Respiratory comfort of automatic tube compensation and inspiratory pressure support in conscious humans

Comparative Efficacy and Safety of Four Different Spontaneous Breathing Trials for Weaning From Mechanical Ventilation: A Systematic Review and Network Meta-Analysis

Background: Spontaneous breathing trial (SBT) has been used to predict the optimal time of weaning from ventilator. However, it remains controversial which trial should be preferentially selected. We aimed to compare and rank four common SBT modes including automatic tube compensation (ATC), pressure support ventilation (PSV), continuous positive airway pressure (CPAP), and T-piece among critically ill patients receiving mechanical ventilation (MV).

Methods: We searched PubMed, EMBASE, and the Cochrane Central Register of Controlled Trials (CENTRAL) to identify studies that investigated the comparative efficacy and safety of at least two SBT strategies among critically ill patients up to May 17, 2020. We estimated the surface under the cumulative ranking curve (SUCRA) to rank SBT techniques, and determined the certainty of evidence using the Grading of Recommendations Assessment, Development and Evaluation method. Primary outcome was weaning success. Secondary outcomes were reintubation, SBT success, duration of acute care, and intensive care unit (ICU) mortality. Statistical analysis was conducted by using RevMan 5.4, Stata, and R software.

Results: We enrolled 24 trials finally. Extubation success rate was significantly higher in ATC than that in T-piece (OR, 0.28; 95% CI, 0.13–0.64) or PSV (OR, 0.53; 95% CI, 0.32–0.88). For SBT success, ATC was better than other SBT techniques, with a pooled OR ranging from 0.17 to 0.42. For reintubation rate, CPAP was worse than T-piece (OR, 2.76; 95% CI, 1.08 to 7.06). No significant difference was detected between SBT modes for the length of stay in ICU or long-term weaning unit (LWU). Similar result was also found for ICU mortality between PSV and T-piece. Majority direct results were confirmed by network meta-analysis. Besides, ATC ranks at the first, first, and fourth place with a SUCRA of 91.7, 99.7, and 39.9%, respectively in increasing weaning success and SBT success and in prolonging ICU or LWU length of stay among four SBT strategies. The confidences in evidences were rated as low for most comparisons.

Conclusion: ATC seems to be the optimal choice of predicting successful weaning from ventilator among critically ill patients. However, randomized controlled trials (RCTs) with high quality are needed to further establish these findings.

Pressure-flow characteristics of breathing systems and their components for pediatric and adult patients

BACKGROUND

Breathing circuits connect the ventilator to the patients' respiratory system. Breathing tubes, connectors, and sensors contribute to artificial airway resistance to a varying extent. We hypothesized that the flow-dependent resistance is higher in pediatric breathing systems and their components compared to respective types for adults.

AIMS

We aimed to characterize the resistance of representative breathing systems and their components used in pediatric patients (including devices for adults) by their nonlinear pressure-flow relationship.

METHODS

We used a physical model to measure the flow-dependent pressure gradient (∆P) across breathing tubes, breathing tube extensions, 90°- and Y-connectors, flow- and carbon dioxide sensors, water traps and reusable, disposable and coaxial breathing systems for pediatric and for adult patients. ∆P was analyzed for usual flow ranges and statistically compared at a representative flow rate of 300 mL∙s^-1 (∆P₃₀₀ ).

RESULTS

∆P across pediatric devices always exceeded ∆P across the corresponding devices for adult patients (all P < .001 [no 95% CI includes 0]). ∆P₃₀₀ across breathing system components for adults was always below 0.2 cmH₂ O but reached up to 4.6 cmH₂ O in a flow sensor for pediatric patients. ∆P₃₀₀ was considerably higher across the reusable compared to the disposable pediatric breathing systems (1.9 vs 0.3 cmH₂ O, P < .001, [95% CI -1.59 to -1.56]).

CONCLUSION

The resistances of pediatric breathing systems and their components result in pressure gradients exceeding those for adults several fold. Considering the resistance of individual components is crucial for composing a breathing system matching the patient's needs. Compensation of the additional resistance should be considered if a large composed resistance is unavoidable.

Advanced Modes of Mechanical Ventilation: Introduction for the Critical Care Pharmacist

Mechanical ventilation continues to be an evolving modality in the critical care environment. Technological advances in microprocessor-controlled ventilation integrated with the complexity of new ventilator modes has provided the multidisciplinary team opportunities to further improve the care of the critically ill ventilator patients. As members of the critical care multidisciplinary team, pharmacists require a basic understanding of both conventional and advanced modes of mechanical ventilation in order to assist in optimizing medication use and ultimately patient health-care outcomes. Pharmacists have a key responsibility to practice vigilance to maintain safe drug therapy use by preventing drug-drug or drug-disease interactions and optimal dose selection based upon pharmacokinetics and pharmacodynamics principles. Pharmacists also assist in the development of drug utilization guidelines and pharmacological ventilator-weaning protocols based upon evidence-based practice. The result of these responsibilities must include the continued longitudinal assessment and reporting of quality measures to assess ventilator weaning, time to liberation of mechanical ventilation, and length of care in intensive care unit. The purpose of this article is to provide the clinical pharmacist a guide to a basic understanding of advanced modes of mechanical ventilation in adults and to apply the knowledge gained to assist in the care of the critical care patients.

Flow-controlled expiration: a novel ventilation mode to attenuate experimental porcine lung injury

BACKGROUND

Whereas the effects of various inspiratory ventilatory modifications in lung injury have extensively been studied, those of expiratory ventilatory modifications are less well known. We hypothesized that the newly developed flow-controlled expiration (FLEX) mode provides a means of attenuating experimental lung injury.

METHODS

Experimental acute respiratory distress syndrome was induced by i.v. injection of oleic acid in 15 anaesthetized and mechanically ventilated pigs. After established lung injury ([Formula: see text]ratio <27 kPa), animals were randomized to either a control group receiving volume-controlled ventilation (VCV) or a treatment group receiving VCV with additional FLEX (VCV+FLEX). At predefined times, lung mechanics and oxygenation were assessed. At the end of the experiment, the pigs were killed, and bronchoalveolar fluid and lung biopsies were taken. Expression of inflammatory cytokines was analysed in lung tissue and bronchoalveolar fluid. Lung injury score was determined on the basis of stained tissue samples.

RESULTS

Compared with the control group (VCV; n=8), the VCV+FLEX group (n=7) demonstrated greater dynamic lung compliance and required less PEEP at comparable [Formula: see text] (both P<0.05), had lower regional lung wet-to-dry ratios and lung injury scores (both P<0.001), and showed less thickening of alveolar walls (an indicator of interstitial oedema) and de novo migration of macrophages into lung tissue (both P<0.001).

CONCLUSIONS

The newly developed FLEX mode is able to attenuate experimental lung injury. FLEX could provide a novel means of lung-protective ventilation.

Weaning from mechanical ventilation: why are we still looking for alternative methods?

