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CRITICAL CARE AND TRAUMA

Automatic "Respirator/Weaning" with Adaptive Support Ventilation: The Effect on Duration of Endotracheal Intubation and Patient Management

Alexander H. Petter, MD^*, René L. Chioléro, MD^*, Tiziano Cassina, MD^*, Pierre-Guy Chassot, MD, Xavier M. Müller, MD, and Jean-Pierre Revelly, MD^*

*Surgical Intensive Care Unit, Department of Anesthesiology, and Department of Cardiac Surgery, University Hospital, Lausanne, Switzerland

Address correspondence and reprint requests to Jean-Pierre Revelly, MD, Surgical Intensive Care Unit, Room 08.652, Lausanne University Hospital, CH-1011-Lausanne, Switzerland. Address e-mail to jrevelly{at}chuv.hospvd.ch

	Abstract

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Adaptive support ventilation (ASV) provides an automatic adaptation of the ventilator settings to patient’s passive and active respiratory mechanics. In a randomized controlled study, we evaluated automatic respiratory weaning in ASV for early tracheal extubation after cardiac surgery. Eligible patients were assigned to either an ASV protocol or a standard one consisting of synchronized intermittent ventilation followed by pressure support. Eighteen patients completed the ASV protocol, and 16 completed the standard one. There were no differences between groups in perioperative characteristics, lengths of tracheal intubation and intensive care unit stay, and ventilatory variables, except less peak inspiratory pressure during the initial phase in ASV (17.5 ± 0.8 versus 22.2 ± 0.8 cm H₂O; P < 0.01). ASV patients required fewer ventilatory settings manipulations (2.4 ± 0.7 versus 4.0 ± 0.8 manipulations per patient; P < 0.05) and endured less high-inspiratory pressure alarms (0.7 ± 2.4 versus 2.9 ± 3.0; P < 0.05). These results suggest that in this specific population of patients, automation of postoperative ventilation with ASV resulted in an outcome similar to the control group. The internal logic of the new device resulted in less manipulation of the setting and alarms that could simplify respiratory management.

IMPLICATIONS: Adaptive support ventilation (ASV), a ventilatory mode providing automatic adjustment of the settings was compared with standard management for rapid tracheal extubation after cardiac surgery. The two approaches were equal in terms of outcome. In ASV, we observed fewer ventilator settings manipulations and a smaller amount of alarms, suggesting that this automatic mode may simplify postoperative respiratory management without delaying extubation.

	Introduction

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Rapid progress in microprocessor technology has markedly increased the capabilities and complexity of intensive care ventilators. New respiratory modes provide automatic selection of ventilatory settings and continuous adaptation. Few reports suggest that such technology can be used in selected patients (1,2). However, its place in clinical practice and its impact on patient process of care has received limited attention. Adaptive support ventilation (ASV) is a microprocessor-controlled mode based on pressure-controlled (PCV) and pressure-support (PS) ventilation, with an automatic adaptation of respiratory rate and level of pressure to patient passive and active respiratory mechanics (1,3). It guarantees that a predefined minute ventilation is delivered and could be considered as an electronic mandatory minute ventilation (4). We have evaluated a weaning protocol based on ASV in fast-track cardiac surgery (5). The length of mechanical ventilation was significantly shorter in the ASV group than in the group receiving the standard of care. In this protocol, the amount of human intervention was considerable, reflecting our preliminary experience with this new mode. Conceivably, the rules of ASV algorithm can automatically deal with any respiratory condition, from full mechanical support to complete weaning. We hypothesized that a simplified protocol, based on ASV automatic ventilation throughout the postoperative period, could reduce human intervention related to respiratory management without delaying extubation. The aims of this randomized, controlled study were to test the capability of ASV to perform respiratory weaning automatically and to evaluate its effect on ventilator management in cardiac surgery patients.

	Methods

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After approval from the Ethics Committee of the Faculty of Medicine of the University of Lausanne, preoperative written informed consent was obtained from patients programmed for elective cardiac surgery (coronary artery bypass grafting or valve surgery) under cardiopulmonary bypass (CPB). The preoperative exclusion criteria were age >80 yr, preoperative left ventricle ejection fraction <30%, chronic obstructive pulmonary disease requiring bronchodilator therapy (i.e., likely to delay rapid tracheal extubation), significant hepatic disease (alanine aminotransferase or aspartate aminotransferase >150 IU/L), renal failure (creatinine >200 µmol/L), history of seizure, and stroke.

