of halothane and N2O was very similar. During enflurane anaesthesia, PetCO2 was less increased when N2O was substituted for enflurane, owing to a significant increase in respiratory frequency. A marked decrease in VE together with an increase in PetCO2 was observed during enflurane anaesthesia (states 3 and 4) when compared to the corresponding states during halothane anaesthesia (states 1 and 2). The respiratory depressant effect of enflurane is greater than that of halothane in unpremedicated children, even when substituting N2O for an equal MAC fraction of enflurane.2+ The effect of N2O on respiratory patterns seems to depend on the inhalational agent used and/or on the vesting respiratory frequency.     

Closed-loop feedback control of methohexital anesthesia by quantitative EEG analysis in humans

A combined pharmacokinetic and pharmacodynamic model of methohexital was used to establish and evaluate feedback control of methohexital anesthesia in 13 volunteers. The median frequency of the EEG power spectrum served as the pharmacodynamic variable constituting feedback. Median frequency values from 2-3 Hz were chosen as the desired EEG level (set-point). In 11 volunteers, the feedback system succeeded in maintaining a satisfactory depth of anesthesia (i.e., unresponsiveness to verbal commands and tactile stimuli). During feedback control, 75% of all measured median frequency values were in the preset range of 2-3 Hz. This distribution of median frequency was obtained by applying random stimulation (six different acoustic and tactile stimuli) to the volunteers approximately every 1.5 min. The decrease of median frequency from baseline to anesthetic values was primarily induced by increasing the fractional power in the frequency band of 0.5-2 Hz from 12.6 +/- 4.5% (mean +/- SD) to 46.0 +/- 2.5%. The median time to recovery (as defined by opening eyes on command) after cessation of the feedback control period was 20.6 min (10.7-44.5 min) when median EEG frequency was 5.2 Hz (4.7-8.4 Hz). The average requirement of methohexital (mean +/- SD) during the 2 h was 1.02 +/- 0.16 g. It is concluded that pharmacokinetic-pharmacodynamic models of intravenous anesthetics established previously may be used to form a suitable background for model-based feedback control of anesthesia by quantitative EEG analysis. This approach gives a possible solution to the problem of adapting pharmacokinetic and pharmacodynamic data to individuals when using population mean data as starting values for drug therapy.     

Arterial to end-tidal carbon dioxide tension difference in anaesthetized adults mechanically ventilated via a laryngeal mask or a cuffed oropharyngeal airway.

To evaluate arterial (PaCO2), end-tidal (PETCO2) and carbon dioxide tension difference during mechanical ventilation with extratracheal airways, 60 patients ASA physical status I-II, receiving general anaesthesia for minor extra-abdominal procedures were randomly allocated to receive either a cuffed oropharyngeal airway (group COPA, n = 30) or a laryngeal mask (group LMA, n = 30). The lungs were mechanically ventilated by IPPV using a 60% nitrous oxide and 1-1.5% isoflurane in oxygen mixture (VT = 8 mL kg-1; RR = 12 b min-1; l/E = 1/2). After PETCO2 had been stable for at least 10 min after airway placement, haemodynamic variables and PETCO2 were recorded and an arterial blood sample was obtained for measurement of PaCO2. No differences in anthropometric parameters, smoking habit, haemodynamic variables and incidence of untoward events were observed between the two groups. Airway manipulation, to maintain adequate ventilation, was required in only nine patients in the cuffed oropharyngeal airway group (30%) (P < 0.0005); however, in no case was it necessary to remove the designated extratracheal airway due to unsuccessful mechanical ventilation. The mean difference between arterial and end-tidal carbon dioxide partial pressure was 0.4 +/- 0.3 KPa in the laryngeal mask group (95% confidence intervals: 0.3-0.5 KPa) and 0.3 +/- 0.26 KPa in the cuffed oropharyngeal airway group (95% confidence intervals: 0.24-0.4 KPa) (P = NS). We conclude that in healthy adults who are mechanically ventilated via the cuffed oropharyngeal airway, the end-tidal carbon dioxide determination is as accurate an indicator of PaCO2 as that measured via the laryngeal mask, allowing capnometry to be reliably used to evaluate the adequacy of ventilation.     