Most patients who require mechanical ventilation for longer than 24 hours, and who improve the condition leading to the indication of ventilatory support, can be weaned after passing a first spontaneous breathing test. The challenge is to improve the weaning of patients who fail that first test. We have methods that can be referred to as traditional, such as the T-tube, pressure support or synchronized intermittent mandatory ventilation (SIMV). In recent years, however, new applications of usual techniques as noninvasive ventilation, new ventilation methods such as automatic tube compensation (ATC), mandatory minute ventilation (MMV), adaptive support ventilation or automatic weaning systems based on pressure support have been described. Their possible role in weaning from mechanical ventilation among patients with difficult or prolonged weaning remains to be established.

Rohrer's constant, K2, as a factor of determining inspiratory resistance of common adult endotracheal tubes

The aim of the study was to calculate the in vitro inspiratory resistance (R(ETT)) of adult endotracheal tubes (ETT), via the end-inspiratory occlusion method, and to apply this method in vivo in order to estimate R(ETT) value in real time. By plotting R(ETT) over inspiratory flow (V) and calculating Rohrer's coefficients of linear and nonlinear resistance, K1 and K2 respectively, we determined the resistive behaviour of each ETT. Peak and plateau pressures were recorded at both proximal and distal sites of the ETT after applying a three-second occlusion under constant flow. Distal pressure was obtained via an intraluminal catheter R(ETT) was calculated as (P(peak) - P(plateau))/(V), at both sites. R(ETT) value resulted from the difference R(proximal) - R(distal). Graph R(ETT) over (V) was plotted and Rohrer's constants were calculated by the method of least squares. For ETTs with inner diameter 9.0, 8.5, 8.0, 7.5, 7.0 and 6.5 mm, K2 was 2.42, 3.05, 4.65, 6.01, 9.17 and 12.80 cmH2O/l/s, respectively. The intraluminal catheter increased R(ETT) No.7.0 by an average of 49%. Finally, ten patients with partially obstructed ETTs were tested and K2 in vivo constants found to be higher than their corresponding in vitro values (P value 0.00012). Therefore, knowing the performing size of an ETT may help the clinicians identify ETT obstruction and deal with weaning problems.

Prediction of extubation outcome: a randomised, controlled trial with automatic tube compensation vs. pressure support ventilation

INTRODUCTION

Tolerance of a spontaneous breathing trial is an evidence-based strategy to predict successful weaning from mechanical ventilation. Some patients may not tolerate the trial because of the respiratory load imposed by the endotracheal tube, so varying levels of respiratory support are widely used during the trial. Automatic tube compensation (ATC), specifically developed to overcome the imposed work of breathing because of artificial airways, appears ideally suited for the weaning process. We further evaluated the use of ATC in this setting.

METHODS

In a prospective study, patients who had received mechanical ventilation for more than 24 hours and met defined criteria for a weaning trial, underwent a one-hour spontaneous breathing trial with either ATC (n = 87) or pressure support ventilation (PSV; n = 93). Those tolerating the trial were immediately extubated. The primary outcome measure was the ability to maintain spontaneous, unassisted breathing for more than 48 hours after extubation. In addition, we measured the frequency/tidal volume ratio (f/VT) both with (ATC-assisted) and without ATC (unassisted-f/VT) at the start of the breathing trial as a pretrial predictor of extubation outcome.

RESULTS

There were no significant differences in any of the baseline characteristics between the two groups apart from a significantly higher Acute Physiology and Chronic Health Evaluation (APACHE) II score in the ATC group (p = 0.009). In the PSV group, 13 of 93 (14%) patients failed the breathing trial compared with only 6 of 87 (6%) in the ATC group; this observed 8% difference, however, did not reach statistical significance (p = 0.12). The rate of reintubation was not different between the groups (total group = 17.3%; ATC = 18.4% vs. PSV = 12.9%, p = 0.43). The percentage of patients who remained extubated for more than 48 hours was similar in both groups (ATC = 74.7% vs. PSV = 73.1%; p = 0.81). This represented a positive predictive value for PSV of 0.85 and ATC of 0.80 (p = 0.87). Finally, the ATC-assisted f/VT was found to have a significant contribution in predicting successful liberation and extubation compared with the non-significant contribution of the unassisted f/VT (unassisted f/VT, p = 0.19; ATC-assisted f/VT, p = 0.005).

CONCLUSIONS

This study confirms the usefulness of ATC during the weaning process, being at least as effective as PSV in predicting successful extubation outcome and significantly improving the predictive value of the f/VT.

TRIAL REGISTRATION

Current Controlled Trials ISRCTN16080446.

[Monitoring airway pressure in pediatric anesthesia: an experimental model of intratracheal medication and pressure-volume loops]

BACKGROUND

In the monitoring of anesthesia, airway pressure is measured in the ventilator or at the closest possible connection to the endotracheal tube.

OBJECTIVE

To compare the airway pressures and pressure-volume loops obtained before connection to the endotracheal tube with those obtained in the trachea.

MATERIAL AND METHODS

We carried out a single-blind prospective observational study on ASA 1 patients between the ages of 7 and 12 years ventilated in volume-control mode with an inspiration-to-expiration ratio of 1:2. Intratracheal and extratracheal peak and plateau pressures and pressure-volume loops were recorded. A special device was designed to monitor intratracheal pressure. Both sensors were connected to the same spirometric analysis system. The variables were measured on intubation and 5, 10, 15, 20, 30, 40, 50, and 60 minutes after intubation. The recorded pressures were compared using the t test, the Pearson product moment correlation coefficient (r), and the Spearman rank correlation coefficient (p), and regression models were fit to the data.

RESULTS

Seventy-one patients were enrolled. The mean (SD) pressure difference between the 2 systems was 3.5 (0.35) cm H2O (P < .01) and no differences between the endotracheal peak pressures and the plateau pressures were observed. The intratracheal areas of the pressure-volume loops were 15% lower than the extratracheal areas. The value of r for the correlation between the intratracheal peak and plateau pressures was 0.998 (P < .01). The value of r for the correlation between the intratracheal and extratracheal peak pressures was 0.981 (P < .01). Analysis of variance confirmed the linear relationship.

CONCLUSIONS

The difference between the intratracheal and extratracheal pressure measurements is due to the different locations at which the measurements are taken.

Extubation outcome following a spontaneous breathing trial with automatic tube compensation versus continuous positive airway pressure

OBJECTIVE

We hypothesized that the additional use of automatic tube compensation (ATC) during a spontaneous breathing trial with continuous positive airway pressure (CPAP), by minimizing respiratory work, would result in more patients undergoing successful extubation.

DESIGN

Prospective, randomized, controlled study.

SETTING

A ten-bed, general intensive care department at a tertiary-care hospital.

PATIENTS

Adult patients (n=99) who had undergone mechanical ventilation for >24 hrs and met defined criteria for a weaning trial.

