Effects of albuterol inhalation on the work of breathing during weaning from mechanical ventilation

Comparison of Vibrating Mesh and Jet Nebulizers During Noninvasive Ventilation in Acute Exacerbation of Chronic Obstructive Pulmonary Disease

Background: Advances in aerosol technology have improved drug delivery efficiency during noninvasive ventilation (NIV). Clinical evaluation of the efficacy of aerosol therapy during NIV in the treatment of acute exacerbation of chronic obstructive pulmonary disease (COPD) is very limited. The aim of our study was to compare the efficacy of bronchodilators administered through a vibrating mesh nebulizer (VMN) and jet nebulizer (JN) during NIV in patients with acute exacerbation of COPD. Methods: Prospective randomized cross-over study included 30 patients treated with NIV for acute exacerbation of COPD in an acute care hospital. Patients were consented and enrolled after stabilization of acute exacerbation (3-5 days after admission). Subjects were randomly assigned into two treatment arms receiving salbutamol (2.5 mg): with VMN (Aerogen Solo) and JN (Sidestream) positioned between the leak port and the nonvented oronasal mask during bilevel ventilation with a single-limb circuit. Measurements (clinical data, pulmonary function tests [PFTs], and arterial blood gases) were performed at baseline, 1, and 2 hours after treatment. Results: All measured PFT parameters significantly increased in both groups, but numerically results were better after inhalation with VMN than with JN: for forced expiratory volume in 1 second (FEV₁) (mean increase from baseline to 120 minutes-165 ± 64 mL vs. 116 ± 46 mL, p = 0.001) and for forced vital capacity (FVC) (mean increase-394 ± 154 mL vs. 123 ± 57 mL, p < 0.001). There was also a statistically significant reduction in respiratory rate and in Borg dyspnea score after therapy with VMN in comparison with the conventional JN. In both groups, there were improvements in PaCO₂, but with VMN these changes were significantly higher. Conclusion: Bronchodilator administration in patients with acute exacerbation of COPD during NIV with VMN resulted in clinically significant improvements in FVC and in Borg dyspnea score. Additional studies required to determine the impact on clinical outcomes.

In Vitro Measurements of Spiriva Respimat Dose Delivery in Mechanically Ventilated Tracheostomy Patients

Background: For patients with severe chronic obstructive pulmonary disease under invasive mechanical ventilation, medication for aerosol therapy is delivered through tracheostomy or endotracheal airways. Typically, these medications (such as bronchodilators) are long-acting formulations that are delivered through Soft Mist^™ Inhalers (SMI), or Pressurized Metered-Dose Inhalers. The Respimat^® SMI has been shown to have increased efficiency because of its slow and prolonged aerosol mist and has gained popularity in clinical settings. However, the Respimat was not designed for drug delivery through artificial airways. Therefore, there is a need for SMI adapters in intensive care for use in mechanical ventilator circuits. The purpose of this study was to evaluate the performance of a new Respimat adapter (ODAPT^™ for mechanical ventilator [ODAPT MV]) for use in mechanical ventilator circuits which, in combination with a Pulmodyne T-piece adapter, allows use without interruption of the circuit in case of medication replacement. Methods: Tiotropium delivery to the lungs, using Respimat, was assessed using the ODAPT MV adapter within an in vitro setup, including a three-dimensional printed trachea model and a mechanical ventilator. Medication deposition and losses were investigated using two commonly used tracheostomy tube (TT) sizes (6 and 8 mm inner canula) for two flow rates (45 and 60 L/min) under different conditions (30%-50%. and 100% relative humidity [RH]). Medication delivery using the ODAPT MV adapter was compared with the RTC-26C in-line adapter under similar conditions (8 mm TT size, 100% RH at 45 L/min). Results: It was found that 7.1%-13.4% of the nominal dose (ND) was lost in the ODAPT MV adapter for different TT size, RH, and flow rates used. Higher losses were found in the inhaler's mouthpiece ranging from 15.7% to 29.1% ND. The percentage of the delivered medication reaching the lungs was determined to be 13.7%-18.5% ND delivered without significant differences between the experimental conditions tested. The ODAPT MV performed well compared with the RTC-26C under similar conditions (17.9% and 16.6% ND, respectively). Conclusion: The medication delivered through mechanical ventilation using the ODAPT MV adapter represents about one third the dose delivered directly through the Respimat SMI in vivo.