Changes in occlusion pressure (P0.1) and breathing pattern during pressure support ventilation

BACKGROUND: The purpose of this study was to investigate changes in breathing pattern, neuromuscular drive (P0.1), and activity of the sternocleidomastoid muscles (SCM) during a gradual reduction in pressure support ventilation (PSV) in patients being weaned off controlled mechanical ventilation. METHODS: Eight non-COPD patients recovering from acute respiratory failure were included in this prospective interventional study. All patients were unable to tolerate discontinuation from mechanical ventilation. Each patient was evaluated during a period of spontaneous breathing and during PSV. Four successive levels of PSV were assessed in the following order: 20 cm H2O (PS20), 15 cm H2O (PS15), 10 cm H2O (PS10), and 5 cm H2O (PS5). RESULTS: When pressure support was reduced from PS20 to PS10 the respiratory rate (f) and the rapid shallow breathing index (f/VT) significantly increased and tidal volume (VT) significantly decreased. These parameters did not vary when pressure support was reduced from PS10 to PS5. Conversely, P0.1 varied negligibly between PS20 and PS15 but increased significantly at low PSV levels. P0.1 values were always greater than 2.9 cm H2O (4.1 (1.1) cm H2O) when SCM activity was present. When contraction of the SCM muscles reappeared the P0.1 was the only parameter that changed significantly. CONCLUSIONS: In postoperative septic patients the value of P0.1 seems to be more useful than breathing pattern parameters for setting the optimal level of pressure assistance during PSV.     

Dynamics of re-expansion of atelectasis during general anaesthesia

A major cause of impaired gas exchange during general anaesthesia is atelectasis, causing pulmonary shunt. A 'vital capacity' (VC) manoeuvre (i.e. inflation of the lungs up to 40 cm H2O, maintained for 15 s) may re-expand atelectasis and improve oxygenation. However, such a manoeuvre may cause adverse cardiovascular effects. Reducing the time of maximal inflation may improve the margin of safety. The aim of this study was to analyse the change over time in the amount of atelectasis during a VC manoeuvre in 12 anaesthetized adults with healthy lungs. I.v. anaesthesia with controlled mechanical ventilation (VT 9 (SD 1) ml kg-1) was used. For the VC manoeuvre, the lungs were inflated up to an airway pressure (Paw) of 40 cm H2O. This pressure was maintained for 26 s. Atelectasis was assessed by analysis of computed x-ray tomography. The amount of atelectasis, measured at the base of the lungs, was 4.0 (SD 2.7) cm2 after induction of anaesthesia. The decrease in the amount of atelectasis over time during the VC manoeuvre was described by a negative exponential function with a time constant of 2.6 s. At an inspired oxygen concentration of 40%, PaO2 increased from 17.2 (4.0) kPa before to 22.2 (6.0) kPa (P = 0.013) after the VC manoeuvre. Thus in anaesthetized adults undergoing mechanical ventilation with healthy lungs, inflation of the lungs to a Paw of 40 cm H2O, maintained for 7-8 s only, may re-expand all previously collapsed lung tissue, as detected by lung computed tomography, and improve oxygenation. We conclude that the previously proposed time for a VC manoeuvre may be halved in such subjects.      

Ventilatory changes during nitrous oxide isoflurane anaesthesia in children

The changes in ventilatory variables under nitrous oxide isoflurane anaesthesia were studied in 10 children (mean age 46 +/- 13.4 months, mean weight 16.2 +/- 2.1 kg). Measurements of flow and volume were performed by pneumotachography. PE'CO2 was measured by capnography. The following variables (VE, VT, TI/TTOT, VI, PE'CO2) were measured or calculated under three increasing inspired isoflurane concentrations (0.75%, 1.5%, 2.25%). At each level of anaesthesia, ventilatory changes during exposure to an inspired CO2 fraction of 2% were studied. The increase in the inspired concentration of isoflurane was associated with a decrease in alveolar ventilation. PE'CO2 increased significantly with increasing depth of anaesthesia. The respiratory rate was slightly increased under light nitrous oxide isoflurane anaesthesia, but no further changes were observed with increasing depth of anaesthesia, although the children were breathing a mixture of nitrous oxide and oxygen. The ventilatory response to a raised inspired CO2 is markedly decreased under light nitrous oxide isoflurane anaesthesia, and decreases significantly with increasing depth of anaesthesia. In response to a raised CO2, VE, VT and VI increase, but respiratory rate decreases or remains constant and TI/TTOT is unchanged.     