INTERVENTIONS

Patients were randomized to undergo a 1-hr spontaneous breathing trial with either ATC with CPAP (ATC group, n=51) or CPAP alone (CPAP group, n=48). ATC was provided by commercially available mechanical ventilators. Patients tolerating the spontaneous breathing trial underwent immediate extubation. The primary outcome measure was successful extubation, defined as the ability to maintain spontaneous breathing for 48 hrs after discontinuation of mechanical ventilation and extubation.

MEASUREMENTS AND MAIN RESULTS

There were no significant differences in demographic, respiratory, or hemodynamic characteristics between the two groups at the start of the spontaneous breathing trial. There was a trend for more patients in the ATC group to tolerate the breathing trial and undergo extubation (96% vs. 85%; p=.08). The rate of reintubation was 14% in the ATC group and 24% in the CPAP group (p=.28). Significantly more patients in the ATC group thus met the criteria for successful extubation (82% vs. 65%; p=0.04).

CONCLUSION

This is the largest single-center study to date assessing the use of commercially available ATC and suggests that this might be a useful mode for performing a spontaneous breathing trial preceding extubation in a general intensive care population.

Proportional assist versus pressure support ventilation in patients with acute respiratory failure: Cardiorespiratory responses to artificially increased ventilatory demand*

OBJECTIVE

To test the hypothesis that in response to increased ventilatory demand, dynamic inspiratory pressure assistance better compensates for increased workload compared with static pressure support ventilation (PSV).

DESIGN

Randomized clinical crossover study.

SETTING

General intensive care u nits of a university hospital.

PATIENTS

Twelve patients with acute respiratory failure.

INTERVENTIONS

Patients received PSV, proportional assist ventilation (PAV), and PAV+ automatic tube compensation (ATC) in random order while maintaining mean inspiratory airway pressure constant. During each setting, ventilatory demand was increased by adding deadspace without ventilator readjustment.

MEASUREMENTS AND MAIN RESULTS

Cardiorespiratory, ventilatory, and work of breathing variables were assessed by routine monitoring plus pneumotachography; airway, esophageal, and abdominal pressure measurements; and nitrogen washout. After deadspace addition, tidal volume and end-expiratory lung volume increased similarly in all ventilatory modalities. Ventilator work, peak inspiratory flow, and maximum airway pressure increased significantly during PAV+ATC when compared with PSV after deadspace addition. However, increase in ventilator work did not result in a smaller increase in patients' work of breathing with elevated ventilatory demand during PAV+ATC (PSV 807 +/- 204 mJ/L, PAV 802 +/- 193 mJ/L, and PAV+ATC 715 +/- 202 mJ/L, p = .11). Increase in patients' work of breathing was mainly caused by a significantly higher resistive workload during PAV and PAV+ATC.

CONCLUSION

In patients with acute respiratory failure, dynamic inspiratory pressure assistance modalities are not superior to PSV with respect to cardiorespiratory function and inspiratory muscles unloading after increasing ventilatory demand. The latter might be explained by higher peak flows resulting in nonlinearly increased resistive workload that was incompletely compensated by PAV+ATC.

Other approaches to open-lung ventilation: Airway pressure release ventilation

OBJECTIVE

To review the use of airway pressure release ventilation (APRV) in the treatment of acute lung injury/acute respiratory distress syndrome.

DATA SOURCE

Published animal studies, human studies, and review articles of APRV.

DATA SUMMARY

APRV has been successfully used in neonatal, pediatric, and adult forms of respiratory failure. Experimental and clinical use of APRV has been shown to facilitate spontaneous breathing and is associated with decreased peak airway pressures and improved oxygenation/ventilation when compared with conventional ventilation. Additionally, improvements in hemodynamic parameters, splanchnic perfusion, and reduced sedation/neuromuscular blocker requirements have been reported.

CONCLUSION

APRV may offer potential clinical advantages for ventilator management of acute lung injury/acute respiratory distress syndrome and may be considered as an alternative "open lung approach" to mechanical ventilation. Whether APRV reduces mortality or increases ventilator-free days compared with a conventional volume-cycled "lung protective" strategy will require future randomized, controlled trials.

Constant positive airway pressure reduces hypoventilation induced by inhalation anesthesia

STUDY OBJECTIVE

To discover if reducing respiratory system impedance would increase tidal volume and improve ventilation during inhalation anesthesia.

DESIGN

Prospective, randomized cross-over study.

SUBJECTS

Nine ASA physical status I and II adult female oncology patients undergoing breast operations with or without lymph node dissection and general anesthesia while breathing spontaneously.

INTERVENTIONS AND MEASUREMENTS

Patients underwent alternating trials of constant positive airway pressure, with or without pressure support. Constant positive airway pressure and pressure support were titrated to maximize respiratory system compliance and equal inspiratory pressure gradient across tracheal tube, respectively. Variables reflecting cardiovascular function, pulmonary mechanics and lung gas exchange, and respired gases and isoflurane concentrations were measured.

MAIN RESULTS

End-tidal concentration of isoflurane (1.3 +/- 0.2%), Fio(2) (0.43 +/- 0.09 ), and CO(2) elimination (209 +/- 42 mL min(-1)) was unchanged throughout study in patients aged 63 +/- 12 years, weighing 72 +/- 12 kg. Constant positive airway pressure (12 +/- 2 cm H(2)O) increased respiratory system compliance from 52 +/- 8 to 80 +/- 9 mL cm H(2)O(-1) (P < .001), tidal volume from 156 +/- 32 to 325 +/- 52 mL (P < .001), and minute ventilation from 4.37 +/- 0.86 to 6.18 +/- 0.92 L min(-1) (P < .001). Respiratory rate decreased from 29 +/- 7 to 19 +/- 2 min(-1) (P < .001), Paco(2) decreased from 54 +/- 8 to 44 +/- 6 mm Hg (P < .001), and Pao(2) increased from 137 +/- 37 to 160 +/- 64 mm Hg (P < .001). Pressure support (3.1 +/- 0.3 cm H(2)O) did not alter ventilation or gas exchange.

CONCLUSION

We conclude that constant positive airway pressure titrated to optimal respiratory system compliance will increase efficiency of inspiratory muscles and improve ventilation. Constant positive airway pressure facilitates a pattern of breathing that minimizes some of the adverse pulmonary effects of inhalation anesthesia.

How to choose a mechanical ventilator

PURPOSE OF REVIEW

To provide some practical and clinical considerations that may guide users through the decision process when choosing mechanical ventilators

RECENT FINDINGS

Although the complexity of mechanical ventilators is steadily increasing, the importance of many devices developed over the course of the technical evolution is still a matter of discussion. Recent data demonstrate that the technical performance of equivalent ventilators (ie, machines of the same generation and category) is pretty similar, suggesting that the different manufacturers keep in step with new developments. Thus, other factors than technical limitations will probably influence the choice of ventilators. Among them the ability of the staff to understand the rationale of the different devices and controls as well as deal with the complexity of the ventilator may be particularly important.