After enrollment, patients presenting any condition hindering rapid extubation were excluded from the study. Specific postoperative exclusion criteria were severe postoperative hemorrhage (chest tube drainage >500 mL/h, 350/h for 2 h, or >1000 mL in total), surgical complication requiring reoperation, postoperative cardiac failure requiring large-dose inotropes, refractory hypoxemia (PaO₂/fraction of inspired oxygen [FIO₂] ratio <150 mm Hg), and neurological complication precluding patient’s collaboration. The study was conducted between October 2000 and March 2001 at our surgical intensive care unit (ICU).

Patients were assigned at random to one of two parallel groups. In one group, the patients were ventilated with a protocol based on ASV, whereas a standard protocol was applied in the control group.

Patient management was performed by the attending anesthesiologists and intensivists, whereas data regarding mechanical ventilation, arterial blood gas, and drugs administered were collected by a research fellow (AHP). Treatment group assignment was in a sealed envelope that was opened upon ICU arrival; it was blinded to the anesthesiologist, but it was known during postoperative management. The patients were anesthetized with etomidate, fentanyl, and vecuronium for induction and midazolam and small-dose fentanyl for maintenance. CPB was performed under moderate hypothermia (28°C–32°C). Patients were transferred tracheally intubated to the ICU where management included the following: continuous monitoring of arterial pressure, electrocardiogram, and SpO₂, blood transfusion to maintain hemoglobin concentration 8.0 g/dL, dopamine and norepinephrine to maintain mean arterial blood pressure >70 mm Hg, and sodium nitro prussiate to treat hypertension more than a mean arterial blood pressure of 100 mm Hg. Requirements for analgesia were assessed by the nurses. Morphine was given as a 1- or 2-mg IV bolus, followed by a continuous infusion of 1 or 2 mg/h when the patient complained or expressed autonomic signs of pain. During Phase 1 of respiratory weaning, propofol was given for sedation in boluses of 20 or 30 mg to achieve a Ramsay score of sedation of 3 (i.e., responsive to commands) (6). Shivering was treated with 25 mg of IV pethidine.

ASV has been comprehensively described elsewhere (3). In brief, it guarantees that a predefined value of minute ventilation is delivered. The initial settings include three variables: ideal body weight (IBW), the desired minute ventilation (vol min) expressed as a percentage of the default value of 100 mL · min^-1 · kg^-1 of IBW (7), and the maximal inspiratory pressure tolerated. The ventilator initially delivers PCV. Optimal respiratory rate is computed from the expiratory time constant using the Otis et al. (8) formula. The target for tidal volume is defined accordingly, and the ventilatory variables are adapted via negative feed-back loops. The ventilatory settings are automatically modified, according to changes in measured respiratory mechanics. Every time the patient performs an inspiratory effort, the algorithm switches automatically from PCV to PS mode, thus placing respiratory rate under the control of the patient. The ventilator reverts to PCV if spontaneous respiratory rate decreases to less than the optimal value. The level of PS is continuously adapted to deliver the desired vol min. If rapid shallow breathing occurs, suggesting insufficient ventilatory support (9), PS is automatically increased. However, ASV progressively decreases PS if the patient’s respiratory rate and tidal volume remain adequate, leading to complete removal of mechanical ventilation.

The weaning protocol, consisting of three phases, is depicted in Figure 1. In the ASV group, the initial settings (Phase 1) were IBW, vol min set at the default value, and peak airway pressure (Ppeak) less than 25 cm H₂O, corresponding to a high-pressure alarm of 35 cm H₂O. An arterial blood gas (ABG) analysis (RapidlabTM 865 blood gas analyzer, Ciba-Corning Diagnostics AG, Dietlikon, Switzerland) was performed 10 min after connection to the ventilator (Galileo with software version GMP 02.10C, Hamilton Medical AG, Rhäzüns, Switzerland). If PaCO₂ was 38 mm of mercury or 50 mm of mercury, percent vol min was decreased or respectively increased by 20%. Each change in the settings of the ventilator was controlled 10 min later. Phase 1 ended with recovery of sustained spontaneous ventilation (no controlled breath for 20 min). Phase 2 ended when PS had decreased to a minimal value of 10 cm H₂O (within 2 cm H₂O for 20 min). At this stage, ABG (i.e., PaCO₂ >50 mm of mercury) and clinical criteria of poor tolerance to weaning were excluded (see Appendix). If the evaluation was satisfactory, the weaning progressed to Phase 3, where PS was set manually at a level of 5 cm H₂O. Ten minutes later, contraindications were excluded (Appendix), and the patient was tracheally extubated.