Airway Hygiene in COVID-19 Pneumonia: Treatment Responses of 3 Critically Ill Cruise Ship Employees

BACKGROUND COVID-19, the disease entity caused by the novel severe acute respiratory coronavirus 2 (SARS-CoV-2), continues to pose a major therapeutic challenge for clinicians. At present, an effective treatment regimen and vaccination has not been established. Many patients develop severe symptoms requiring endotracheal intubation and a prolonged stay in the Intensive Care Unit (ICU). In early postmortem examinations of COVID-19 patients, profuse viscous secretions were observed throughout the respiratory tract. Thus, oxygen supplementation without aggressive pulmonary hygiene management may be suboptimal. In the present case series, pulmonary hygiene management encompassed mucolytics, bronchodilators, and tracheal suctioning. We report 3 severe cases of COVID-19 pneumonia in cruise ship employees who were admitted to the ICU and responded to supportive mechanical ventilation and pulmonary hygiene management. CASE REPORT Three cruise ship employees with COVID-19 underwent endotracheal intubation and were admitted to the ICU for acute hypoxemic respiratory failure. Initial chest X-rays suggested multifocal pneumonia with superimposed acute respiratory distress syndrome (ARDS). A regimen of hydroxychloroquine, azithromycin, and dexamethasone was initiated on admission in all cases. Additionally, medications used for pulmonary hygiene were administered through a metered-dose inhaler (MDI) in line with the ventilator circuit. Endotracheal suctioning was performed prior to medication administration. The duration from endotracheal intubation to extubation ranged from 9 to 24 days. All 3 patients reached 30-day survival. CONCLUSIONS The cases reported highlight the importance of the use of airway hygiene with mucolytics, bronchodilators, and tracheal suctioning for patients with COVID-19 pneumonia requiring ventilatory support.

Assessment of Work of Breathing in Patients with Acute Exacerbations of Chronic Obstructive Pulmonary Disease

The assessment of the work of breathing (WOB) of patients with acute exacerbations of chronic obstructive pulmonary disease (COPD) is difficult, particularly when the patient first presents with acute hypercapnia and respiratory acidosis. Acute exacerbations of COPD patients are in significant respiratory distress and noninvasive measurements of WOB are easier for the patient to tolerate. Given the interest in using alternative therapies to noninvasive ventilation, such as high flow nasal oxygen therapy or extracorporeal carbon dioxide removal, understanding the physiological changes are key and this includes assessment of WOB. This narrative review considers the role of three different methods of assessing WOB in patients with acute exacerbations of COPD. Esophageal pressure is a very well validated measure of WOB, however the ability of patients with acute exacerbations of COPD to tolerate esophageal tubes is poor. Noninvasive alternative measurements include parasternal electromyography (EMG) and electrical impedance tomography (EIT). EMG is easily applied and is a well validated measure of neural drive but is more likely to be degraded by the electrical environment in intensive care or high dependency. EIT is less well validated as a tool for WOB in COPD but extremely well tolerated by patients. Each of the different methods assess WOB in a different way and have different advantages and disadvantages. For research into therapies treating acute exacerbations of COPD, combinations of EIT, EMG and esophageal pressure are likely to be better than only one of these.

Aerosol therapy in intensive and intermediate care units: prospective observation of 2808 critically ill patients

PURPOSE

Unlike in the outpatient setting, delivery of aerosols to critically ill patients may be considered complex, particularly in ventilated patients, and benefits remain to be proven. Many factors influence aerosol delivery and recommendations exist, but little is known about knowledge translation into clinical practice.