SUMMARY

Choosing mechanical ventilators should begin by defining the algorithms of how to ventilate a patient. Once this is done, a ventilator should allow the transformation of specific strategies into practice and the adaptation of the mechanical support to the needs of the individual patient. This procedure is crucially important, because ventilator therapy should always be determined by the physician and based on solid physiologic rationales rather than by the technical features of the machine.

Understanding and implementing advances in ventilator capabilities

PURPOSE OF REVIEW

To review the changes in mechanical ventilation technology over the past year and identify areas that provide a benefit.

RECENT FINDINGS

The literature demonstrates a continued effort to improve patient ventilator synchrony though the development of new triggering and cycling methods. These techniques include using new signals and using closed loop techniques to respond to changes in patient breathing pattern. New modes of ventilation continue to be introduced, often without proof of efficacy. Fortunately, clinicians have developed alterations to new modes that improve utility and they continue to study these techniques clinically to determine appropriate use. Monitoring the patient remains an important area of investigation, with a flurry of activity surrounding pressure volume curves of the respiratory system. Finally, new ventilators have been introduced that combine high-end performance with small size and weight, while providing an on-board source of air.

SUMMARY

Mechanical ventilation is ubiquitous to intensive care. Advances in ventilator technology are rapid, and clinicians must keep abreast of changes in ventilator performance and application.

Accuracy of automatic tube compensation in new-generation mechanical ventilators*

OBJECTIVE

To compare performance of flow-adapted compensation of endotracheal tube resistance (automatic tube compensation, ATC) between the original ATC system and ATC systems incorporated in commercially available ventilators.

DESIGN

Bench study.

SETTING

University research laboratory.

SUBJECTS

The original ATC system, Dräger Evita 2 prototype, Dräger Evita 4, Puritan-Bennett 840.

INTERVENTIONS

The four ventilators under investigation were alternatively connected via different sized endotracheal tubes and an artificial trachea to an active lung model. Test conditions consisted of two ventilatory modes (ATC vs. continuous positive airway pressure), three different sized endotracheal tubes (inner diameter 7.0, 8.0, and 9.0 mm), two ventilatory rates (15/min and 30/min), and four levels of positive end-expiratory pressure (0, 5, 10, and 15 cm H2O).

MEASUREMENTS AND MAIN RESULTS

Performance of tube compensation was assessed by the amount of tube-related (additional) work of breathing (WOBadd), which was calculated on the basis of pressure gradient across the endotracheal tube. Compared with continuous positive airway pressure, ATC reduced inspiratory WOBadd by 58%, 68%, 50%, and 97% when using the Evita 4, the Evita 2 prototype, the Puritan-Bennett 840, and the original ATC system, respectively. Depending on endotracheal tube diameter and ventilatory pattern, inspiratory WOBadd was 0.12-5.2 J/L with the original ATC system, 1.5-28.9 J/L with the Puritan-Bennett 840, 10.4-21.0 J/L with the Evita 2 prototype, and 10.1-36.1 J/L with the Evita 4 (difference between each ventilator at identical test situations, p <.025). Expiratory WOBadd was reduced by 5%, 26%, 1%, and 70% with the Evita 4, the Evita 2 prototype, the Puritan-Bennett 840, and the original ATC system, respectively. The expiratory WOBadd caused by an endotracheal tube of 7.0 mm inner diameter was 5.5-42.2 J/L at a low ventilatory rate and 19.6-82.3 J/L at a high ventilatory rate. It was lowest with the original ATC system and highest with the Evita 4 ventilator (p <.025).

CONCLUSIONS

Flow-adapted tube compensation by the original ATC system significantly reduced tube-related inspiratory and expiratory work of breathing. The commercially available ATC modes investigated here may be adequate for inspiratory but probably not for expiratory tube compensation.

Cricoid yoke: the effect of surface area and applied force on discomfort experienced by conscious volunteers

BACKGROUND AND OBJECTIVE

The application of cricoid force is central to techniques that reduce the risk of gastric regurgitation and the subsequent pulmonary aspiration associated with obstetric and emergency anaesthesia. The discomfort associated with cricoid force in awake preoperative patients increases the incidence of coughing, struggling and pain during induction of anaesthesia. This study determined if increasing the surface area of a cricoid yoke reduced the associated discomfort in volunteers.

METHODS

Fifty volunteers participated in a randomized single-blinded study. The cricoid yoke was positioned using standard anatomical landmarks and forces of 10, 20, 30 and 40 N were applied in a random order for 20s, using two different yoke attachments with surface areas of 3 and 10 cm2. A rest of 30s was allowed between the application of forces. Discomfort was graded by volunteers on a scale from 0 to 10 (0: no discomfort; 10: worse discomfort imaginable). A score of 10 was allocated if the volunteers could not tolerate the applied force for 20s.

RESULTS

Median scores for the small yoke were always higher than those for the large yoke at each force. There were significant differences between the scores for the small and large yokes at 10 and 20 N (P < 0.001) and 30 N (P = 0.0233), but there was no significant difference at 40 N.

CONCLUSIONS

The larger yoke was tolerated better by volunteers when clinically relevant cricoid forces were applied.

Tracheal intubation without muscle relaxants: remifentanil or alfentanil in combination with propofol

BACKGROUND AND OBJECTIVE

In some situations, the use of muscle relaxants (neuromuscular blocking drugs) are undesirable or contraindicated. We compared intubating conditions without muscle relaxants in premedicated patients receiving either alfentanil 40 microg kg(-1) or remifentanil 2, 3 or 4 microg kg(-1) followed by propofol 2 mg kg(-1).

METHODS

In a randomized, double-blind study, 80 healthy patients were assigned to one of four groups (n = 20). After intravenous atropine, alfentanil 40 microg kg(-1) or remifentanil 2, 3 or 4 microg kg(-1) were injected over 90 s followed by propofol 2 mg kg(-1). Ninety seconds after administration of the propofol, laryngoscopy and tracheal intubation were attempted. Intubating conditions were assessed as excellent, good or poor on the basis of ease of lung ventilation, jaw relaxation, laryngoscopy, position of the vocal cords, and patient response to intubation and slow inflation of the endotracheal tube cuff.

RESULTS

Seven patients who received remifentanil 2 microg kg(-1) and one patient who received remifentanil 3 microg kg(-1) could not be intubated at the first attempts. Excellent intubating conditions (jaw relaxed, vocal cords open and no movement in response to tracheal intubation and cuff inflation) were observed in those who received either alfentanil 40 microg kg(-1) (45% of patients) or remifentanil in doses of 2 microg kg(-1) (20%), 3 microg kg(-1) (75%) or 4 microg kg(-1) (95%). Overall, intubating conditions were significantly better (P < 0.05), and the number of patients showing excellent conditions were significantly higher (P < 0.05) in patients who received remifentanil 4 microg kg(-1) compared with those who received alfentanil 40 microg kg(-1) or remifentanil 2 microg kg(-1). No patient needed treatment for hypotension or bradycardia.