View larger version (23K): [in this window] [in a new window]   Figure 1. Diagram of the weaning protocol for adaptive support ventilation (ASV; left) and control group (right). In both groups, the weaning process was divided into three phases using explicit criteria. Phase 1 started with controlled ventilation and ended when the transition toward spontaneous ventilation was completed. Transition to Phase 2 occurred automatically in ASV, whereas pressure support (PS) was set manually in the control group. In Phase 3, a minimal value of 5 cm H2O of PS was set manually in both groups. The initial settings (Phase 1) were ideal body weight (IBW), minute ventilation (vol min) set at the theoretical value, and peak airway pressure (Ppeak) less than 25 cm H2O. Other settings included fraction of inspired oxygen (FIO2) of 100%, positive end-expiratory pressure (PEEP) of 4 cm H2O (maintained constant until extubation), and pressure trigger sensitivity of -1 cm H2O. High-pressure alarm was set at 35 cm H2O. FIO2 was adjusted to maintain an arterial saturation 95%. Percent vol min denotes the percentage of vol min set in function of a theoretical value of 100 mL/kg of IBW; Pinsp is the level of PS automatically set by the ventilator; *criteria of poor tolerance and contraindications to extubation are described in the Appendix.

Clinical characteristics including age, sex, anthropometrical data, Parsonnet cardiac surgery risk score (10), intraoperative data including anesthesia, CPB and aortic cross-clamping lengths, postbypass left ventricle ejection fraction determined by transesophageal echocardiography, temperature, sedative, and analgesic doses were recorded. Expired tidal volume, respiratory rate, Ppeak, mean airway pressure, end-expiratory pressure, and inspiratory pressure at 100 ms (P_0.1) were recorded for each breath using a data-acquisition software (Datalogger® Version 2.4, Hamilton Medical AG). For statistical analysis, the values of these variables was averaged during the initial 10 min of Phase 1, the initial (2a) and final (2b) 10 min of Phase 2, and the last 10 min of Phase 3. The length of the three phases of weaning was determined by the research fellow in the control group and from the respiratory data file in ASV. Other elements relevant to mechanical ventilation management included ABG, the number of changes in the settings of the ventilator performed by the health care workers, the number of transitions from controlled to assisted ventilation, and the number of apnea and high-pressure alarms.

The primary outcome variable was the duration of tracheal intubation. The secondary variables included mechanical ventilation, the number of alarms, and the amounts of sedative and analgesic administered.

Statistical analysis was performed using JMP Statistical software version 3.5.1 (SAS Institute, Cary, NC). P < 0.05 was considered statistically significant for all analyses. Lengths of protocol phases, tracheal intubation, and ICU stay were compared by log-rank tests and were expressed as median [quartiles]. Other continuous variables were expressed as mean ± SD. The mean values determined for each phase were compared by two-way analysis of variance for repeated measurements to assess the effect of treatment group and time. When the effect of time was significant, the values at one phase were compared with the preceding values with Dunnett tests. When the interaction was significant, comparisons between groups were performed with Scheffé tests. Nominal variables were compared between groups by ² tests.

	Results

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Of 45 patients enrolled in the study, 34 completed the weaning protocol and figure in the statistical analysis. One patient (ASV) was accidentally tracheally extubated, his evolution was uneventful, and he was excluded from the analysis. Ten patients were excluded for postoperative complication precluding rapid extubation: hemorrhage (0 ASV versus 3 control), hypoxemia (1 versus 3), and cardiac failure (1 versus 2). Two of them died in the ICU: one from refractory cardiac failure, and one from multiple organ failure. The number of patients excluded as well as the reason were not different between groups (P = 0.18). No patient completing the protocol required reintubation for respiratory cause. However, two patients (from the ASV group) required reintubation for nonrespiratory cause: myocardial infarction occurred 18 h after surgery in one and postoperative bleeding requiring reoperation at 24 h in the other.