METHODS

Two-week cross-sectional study to assess the prevalence of aerosol therapy in 81 intensive and intermediate care units in 22 countries. All aerosols delivered to patients breathing spontaneously, ventilated invasively or noninvasively (NIV) were recorded, and drugs, devices, ventilator settings, circuit set-up, humidification and side effects were noted.

RESULTS

A total of 9714 aerosols were administered to 678 of the 2808 admitted patients (24 %, CI95 22-26 %), whereas only 271 patients (10 %) were taking inhaled medication before admission. There were large variations among centers, from 0 to 57 %. Among intubated patients 22 % (n = 262) received aerosols, and 50 % (n = 149) of patients undergoing NIV, predominantly (75 %) inbetween NIV sessions. Bronchodilators (n = 7960) and corticosteroids (n = 1233) were the most frequently delivered drugs (88 % overall), predominantly but not exclusively (49 %) administered to patients with chronic airway disease. An anti-infectious drug was aerosolized 509 times (5 % of all aerosols) for nosocomial infections. Jet-nebulizers were the most frequently used device (56 %), followed by metered dose inhalers (23 %). Only 106 (<1 %) mild side effects were observed, despite frequent suboptimal set-ups such as an external gas supply of jet nebulizers for intubated patients.

CONCLUSIONS

Aerosol therapy concerns every fourth critically ill patient and one-fifth of ventilated patients.

Inhaled β2-Agonist Therapy Increases Functional Residual Capacity in Mechanically Ventilated Children With Respiratory Failure

OBJECTIVES

To test the hypothesis that in mechanically ventilated children with respiratory failure, aerosolized albuterol modifies functional residual capacity, lung mechanics, oxygen consumption, and hemodynamics.

DESIGN

Prospective, self-control clinical trial.

SETTING

A 24-bed PICU in a quaternary care, academic children's hospital.

PATIENTS

25 children (age range, 1-18 yr) undergoing mechanical ventilation to treat respiratory failure. Entry criteria included previously prescribed inhaled β2 agonists. Physiologic measurements were performed prior to and 20 minutes after administration of aerosolized albuterol solution. Functional residual capacity was determined via nitrogen washout.

INTERVENTIONS

Functional residual capacity, oxygen consumption, respiratory mechanics, and vital signs were measured were measured prior to and 20 minutes after administration of aerosolized albuterol solution. Functional residual capacity was determined via nitrogen washout.

MEASUREMENT AND MAIN RESULTS

At baseline, functional residual capacity is only 53% of predicted. After aerosolized albuterol, functional residual capacity increased by 18.3% (p = 0.008). Overall, aerosolized albuterol had no effect on airway resistance. However, in patients with an endotracheal tube size of more than or equal to 4.0 mm, resistance decreased from 33 ± 3 to 25 ± 3 (p < 0.02). Inhaled albuterol administration had no effect on oxygen consumption despite an increase in heart rate from 116 ± 2 to 128 ± 2 beats/min (p < 0.0001).

CONCLUSIONS

In pediatric patients with respiratory failure, aerosolized albuterol increases functional residual capacity without a decrease in resistance. In infants and children, aerosolized albuterol might favorably enhance pulmonary mechanics and thereby represent a novel strategy for lung recruitment in children with respiratory failure.

Impact of the anesthetic conserving device on respiratory parameters and work of breathing in critically ill patients under light sedation with sevoflurane

BACKGROUND

Sevoflurane sedation in the intensive care unit is possible with a special heat and moisture exchanger called the Anesthetic Conserving Device (ACD) (AnaConDa; Sedana Medical AB, Uppsala, Sweden). The ACD, however, may corrupt ventilatory mechanics when used during the weaning process of intensive care unit patients. The authors compared the ventilatory effects of light-sedation with sevoflurane administered with the ACD and those of classic management, consisting of a heated humidifier and intravenous sedation, in intensive care unit patients receiving pressure-support ventilation.