CONCLUSIONS

Remifentanil 4 microg kg(-1) and propofol 2 mg kg(-1) administered in sequence intravenously provided good or excellent conditions for tracheal intubation in all patients without the use of muscle relaxants.

Stability of the LMA-ProSeal and standard laryngeal mask airway in different head and neck positions: a randomized crossover study

BACKGROUND AND OBJECTIVE

The LMA-ProSeal laryngeal mask airway is a new laryngeal mask airway with a modified cuff and drainage tube. We compared oropharyngeal leak pressure, intracuff pressure and anatomical position (assessed fibreoptically) for the Size 5 LMA-ProSeal laryngeal mask airway and the classic laryngeal mask airway in different head-neck positions and using different intracuff inflation volumes.

METHODS

Thirty paralysed anaesthetized adult male patients were studied. The LMA-ProSeal laryngeal mask airway and the classic laryngeal mask airway were inserted into each patient in random order. The oropharyngeal leak pressure, intracuff pressure, and anatomical position of the airway tube and drainage tube (LMA-ProSeal laryngeal mask airway only) were documented in four head and neck positions (neutral first, then flexion, extension and rotation in random order), and at 0-40 mL cuff volumes in the neutral position in 10 mL increments.

RESULTS

Compared with the neutral position, the oropharyngeal leak pressure for both the LMA-ProSeal laryngeal mask airway and the classic laryngeal mask airway was higher in flexion and rotation (all P < or = 0.02), but lower in extension (all P < or = 0.01). Changes in head-neck position did not alter the anatomical position of the airway tube or the drainage tube. The oropharyngeal leak pressure was always higher for the LMA-ProSeal laryngeal mask airway (all P < or = 0.005) and anatomical position better for the classic laryngeal mask airway (all P < or = 0.04).

CONCLUSIONS

The anatomical position of the LMA-ProSeal and the classic laryngeal mask airway is stable in different head-neck positions, but head-neck flexion and rotation are associated with an increase, and head-neck extension a decrease, in oropharyngeal leak pressure and intracuff pressure. The Size 5 LMA-ProSeal laryngeal mask airway is capable of forming a more effective seal than the Size 5 classic laryngeal mask airway in males.

Does the tube-compensation function of two modern mechanical ventilators provide effective work of breathing relief?

OBJECTIVE

An endotracheal tube (ETT) imposes work of breathing on mechanically ventilated patients. Using a bellows-in-a-box model lung, we compared the tube compensation (TC) performances of the Nellcor Puritan-Bennett 840 ventilator and of the Dräger Evita 4 ventilator.

MEASUREMENTS AND RESULTS

Each ventilator was connected to the model lung. The respiratory rate of the model lung was set at 10 breaths/min with 1 s inspiratory time. Inspiratory flows were 30 or 60 l/min. A full-length 8 mm bore ETT was inserted between the ventilator circuit and the model lung. The TC was set at 0%, 10%, 50%, and 100% for both ventilators. Pressure was monitored at the airway, the trachea, and the pleura, and the data were recorded on a computer for later analysis of the delay time, of the inspiratory trigger pressure, and of the pressure-time product (PTP). The delay time was calculated as the time between the start of inspiration and minimum airway pressure, and the inspiratory trigger pressure was defined as the most negative pressure level. The same measurements were performed under pressure support ventilation of 4 and 8 cmH2O. The PTP increased according to the magnitude of inspiratory flow. Even with 100% TC, neither ventilator could completely compensate for the PTP imposed by the ETT. At 0% TC the PTP tended to be less with the Nellcor Puritan-Bennett 840 ventilator, while at 100% TC the PTP tended to be less with the Dräger Evita 4 ventilator. A small amount of pressure support can be equally effective to reduce the inspiratory effort compared with the TC.

CONCLUSION

Although both ventilators provided effective TC, even when set to 100% TC they could not entirely compensate for a ventilator and ETT-imposed work of breathing. The effect of TC is less than that of pressure support ventilation. Physicians should be aware of this when using TC in weaning trials.

Breathing pattern and workload during automatic tube compensation, pressure support and T-piece trials in weaning patients

BACKGROUND AND OBJECTIVE

Automatic tube compensation has been designed as a new ventilatory mode to compensate for the non-linear resistance of the endotracheal tube. The study investigated the effects of automatic tube compensation compared with breathing through a T-piece or pressure support during a trial of spontaneous breathing used for weaning patients from mechanical ventilation of the lungs.

METHODS

Twelve patients were studied who were ready for weaning after prolonged mechanical ventilation (10.2 +/- 8.4 days) due to acute respiratory failure. Patients with chronic obstructive pulmonary disease were excluded. Thirty minutes of automatic tube compensation were compared with 30 min periods of 7 cmH2O pressure support and T-piece breathing. Breathing patterns and workload indices were measured at the end of each study period.

RESULTS

During T-piece breathing, the peak inspiratory flow rate (0.65 +/- 0.20 L s(-1)) and minute ventilation (8.9 +/- 2.7L min(-1)) were lower than during either pressure support (peak inspiratory flow rate 0.81 +/- 0.25 L s(-1) minute ventilation 10.2 +/- 2.3 L min(-1), respectively) or automatic tube compensation (peak inspiratory flow rate 0.75 +/- 0.26L s(-1); minute ventilation 10.8 +/- 2.7 L min(-1)). The pressure-time product as well as patients' work of breathing were comparable during automatic tube compensation (pressure-time product 214.5 +/- 104.6 cmH2O s(-1) min(-1), patient work of breathing 1.1 +/- 0.4 J L(-1)) and T-piece breathing (pressure-time product 208.3 +/- 121.6 cmH2O s(-1) min(-1), patient work of breathing 1.1 +/- 0.4 J L(-1)), whereas pressure support resulted in a significant decrease in workload indices (pressure-time product 121.2 +/- 64.1 cmH2O s(-1) min(-1), patient work of breathing 0.7 +/- 0.4 J L(-1)).

CONCLUSIONS

In weaning from mechanical lung ventilation, patients' work of breathing during spontaneous breathing trials is clearly reduced by the application of pressure support 7 cmH2O, whereas the workload during automatic tube compensation corresponded closely to the values during trials of breathing through a T-piece.

Anaesthetic agents in adult day case surgery

This study reports a review of all comparative published studies of adult day case anaesthesia in the English language up to December 2000. Ten databases were searched using appropriate keywords and data were extracted in a standardized fashion. One hundred-and-one published studies were examined. Recovery measurements were grouped as early, intermediate, late, psychomotor and adverse effects. With respect to induction of anaesthesia, propofol was superior to methohexital, etomidate and thiopental, but equal to sevoflurane and desflurane. Desflurane and sevoflurane were both superior to thiopental. There was no detectable difference between sevoflurane and isoflurane. With respect to the maintenance of anaesthesia, isoflurane and halothane were the worst. There were no significant differences between propofol, desflurane, sevoflurane and enflurane. Propofol is the induction agent of choice in day case patients. The use of a propofol infusion and avoidance of nitrous oxide may help to reduce postoperative nausea and vomiting.