Can first responders achieve and maintain normocapnia when sequentially ventilating with a bag-valve device and two oxygen-driven resuscitators? A controlled clinical trial in 104 patients

BACKGROUND AND OBJECTIVE: To evaluate the capability of first responders to achieve and maintain normal ventilation of the lungs of victims employing a bag-valve device and two oxygen-driven resuscitators. METHODS: Prospective, controlled, blinded, single-centre clinical trial using a bag-valve device and one of two FR-300 devices, with 20 cmH2O working pressure, and flows of either 24 or 30 L min(-1). One hundred and four patients were analysed. Induction of anaesthesia followed by ventilation of the lungs with a bag-valve device and an Oxylator in manual and automatic modes performed by a fireman first responder. Each series was repeated for three conditions (anaesthesia; anaesthesia plus muscle relaxation, both with facemask; anaesthesia plus relaxation using an endotracheal tube). RESULTS: Patients age 49 +/- 17 yr; 47% males, 48-132 kg. Normocapnia was achieved and maintained in 66% (bag-valve device), 82% (Oxylator). CONCLUSIONS: The use of an oxygen-driven device improves the ability of first responders to achieve and maintain normocapnia even when distracted. Use of the Oxylators improves performance (P < 0.001) vs. the bag-valve device significantly.     

Epidural block does not worsen oxygenation during one-lung ventilation for lung resections under isoflurane/nitrous oxide anaesthesia

BACKGROUND AND OBJECTIVES: The aim of this prospective, randomized, controlled clinical study was to evaluate the effects of thoracic epidural anaesthesia combined with isoflurane/nitrous oxide anaesthesia on intraoperative oxygenation during one-lung ventilation for lung resections. METHODS: Forty patients were randomly allocated to receive general anaesthesia maintained with isoflurane/nitrous oxide (group General, n = 20) or the same anaesthetic combined with thoracic epidural anaesthesia (group Integrated, n = 20). All patients were mechanically ventilated with the same settings (FiO2 = 0.5; VT = 9 mL kg(-1); inspiratory: expiratory time = 1:1; inspiratory pause = 10%). Effects on oxygenation were evaluated by determining the changes in PaO2/FiO2 ratio at 10, 30, 45 and 60 min of one-lung ventilation as compared to values obtained after induction of anaesthesia (supine, two-lung ventilation). RESULTS: The PaO2/FiO2 ratio was decreased in both groups during one-lung ventilation until the end of surgery. No differences were found at any observation time between the groups. Ventilation with 100% oxygen because of SpO2 decrease <92% was required in nine patients of group General (45%) and in eight patients of group Integrated (40%) (P = 0.64). Manual re-inflation of the operated lung was required in one patient of group General only (P = 0.99). Heart rate was lower in group Integrated than in group General throughout the study. No differences between the two groups in mean arterial pressure were observed. CONCLUSIONS: Adding a thoracic epidural block to isoflurane/nitrous oxide anaesthesia during one-lung ventilation for lung resections does not result in clinically relevant detrimental effects on intraoperative oxygenation.     

Predictors of weaning after acute respiratory failure

Although weaning predictors have been extensively explored in weaning research, their use is currently under debate. From all the stages of mechanical ventilation, the measurements of weaning predictors have been considered by some authors as imperative in order to progress weaning and initiate a weaning trial. However, this practice is rejected by other authors who considered that these tests are not necessary to perform a weaning trial, based in a meta-analysis study from the American College of Chest Physicians. Among all the weaning predictors, the frequency-to-tidal volume ratio (f/VT) remains the most important predictor of weaning. Other predictors have been defined, but their narrow predictive capacity or the requirement of specific technology, have limited their use. The variability of the results obtained by the efficacy of f/VT is probably explained because in most cases weaning is initiated late, when pre-test probability of weaning success is high. In order to reduce weaning duration, weaning strategies must be performed earlier, when a failed weaning trial probably is poor tolerated and the use of f/VT could have a role. New applications of weaning predictors must be clarified in future research, in order to progress in weaning in the context of new studies. Weaning is still a challenging period during mechanical ventilation.