METHODS

Fifteen intensive care unit patients without chronic pulmonary disease were included. A target Richmond Agitation Sedation Scale level of -1/-2 was obtained with intravenous remifentanil (baseline 1-condition). Two successive interventions were tested: replacement of the heated humidifier by the ACD without sedation change (ACD-condition) and sevoflurane with the ACD with an identical target level (ACD-sevoflurane-condition). Patients finally returned to baseline (baseline 2-condition). Work of breathing, ventilatory patterns, blood gases, and tolerance were recorded. A steady state of 30 min was achieved for each experimental condition.

RESULTS

ACD alone worsened ventilatory parameters, with significant increases in work of breathing (from 1.7 ± 1.1 to 2.3 ± 1.2 J/l), minute ventilation, P0,1, intrinsic positive end-expiratory pressure (from 1.3 ± 2.6 to 4.7 ± 4.2 cm H2O), inspiratory pressure swings, and decreased patient comfort. Sevoflurane normalized work of breathing (from 2.3 ± 1.2 to 1.8 ± 1 J/l), intrinsic positive end-expiratory pressure (from 4.7 ± 4.2 to 1.8 ± 2 cm H2O), inspiratory pressure swings, other ventilatory parameters, and patient tolerance.

CONCLUSIONS

ACD increases work of breathing and worsens ventilatory parameters. Sevoflurane use via the ACD (for a light-sedation target) normalizes respiratory parameters. In this patient's population, light-sedation with sevoflurane and the ACD may be possible during the weaning process.

Measurement of esophageal pressure at bedside: pros and cons

PURPOSE OF REVIEW

Esophageal pressure measurement well estimates pleural pressure. The interpretation of absolute values is often debated for various reasons, but the changes in pressure measured are considered very accurate provided that a number of precautions are taken. The information provided by these measurements is unique in nature and has an enormous potential to influence management. It allows to study the exact influence of the chest wall and to determine the real lung distending pressure. It is also the only way to quantify respiratory muscle activity and the work of breathing.

RECENT FINDINGS

The application of esophageal pressure monitoring potentially covers a large field, especially for what concerns mechanical ventilation. This goes from the acute phase of the acute respiratory distress syndrome (ARDS) to weaning and patient-ventilator interactions. During ARDS, recent findings indicate that this measurement may help titrating the level of positive end-expiratory pressure or to determine the well tolerated upper limit of airway pressure.

SUMMARY

Application of esophageal pressure monitoring is limited by technical issues, the need for background physiological knowledge and the fact that very few studies have assessed a direct influence of this measurement on patients' outcome. The technique is underused in everyday practice.

Clinical review: Respiratory monitoring in the ICU - a consensus of 16

Monitoring plays an important role in the current management of patients with acute respiratory failure but sometimes lacks definition regarding which 'signals' and 'derived variables' should be prioritized as well as specifics related to timing (continuous versus intermittent) and modality (static versus dynamic). Many new techniques of respiratory monitoring have been made available for clinical use recently, but their place is not always well defined. Appropriate use of available monitoring techniques and correct interpretation of the data provided can help improve our understanding of the disease processes involved and the effects of clinical interventions. In this consensus paper, we provide an overview of the important parameters that can and should be monitored in the critically ill patient with respiratory failure and discuss how the data provided can impact on clinical management.

Inhalation therapy in patients receiving mechanical ventilation: an update

Incremental gains in understanding the influence of various factors on aerosol delivery in concert with technological advancements over the past 2 decades have fueled an ever burgeoning literature on aerosol therapy during mechanical ventilation. In-line use of pressurized metered-dose inhalers (pMDIs) and nebulizers is influenced by a host of factors, some of which are unique to ventilator-supported patients. This article reviews the impact of various factors on aerosol delivery with pMDIs and nebulizers, and elucidates the correlation between in-vitro estimates and in-vivo measurement of aerosol deposition in the lung. Aerosolized bronchodilator therapy with pMDIs and nebulizers is commonly employed in intensive care units (ICUs), and bronchodilators are among the most frequently used therapies in mechanically ventilated patients. The use of inhaled bronchodilators is not restricted to mechanically ventilated patients with chronic obstructive pulmonary disease (COPD) and asthma, as they are routinely employed in other ventilator-dependent patients without confirmed airflow obstruction. The efficacy and safety of bronchodilator therapy has generated a great deal of interest in employing other inhaled therapies, such as surfactant, antibiotics, prostacyclins, diuretics, anticoagulants and mucoactive agents, among others, in attempts to improve outcomes in critically ill ICU patients receiving mechanical ventilation.