Prediction of difficult tracheal intubation

BACKGROUND AND OBJECTIVE

Preoperative bedside screening tests for difficult tracheal intubation may be neither sensitive nor specific enough for clinical use. The aim was to investigate if a combination of the Mallampati classification of the oropharyngeal view with either the thyromental or sternomental distance measurement improved the predictive value.

METHODS

A total of 212 (109 male, 103 female) non-obstetric surgical patients, aged >18 yr, undergoing elective surgical procedures requiring tracheal intubation were assessed preoperatively with respect to the oropharyngeal (modified Mallampati) classification, thyromental and sternomental distances. An experienced anaesthetist, blinded to the preoperative airway assessment, performed laryngoscopy and graded the view according to Cormack and Lehane's classification.

RESULTS

Twenty tracheal intubations (9%) were difficult as defined by a Cormack and Lehane Grade 3 or 4, or the requirement for a bougie in patients with Cormack and Lehane Grade 2. Used alone, the Mallampati oropharyngeal view, and thyromental and sternomental distances were associated with poor sensitivity, specificity and positive predictive values. Combining the Mallampati Class III or IV with either a thyromental distance <6.5cm or a sternomental distance <12.5cm decreased the sensitivity (from 40 to 25 and 20%, respectively), but maintained a negative predictive value of 93%. The specificity and positive predictive values increased from 89 and 27% respectively for Mallampati alone to 100%.

CONCLUSIONS

The findings suggest that the Mallampati classification, in conjunction with measurement of the thyromental and sternomental distances, may be a useful routine screening test for preoperative prediction of difficult tracheal intubation.

Remifentanil versus alfentanil in total intravenous anaesthesia for day case surgery

BACKGROUND AND OBJECTIVE

We assessed the intraoperative haemodynamic responses and recovery profiles of total intravenous anaesthesia with remifentanil and alfentanil for outpatient surgery.

METHODS

Patients in Group 1 (n = 20) received alfentanil 20 microg kg(-1) followed by 2 microg kg(-1) min(-1) intravenously; patients in Group 2 (n = 20) received remifentanil 1 microg kg(-1) followed by 0.5 microg kg(-1) min(-1) intravenously. Both groups then received propofol 2 mg kg(-1) followed by 9 mg kg(-1) h(-1) intravenously. Five minutes after skin incision, infusion rates were decreased, and at the end of surgery, all infusions were discontinued. Early recovery was assessed by the Aldrete score, whereas intermediate recovery was assessed with the postanaesthetic discharge scoring system (PADS).

RESULTS

Perioperative arterial pressure was similar in both groups; heart rate was lower in Group 2 (P < 0.05). The times to spontaneous and adequate respiration, response to verbal commands, extubation and times for Aldrete score > or = 9 were shorter in Group 2 patients (P < 0.05). Pain scores were higher in Group 2 patients (P < 0.05). Overall times for postanaesthetic discharge scores > or = 9 were similar.

CONCLUSIONS

Early recovery of patients after day surgery is significantly shorter after total intravenous anaesthesia with remifentanil compared with that with alfentanil but postoperative pain management must be planned ahead.

Effects of different doses of oral ketamine for premedication of children

BACKGROUND AND OBJECTIVE

A need exists for a safe and effective oral preanaesthetic medication for use in children undergoing elective surgery. The study sought to define the dose of oral ketamine that would facilitate induction of anaesthesia without causing significant side-effects.

METHODS

We studied 80 children undergoing elective surgery under general anaesthesia who received oral ketamine 4, 6 or 8 mg kg(-1) in a prospective, randomized, double-blind placebo controlled study. We compared the reaction to separation from parents, transport to the operating room, the response to intravenous cannula insertion and application of an anaesthetic facemask, the induction of anaesthesia and recovery from anaesthesia.

RESULTS

In the group receiving ketamine 8 mg kg(-1), the children were significantly calmer than those of the other groups, and anaesthesia induction was more comfortable. Recovery from anaesthesia was longer in the group receiving ketamine 8 mg kg(-1) compared with the other groups, but no differences between the groups were observed after 2 h in the recovery room.

CONCLUSIONS

It is concluded that oral ketamine 8 mg kg(-1) is an effective oral premedication in inpatient children undergoing elective surgery.

Trendelenburg positioning after cardiac surgery: effects on intrathoracic blood volume index and cardiac performance

BACKGROUND AND OBJECTIVE

The efficacy of the Trendelenburg position, a common first step to treat suspected hypovolaemia, remains controversial. We evaluated its haemodynamic effects on cardiac preload and performance in patients after cardiac surgery.

METHODS

Twelve patients undergoing mechanical ventilation of the lungs who demonstrated left ventricular 'kissing papillary muscles' by transoesophageal echocardiography, thus suggesting hypovolaemia, were positioned 30 degrees head down for 15 min immediately after cardiac surgery. Cardiac output by thermodilution, central venous pressure, pulmonary artery occlusion pressure, left ventricular end-diastolic area by transoesophageal echocardiography and intrathoracic blood volume by thermo- and dye dilution were determined before, during and after this Trendelenburg manoeuvre.

RESULTS

Trendelenburg's manoeuvre was associated with increases in central venous pressure (9 +/- 2 to 12 +/- 3 mmHg) and pulmonary artery occlusion pressure (8 +/- 2 to 11 +/- 3 mmHg). The intrathoracic blood volume index increased slightly (dye dilution from 836 +/- 129 to 872 +/- 112 mL m(-2); thermodilution from 823 +/- 129 to 850 +/- 131 mL m(-2)) as did the left ventricular end-diastolic area index (7.5 +/- 2.1 to 8.1 +/- 1.7 cm2 m(-2)), whereas mean arterial pressure and the cardiac index did not change significantly. After supine repositioning, the cardiac index decreased significantly below baseline (3.0 +/- 0.6 versus 3.5 +/- 0.8 L min(-1) m(-2)) as did mean arterial pressure (76 +/- 12 versus 85 +/- 11 mmHg), central venous pressure (8 +/- 2 mmHg) and pulmonary artery occlusion pressure (6 +/- 4 mmHg). The intrathoracic blood volume index and left ventricular end-diastolic area index did not differ significantly from baseline.

CONCLUSIONS

Trendelenburg's manoeuvre caused only a slight increase of preload volume, despite marked increases in cardiac-filling pressures, without significantly improving cardiac performance.

Anaesthetic and haemodynamic effects of continuous spinal versus continuous epidural anaesthesia with prilocaine

BACKGROUND AND OBJECTIVE

To compare, using prilocaine, the effects of continuous spinal anaesthesia (CSA) and continuous epidural anaesthesia (CEA) on haemodynamic stability as well as the quality of anaesthesia and recovery in patients undergoing transurethral resection of the prostate gland.