Clinical characteristics are reported in Table 1 and 2. Besides a shorter time of aortic cross-clamping in the control group, there were no differences in perioperative variables between groups. There were no differences between groups in the length of tracheal intubation, ICU stay, and amounts of postoperative sedation (Table 2). A post hoc analysis indicated that the power to detect a difference of 1 h between groups was 76%. All patients except two (one ASV and one control) could be extubated within 6 h, a duration considered as a successful early extubation (11).

View larger version (24K): [in this window] [in a new window]   Figure 2. Respiratory variables during the three phases of weaning. Open circles denote mean values (± SD) of adaptive support ventilation (ASV) group and closed triangles the control group. The horizontal axis figures the phases of respiratory weaning. 2a and 2b denote the initial and terminal part of Phase 2 (see Methods). adifferent from ASV group during the same phase; bdifferent from Phase 1; cdifferent from Phase 2a; ddifferent from Phase 2b (P < 0.05).

	Discussion

Top Abstract Introduction Methods Results Discussion Appendix References

 
The major finding of the present study was that ASV, an automatic mode of mechanical ventilation, may have simplified patient’s management, whereas the outcome was equal to the control group in fast-track cardiac surgery.

Early tracheal extubation constitutes a major element of fast-track after cardiac surgery (12). Numerous interventions have been implemented to accelerate recovery, such as shortening CPB time, minimizing hypothermia, and using short-acting anesthesia drugs. An appropriate ventilation strategy can reduce the duration of mechanical ventilation after cardiac surgery, although this aspect has been incompletely investigated (13,14).

Different approaches have been used to automate mechanical ventilation. Dojat et al. (15) have applied a knowledge-based system to adjust PS automatically in medical patients requiring prolonged mechanical ventilation. They have shown that automatic adaptation of PS provided more favorable respiratory variables than physician’s management. ASV appropriately decreased PS to a minimal value in patients requiring prolonged mechanical ventilation who successfully passed a conventional weaning trial, whereas it maintained larger levels of PS in those failing the weaning trial (2). Thus, there is evidence that some aspects of mechanical ventilation can be successfully automated.

We have tested ASV use in postoperative mechanical ventilation because this situation is conceivably ideal for automation (16). ASV automatically sets the initial ventilatory variables, detects patient’s inspiratory effort breath-by-breath, and switches without human intervention to PS. It transfers the control of respiration to the patient while continuously assessing respiratory rate and reestablishing PCV if the spontaneous rate decreases to less than the optimal value, thus preventing by design hypoventilation (see Methods). During spontaneous respiration, the level of PS is continuously adapted to a patient’s respiratory rate and tidal volume. If the patient maintains a favorable respiratory pattern, PS is reduced automatically. As such, ASV performs a continuous weaning trial, with a constant adjustment to a patient’s tolerance. The present study is the first demonstrating the capability of ASV to achieve a completely automatic weaning. The duration of ventilation was equal to that of the control group. The absence of difference in lengths of intubation in the present study contrasts with the result of a previous study, where we found a more rapid extubation in ASV (5). The main reason for the difference is likely related to different study designs. In the first study, the target of vol min was reduced manually in Group ASV once the patient had recovered a spontaneous ventilation (Phase 2), leading to a rapid reduction of pressure support. In the present study, the target of vol min was left unchanged, i.e., at 100% of the predicted value. In such condition, the algorithm may have been overzealous in maintaining vol min and may have delayed respiratory weaning. Although inherently safe, this relative prolongation of Phase 2 may have contributed to the lack of difference in length of intubation with the control group observed in the present study.

Besides the duration of mechanical ventilation, we evaluated the impact of ASV on patient respiratory management, based on the hypothesis that automation would reduce workload. In ASV, the only settings manipulated were related to the initiation of controlled ventilation and the final adjustment to set PS at 5 cm H₂O. Accordingly, the nurses performed significantly less handling of the ventilator’s settings (Table 3). Because each change in the setting was verified by an ABG determination, we observed a concomitant reduction in ABG sampling. The latter reduced the amount of blood taken and nurses’ work related to sample manipulation and analysis (17). The absolute number of manipulations was limited because of the short duration of mechanical ventilation in these postoperative patients. Clearly, the potential benefit of ASV should be verified in patients more difficult to wean.