Randomized, placebo-controlled clinical trial of an aerosolized β₂-agonist for treatment of acute lung injury

RATIONALE

β₂-Adrenergic receptor agonists accelerate resolution of pulmonary edema in experimental and clinical studies.

OBJECTIVES

This clinical trial was designed to test the hypothesis that an aerosolized β₂-agonist, albuterol, would improve clinical outcomes in patients with acute lung injury (ALI).

METHODS

We conducted a multicenter, randomized, placebo-controlled clinical trial in which 282 patients with ALI receiving mechanical ventilation were randomized to receive aerosolized albuterol (5 mg) or saline placebo every 4 hours for up to 10 days. The primary outcome variable for the trial was ventilator-free days.

MEASUREMENTS AND MAIN RESULTS

Ventilator-free days were not significantly different between the albuterol and placebo groups (means of 14.4 and 16.6 d, respectively; 95% confidence interval for the difference, -4.7 to 0.3 d; P = 0.087). Rates of death before hospital discharge were not significantly different between the albuterol and placebo groups (23.0 and 17.7%, respectively; 95%confidence interval for the difference,-4.0 to 14.7%;P = 0.30). In the subset of patients with shock before randomization, the number of ventilator-free days was lower with albuterol, although mortality was not different. Overall, heart rates were significantly higher in the albuterol group by approximately 4 beats/minute in the first 2 days after randomization, but rates of new atrial fibrillation (10% in both groups) and other cardiac dysrhythmias were not significantly different.

CONCLUSIONS

These results suggest that aerosolized albuterol does not improve clinical outcomes in patients with ALI. Routine use of β₂-agonist therapy in mechanically ventilated patients with ALI cannot be recommended. Clinical trial registered with www.clinicaltrials.gov (NCT 00434993).

In vitro validation of a Respimat® adapter for delivery of inhaled bronchodilators during mechanical ventilation

BACKGROUND

Inhaled bronchodilators are frequently used in patients with chronic obstructive pulmonary disease (COPD). However, there has been no efficient way to administer the long-acting anticholinergic tiotropium to mechanically ventilated patients. The aim of this in vitro study was to compare the fine particle dose (FPD) output of a specifically designed adapter with other accessory devices for the delivery of bronchodilators using the Respimat® (RMT) inhaler by simulating the specific inhalation flow profiles of patients with COPD.

METHODS

Using characteristic flow profiles from COPD patients being weaned off mechanical ventilation, an in vitro study was performed analyzing the FPD achieved with different accessory devices (connectors, spacers, AeroTrachPlus valved holding chamber), which can be used to deliver drugs from pressurized metered dose inhalers (pMDI) and RMT inhalers to artificial airways. Fenoterol pMDI, tiotropium RMT, and a fixed-dose combination of salbutamol and ipratropium delivered by pMDI or RMT, were used as bronchodilators. Aerosols were collected by a next-generation impactor.

RESULTS

The RMT inhaler, combined with a new in-line adapter, was superior to other inhaler device connector or spacer combinations in FPD delivery during simulated mechanical ventilation (p<0.01). The outcome with the RMT inhaler/RMT adapter combination during simulation of mechanical ventilation was comparable to the measurements with the RMT/AeroTrachPlus valved holding chamber during simulation of spontaneous breathing. The delivery rates of the RMT adapter were not significantly affected by the administered bronchodilators or by the type of artificial airway (endotracheal or tracheostomy tube) employed.

CONCLUSIONS

The RMT inhaler combined with the prototype in-line adapter was better than the other accessory device combinations in fine particle deposition of inhaled bronchodilators during mechanical ventilation. Further research is required to determine the clinical relevance of these in vitro findings.