METHODS

Thirty patients (>60 yr) were randomized into two groups. Prilocaine, 2% 40 mg, was given to patients in the CSA group, and prilocaine 1% 150mg was given to patients in the CEA group. Incremental doses were given if the level of sensory block was lower than T10 or if needed during surgery.

RESULTS

There was a significant decrease in mean arterial pressure in Group CEA compared with Group CSA (P < 0.01). The decrease in heart rate in Group CSA occurred 10 min after the first local anaesthetic administration and continued through the operation (P < 0.05). The level of sensory anaesthesia was similar in both groups. The times to reach the level of T10 and the upper level of sensory blockade (Tmax) were 18.0 +/- 4.7 and 25.3 +/- 7.0 min in Groups CSA and CEA, respectively, and were significantly longer in Group CEA. The duration of anaesthesia was 76.8 +/- 4min and was shorter in Group CSA (P < 0.01).

CONCLUSIONS

Spinal or epidural anaesthesia administered continuously was reliable in elderly patients undergoing transurethral resection of the prostate. Continuous spinal anaesthesia had a more rapid onset of action, produced more effective sensory and motor blockade and had a shorter recovery period. Prilocaine appeared to be a safe local anaesthetic for use with either continuous spinal anaesthesia or continuous epidural anaesthesia.

Cortical activity assessed by Narcotrend in relation to haemodynamic responses to tracheal intubation at different stages of cortical suppression and reflex control

BACKGROUND AND OBJECTIVE

Many anaesthesiologists still interpret haemodynamic responses as signs of insufficient cortical suppression. The aim was to illustrate how haemodynamics may only poorly reflect the level of cortical suppression and that electroencephalographic monitoring could indicate different relationships between cortical effects and haemodynamics.

METHODS

Anaesthesia was induced with thiopental (7 mg kg(-1)), and fentanyl (2 microg kg(-1)) with succinylcholine (1.5 mg kg(-1)) for neuromuscular blockade in the 11 patients of Group 1. In Group 2 (n = 15), thiopental (7 mg kg(-1)) and succinylcholine (1.5 mg kg(-1)) were given. In Group 3, the patients (n = 13) received thiopental (7 mg kg(-1)), fentanyl (2 microg kg(-1)) and cisatracurium (0.1 mg kg(-1)), and they were intubated 3 min later than the patients in Groups 1 and 2. We determined conventional electroencephalographic (EEG) variables and classified 14 EEG stages in real-time ranging from A (= 1), indicating full wakefulness, to F1 (= 14), at profound cortical suppression.

RESULTS

All groups had profound cortical suppression 45 s after thiopental administration, which rapidly decreased (EEG stage, 11 (6-13) versus 7 (2-13) at 4 min, P < 0.0001). Decreasing EEG stages were associated with increasing SEF 95, relative alpha and beta power and decreasing relative delta power. During tracheal intubation, profound cortical suppression remained unchanged in Groups 1 and 2. In Group 3, cortical suppression had decreased before laryngoscopy (P < 0.005). In Group 2, 11 patients had heart rate responses to tracheal intubation, whereas only two responded in Group 1 (P = 0.015) and three in Group 3 (P = 0.02). Thirteen patients in Group 2 had arterial pressure responses, and five in Group 1 (P = 0.038). Circulatory responses did not differ between Groups 1 and 3.

CONCLUSIONS

Electroencephalographic monitoring was suitable to indicate in real-time that haemodynamics only poorly reflect rapidly changing levels of cortical suppression, and how haemodynamics and cortical activity depend on the applied combination of hypnotic and analgesic drugs during anaesthesia induction with thiopental.

Endotracheal tubes and imposed work of breathing: what should we do about it, if anything?

Concerns about the work of breathing imposed by the endotracheal tube have led clinicians to routinely use pressure support to overcome this resistive component. More recently, ventilator manufacturers have introduced systems to automatically overcome endotracheal tube resistance, regardless of tube diameter or patient demand for flow. Despite the theoretical advantages, neither method appears to provide superior performance. Stepping back, the real question may be, is overcoming endotracheal tube resistance really important?

Increasing the injection volume by dilution improves the onset of motor blockade, but not sensory blockade of ropivacaine for brachial plexus block

BACKGROUND AND OBJECTIVE

Ropivacaine used for axillary plexus block provides effective motor and sensory blockade. Varying clinical dosage recommendations exist. Increasing the dosage by increasing the concentration showed no improvement in onset. We compared the behaviour of a constant dose of ropivacaine 150 mg diluted in a 30, 40 or 60 mL injection volume for axillary (brachial) plexus block.

METHODS

A prospective, randomized, observer-blinded study on patients undergoing elective hand surgery was conducted in a community hospital. Three groups of patients with a constant dose of ropivacaine 150 mg, diluted in 30,40 or 60 mL NaCl 0.9%, for axillary plexus blockade were compared for onset times of motor and sensory block onset by assessing muscle strength, two-point discrimination and constant-touch sensation.

RESULTS

Increasing the injection volume of ropivacaine 150 mg to 60 mL led to a faster onset of motor block, but not of sensory block, in axillary plexus block, compared with 30 or 40 mL volumes of injection.

CONCLUSIONS

The data show that the onset of motor, but not of sensory block, is accelerated by increasing the injection volume to 60 mL using ropivacaine 150 mg for axillary plexus block. This may be useful for a more rapid determination of whether the brachial plexus block is effective. However, when performing surgery in the area of the block, sensory block onset seems more important.

Automatic tube compensation-assisted respiratory rate to tidal volume ratio improves the prediction of weaning outcome

OBJECTIVE

To assess whether the respiratory rate to tidal volume ratio (RVR) measured while receiving automatic tube compensation (ATC) [RVRATC] would have a better predictive value as a weaning measure than unassisted RVR.

DESIGN

Prospective cohort study.

SETTING

General ICU of a tertiary-care university hospital.

PATIENTS

Forty-three patients who received mechanical ventilation for > 24 h and were considered ready for weaning.

INTERVENTIONS

All patients underwent a 60-min spontaneous breathing trial (SBT) [positive end-expiratory pressure of 5 cm H(2)O; ATC, 100%]. Patients tolerating the trial (n = 35) were extubated immediately. The following parameters were measured at the onset and end of the SBT: RVR, RVRATC, peak airway pressure (Paw), airway occlusion pressure, and minute ventilation. The outcome measure was successful extubation (ability to maintain spontaneous breathing for > 48 h).