Patients in ASV generated less ventilator alarms. ICU alarms can be considered in the perspective of safety and workload. Apnea was, by design, abolished in ASV, whereas it occurred in two control patients. There were no adverse events because the back-up ventilation was activated. Thus, although ASV use required less human intervention, it did not result in different outcome. More surprisingly, the high-pressure alarms were almost eliminated in ASV. Although it could not be directly tested in the present study, this observation may reflect a better synchrony with the ventilator during the phase of controlled ventilation, as suggested in one study (1). The observation of a reduction of alarms in ASV may influence nurses’ working conditions. One study suggested that the ventilator is the major source of alarms in the ICU (18) and the most likely cause of nurse intervention, thus fragmenting work organization (19).

Potential savings incurred by the use of ASV are difficult to assess because ventilator adjustments constitute only part of the workload related to mechanical ventilation (20). The scores validated for the assessment of ICU nurse workload (TISS (21), NEMS (22), and PRN (19)) do not take into account specific actions related to mechanical ventilation. These scores ascribe a fixed amount of time to ventilatory management, rather than considering specific actions such as changes in the settings or responding to an alarm. Only variable costs were likely to be affected by different ventilatory strategy because the equipment and level of staffing were identical in the two groups. Although the number of interventions was reduced, this may not translate into direct savings.

The present study suffers some limitations. For practical reasons, blinding of the treatment group was impossible in the ICU, as in most randomized trials on mechanical ventilation. The protocol of the control group is also arguable, and PCV may have offered a better comparison with ASV. We opted for SIMV because it was the standard in our unit, and this mode has been used in other similar studies (23–25). We also tried to assess patient’s ventilatory comfort during mechanical ventilation using visual analog scales. Unfortunately, in the clinical setting of patients recovering from prolonged anesthesia and receiving IV sedation and analgesia, we were not been able to obtain scores for meaningful data.

We have measured P_0.1 as an index of respiratory drive. We are not aware of the usual values of P_0.1 in the context of rapid postoperative extubation, but these values are similar to those reported in patients successfully weaned after prolonged mechanical ventilation (26).

In a small group of fast-track patients, we have evaluated a protocol of automatic postoperative respiratory weaning in ASV. ASV provided adequate ventilatory variables in all patients. Respiratory weaning with ASV was as rapid as conventional management and it reduced alarms and settings manipulations, suggesting that it may simplify respiratory management. Whether this observation applies to other situations of mechanical ventilation remains to be determined.

	Appendix

Top Abstract Introduction Methods Results Discussion Appendix References

 
Criteria of poor tolerance to weaning:

• Respiratory rate 35 breaths/min
  
• SaO₂ < 90%
  
• Heart rate (HR) > 140 bpm or a sustained increase or decrease in the HR of more than 20%
  
• Systolic blood pressure > 200 mm Hg or < 90 mm Hg
  
• Agitation
  
• Diaphoresis
  
• PaCO₂ > 50 mm Hg
  

Tracheal extubation criteria:

Extubation was postponed if any of the following occurred:

• Patient not responsive and cooperative
  
• FIO₂ > 50%; PaO₂/FIO₂ < 150; PEEP 4 cm H₂O
  
• Hemodynamically unstable, urine output > 0.5 mL · kg^-1 · h^-1
  
• Chest tube drainage > 200 mL in the last hour
  
• Uncontrolled arrhythmia
  
• Rectal temperature > 36.0°C
  

Reintubation criteria:

Reintubation was performed if any of the following was present:

Respiratory cause (cute respiratory failure):

• PaO₂ of < 60 mm Hg with FIO₂ of 60%
  
• PaCO₂ of > 50 mm Hg and pH <7.30
  
• Respiratory rate of >40 breaths/min with physical signs of distress.
  

Nonrespiratory causes:

• Acute deterioration of level of consciousness or new onset of global neurological deficit
  
• Inability to protect the airway from aspiration of oropharyngeal secretions and gastric contents because of neurological dysfunction or altered level of consciousness
  
• Severe hemodynamic instability or cardiogenic shock
  

	Acknowledgments

 
Supported, in part, by a grant from Hamilton Medical AG, Rhäzüns, Switzerland.

The authors are deeply indebted to the nursing team of the surgical ICU for their active collaboration and wish to greatly acknowledge the help of Julien Rossat, MD, for reviewing manuscript syntax.

	References

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Anesthesia & Analgesia® is published for the International Anesthesia Research Society® by Lippincott Williams & Wilkins with the assistance of Stanford University Libraries' HighWire Press®. Copyright 2006 by the International Anesthesia Research Society. Online ISSN: 1526-7598   Print ISSN: 0003-2999