How best to deliver aerosol medications to mechanically ventilated patients

Pressurized metered-dose inhalers (pMDIs) and nebulizers are employed routinely for aerosol delivery to ventilator-supported patients, but the ventilator circuit and artificial airway previously were thought to be major barriers to effective delivery of aerosols to patients receiving mechanical ventilation. In the past two decades, several investigators have shown that careful attention to many factors, such as the position of the patient, the type of aerosol generator and its configuration in the ventilator circuit, aerosol particle size, artificial airway, conditions in the ventilator circuit, and ventilatory parameters, is necessary to optimize aerosol delivery during mechanical ventilation. The best techniques for aerosol delivery during noninvasive positive-pressure ventilation are not well established as yet, and the efficiency of aerosol delivery in this setting is lower than that during invasive mechanical ventilation. The most efficient methods of using the newer hydrofluoroalkane-pMDIs and vibrating mesh nebulizers in ventilator-supported patients also require further evaluation. When optimal techniques of administration are employed, the efficiency of aerosolized drug delivery in mechanically ventilated patients is comparable to that achieved in ambulatory patients.

Inhaled bronchodilator administration during mechanical ventilation: how to optimize it, and for which clinical benefit?

Bronchodilators are frequently used in ICU patients, and are the most common medications administered by inhalation during mechanical ventilation. The amount of bronchodilator that deposits at its site of action depends on the amount of drug, inhaled mass, deposited mass, and particle size distribution. Mechanical ventilation challenges both inhaled mass and lung deposition by specific features, such as a ventilatory circuit, an endotracheal tube, and ventilator settings. Comprehensive in vitro studies have shown that an endotracheal tube is not as significant a barrier for the drug to travel as anticipated. Key variables of drug deposition are attachments of the inhalation device in the inspiratory line 10 to 30 cm to the endotracheal tube, use of chamber with metered-dose inhaler, dry air, high tidal volume, low respiratory frequency, and low inspiratory flow, which can increase the drug deposition. In vivo studies showed that a reduction by roughly 15% of the respiratory resistance was achieved with inhaled bronchodilators during invasive mechanical ventilation. The role of ventilatory settings is not as clear in vivo, and primary factors for optimal delivery and physiologic effects were medication dose and device location. Nebulizers and pressurized metered-dose inhalers can equally achieve physiologic end points. The effects of bronchodilators should be carefully evaluated, which can easily be done with the interrupter technique. With the non-invasive ventilation, the data regarding drug delivery and physiologic effects are still limited. With the bilevel ventilators the inhalation device should be located between the leak port and face mask. Further studies should investigate the effects of inhaled bronchodilators on patient outcome and methods to optimize delivery of inhaled bronchodilators during non-invasive ventilation.

Effect of salbutamol on respiratory muscle function and ventilation in awake canines

The effect of the beta-agonist bronchodilator salbutamol on respiratory muscles and ventilation is uncertain. The presence of beta2 receptors on skeletal muscles and increased diaphragm contractility in vitro with salbutamol predict a significant effect that has not been confirmed, in vivo in non-fatigued diaphragm or in clinical studies using standard bronchodilator dosages. Therefore, we infused salbutamol at a higher dosage (23.3 microg/min) used clinically for treatment of respiratory emergencies, while measuring directly the length, shortening and EMG activation of costal and crural diaphragm, parasternal intercostal and transversus abdominis muscles, in 10 awake canines. At this salbutamol dosage, ventilation and tidal volume increased significantly during both resting and CO2-stimulated breathing. Salbutamol elicited significant increases in respiratory muscle shortening with much smaller increases in EMG activity, so the proportionally greater muscle shortening per unit EMG showed increased muscle contractility. The effects of salbutamol were not extinguished by inspiratory flow resistance or fluid challenge but were reversed specifically by the beta-blocker, propranolol. This study demonstrates that, in sufficient intravenous dosage, the beta-agonist salbutamol elicits increased ventilation and a beta2 receptor-mediated increase in contractility of respiratory muscles.