MEASUREMENTS AND RESULTS

Median age was 55 years (range, 25 to 88 years), median APACHE (acute physiology and chronic health evaluation) II score was 15.5 (range, 3 to 29), and median duration of mechanical ventilation prior to the SBT was 7 days (range, 1 to 40 days). Extubation was successful in 25 patients (72%). There were no significant differences in baseline characteristics between patients successfully extubated (group 1) and those requiring reintubation. On multivariate analysis, RVRATC measured at 60 min (RVR(60)ATC) was most predictive of successful extubation (p = 0.03). The area under the receiver operator characteristic curve was also highest for RVR(60)ATC (0.81 +/- 0.03) as compared to RVR (0.77 +/- 0.03), RVRATC (0.75 +/- 0.04), and RVR measured at 60 min (0.69 +/- 0.05). The ratio of RVR(60)ATC to Paw was the best predictor (0.84 +/- 0.02).

CONCLUSIONS

RVRATC measured at the end of the SBT was the best predictor of successful extubation. A new ratio (ratio of RVRATC to Paw) was most predictive and deserves further study.

Extubation after breathing trials with automatic tube compensation, T-tube, or pressure support ventilation

BACKGROUND

Automatic tube compensation (ATC) is a new option to compensate for the pressure drop across the endotracheal or tracheostomy tube (ETT), especially during ventilator-assisted spontaneous breathing. While several benefits of this mode have so far been documented, ATC has not yet been used to predict whether the ETT could be safely removed at the end of weaning, from mechanical ventilation.

METHODS

We undertook a systematic trial using a randomized block design. During a 2-year period, all eligible patients of a medical intensive care unit were treated with ATC, conventional pressure support ventilation (PSV, 5 cmH2O), or T-tube for 2-h. Tolerance of the breathing trial served as a basis for the decision to remove the endotracheal tube. Extubation failure was considered if reintubation was necessary or if the patient required non-invasive ventilatory assistance (both within 48 h).

RESULTS AND CONCLUSIONS

After the inclusion of 90 patients (30 per group) we did not observe significant differences between the modes. Twelve patients failed the initial weaning trial. However, half of the patients who appeared to fail the spontaneous breathing trial on the T-tube, PSV, or both, were successfully extubated after a succeeding trial with ATC. Extubation was thus withheld from four and three of these patients while breathing with PSV or the T-tube, respectively, but to any patient breathing with ATC. It seems that ATC can be used as an alternative mode during the final phase of weaning from mechanical ventilation. Furthermore, this study may promote a larger multicenter trial on weaning with ATC compared with standard modes.

Automatic tube compensation in patients after cardiac surgery: effects on oxygen consumption and breathing pattern

OBJECTIVE

To evaluate patients without prior pulmonary disease after cardiac surgery and to determine whether resistive unloading by automatic tube compensation, pressure support ventilation, and continuous positive airway pressure has different effects on oxygen consumption, breathing pattern, gas exchange, and hemodynamics.

DESIGN

Prospective, randomized, controlled study.

SETTING

Tertiary care, postoperative intensive care unit.

PATIENTS

Twenty-one patients scheduled for open heart coronary artery bypass graft surgery.

INTERVENTIONS

Each patient was ventilated with all three modes in random order.

MEASUREMENTS AND MAIN RESULTS

Patients were ventilated in three modes, each applied for 30 mins according to computer-generated randomization: pressure support ventilation with 5 cm H2O, continuous positive airway pressure, and automatic tube compensation. Oxygen consumption was calculated by means of indirect calorimetry. The hypnotic state of the patients was monitored by Bispectral Index. For hemodynamic measurements, a fiberoptic pulmonary artery catheter was inserted. The main finding of our study was that oxygen consumption and breathing pattern (tidal volume and respiratory rate) did not differ significantly during automatic tube compensation and pressure support ventilation compared with continuous positive airway pressure (oxygen consumption, 170 +/- 29 vs. 170 +/- 26 vs. 174 +/- 29 mL.min.m, respectively; tidal volume, 466 +/- 132 vs. 484 +/- 125 vs. 470 +/- 119 mL, respectively; respiratory rate, 16 +/- 4 vs. 15 +/- 4 vs. 16 +/- 4 breaths/min, respectively). Automatic tube compensation and pressure support ventilation had no clinical effects on gas exchange and hemodynamic variables compared with continuous positive airway pressure. None of the variables differed significantly during the three ventilatory settings.

CONCLUSION

In postoperative tracheally intubated patients with normal ventilatory demand, automatic tube compensation and pressure support ventilation with 5 cm H2O lead to identical oxygen consumption, breathing patterns, gas exchange, and hemodynamics. We, therefore, suggest that this group of patients does not need any additional positive pressure support from the ventilator to overcome the additional work of breathing imposed by the endotracheal tube during the weaning phase from mechanical ventilation.

Controlled versus assisted mechanical ventilation

On the basis of currently available data, it can be suggested that maintained spontaneous breathing during mechanical ventilation should not be suppressed even in patients with severe pulmonary dysfunction if no contraindications, such as increased intracranial pressure, are present. Improvements in pulmonary gas exchange, systemic blood flow, and oxygen supply to tissues, which have been observed when spontaneous breathing was allowed during ventilatory support, are reflected in the clinical improvement in the patient's condition, as indicated by significantly fewer days with ventilation, earlier extubation, and shorter stays in the intensive care unit. The positive effects of spontaneous breathing have been documented only for some of the available partial ventilatory support modalities. If ventilatory modalities are limited to those whose positive effects have been documented, then partial ventilatory support can be used as a primary modality even in patients with severe pulmonary dysfunction. Whereas controlled mechanical ventilation followed by weaning with partial ventilatory support modalities has been the earlier standard in ventilation therapy, this approach should be reconsidered in view of the available data.

Evaluation of tube compensation in the Bennett 840 ventilator--a new ventilatory mode to support spontaneous breathing

Respiratory care patients frequently require intubation with an endotracheal tube (ETT). Unfortunately, the ETT introduces a pressure drop (deltaPETT) that depends on the respiratory flow rate, thus increasing the work of breathing (WOB). Pressure support ventilation (PSV) cannot adequately compensate for this added WOB, because the degree of inspiratory assistance by PSV is fixed. Therefore, a technique called tube compensation (TC) has been developed to address deltaPETT. We examined the performance of TC and compared it with PSV of 5 cm H2O. The experimental system was constructed from a simulator, a test-lung, flow sensors, and a Bennett 840, and the respiratory parameters were studied. ETTs with IDs 6.5 and 8.0 mm were used. The quadratic approximation obtained for deltaPETT in the 6.5-mm ETT was 2.316 x flow + 7.910 x flow2, while that for the 8.0-mm ETT was 1.881 x flow + 3.353 x flow2. The maximum inspiratory flow (MIF) increased significantly with increasing TC, but tidal volume and inspiratory time did not show marked changes. The MIF for TC of 100% was larger than that for PSV of 5 cm H2O, when the 6.5-mm ID was used, but there was no significant difference between these modes when an ID of 8.0 mm was used. For both the 6.5 and 8.0-mm IDs, the PV loop corresponding to 100% TC was larger than that for PSV of 5 cm H2O. TC only compensated for the WOB caused by the ETT, whereas PSV compensated for the WOB caused by the ETT and the demand valve system. In clinical use, the differences between TC and PSV will demand attention.