The Thoracic Surgical Patient: Initial Postoperative Care

Thoracic surgery patients require complex perioperative care. Accurate preoperative screening of pulmonary conditions can only partially predict the requirement of postoperative mechanical ventilation. In general, extensive lung resections are associated with significant gas exchange abnormalities. In this group of patients, a mechanical ventilation strategy protective from barotrauma and volutrauma and a conservative use of intraoperative and postoperative fluid limit the inevitable increase of extravascular lung water and gas exchange impairment. A wise use of pulmonary vasodilatatory and bronchodilating drugs and airway manipulation including suctioning and bronchoscopy can also significantly affect postoperative respiratory dysfunction and hospital stay. A number of acute postoperative complications have been described specifically related to the type of surgery or pleural space suctioning devices. The role of the intensivist is to maintain a low index of suspicion for such complications, when acute hemodynamic or pulmonary deterioration occurs and be prepared to immediately correct them or alert the thoracic surgeon. In general, when these roles are observed, thoracic surgery can be safely performed with a low perioperative mortality.

Bronchodilator delivery with metered-dose inhaler during mechanical ventilation

The delivery of bronchodilators with metered-dose inhaler (MDI) in mechanically ventilated patients has attracted considerable interest in recent years. This is because the use of the MDI has several advantages over the nebulizer, such as reduced cost, ease of administration, less personnel time, reliability of dosing and a lower risk of contamination. A spacer device is fundamental in order to demonstrate the efficacy of the bronchodilatory therapy delivered by MDI. Provided that the technique of administration is appropriate, MDIs are as effective as nebulizers, despite a significantly lower dose of bronchodilator given by the MDI.

Weaning from mechanical ventilation

For most mechanically ventilated patients, weaning can be accomplished quickly and easily. However, there is a smaller group of ventilated patients who fail to wean and remain ventilator-dependent. These patients account for a significant amount of health care costs and pose a great challenge for clinicians. Detailed knowledge of the etiology and pathophysiology of weaning failure is very important for the "treatment" of difficult to wean patients, and is thoroughly presented in this article. Based on this physiological background, strategies and techniques are proposed that are useful for the gradual transition to spontaneous ventilation.

Contribution of the endotracheal tube and the upper airway to breathing workload

The influence of the endotracheal tube (ETT) during a T-piece trial remains controversial. Our aim was to compare the work of breathing of 14 successfully extubated patients at the end of a 2-h trial (T) and after extubation (E) of the trachea, and to assess, using the acoustic reflection method, the resistance of the endotracheal tube and of the supraglottic airway as well as their related work. We found that the work of breathing of the patients was identical between T and E (1.72 +/- 0.59 versus 1.63 +/- 0.45 J/L; p = 0.50 and 23.5 +/- 10.6 versus 22.6 +/- 9.7 J/min; p = 0.70). There was no significant difference between the beginning and the end of the T-piece trial (1.57 +/- 0.53 versus 1.72 +/- 0.59 J/ L, p = 0.10). The work caused by the ETT amounted to 11.0 +/- 3.9% of the total work of breathing. The supraglottic airway resistance was in the normal range and was significantly smaller than the endotracheal tube resistance (0.79 +/- 0.4 versus 1.43 +/- 0.31 cm H2O x s/L; p = 0.008, flow = 0.25 L/s). We conclude that a 2-h trial of spontaneous breathing through an endotracheal tube well mimics the work of breathing performed after extubation, in patients who pass a weaning trial and do not require reintubation.

Weaning from mechanical ventilation

Use of mechanical ventilation is associated with several major complications despite its lifesaving potential. Timely discontinuation of mechanical ventilation is critical to control of duration of intensive care unit stay and reduction of complications associated with mechanical ventilation. Difficulty in discontinuation (or weaning) of patients from mechanical ventilatory support is in part attributable to inadequate understanding of the mechanisms responsible for unsuccessful outcome and a lack of guidelines regarding the optimal approach to the process of discontinuation of mechanical ventilation. For the first time, results from prospective, randomized, multicenter trials are available comparing common means of discontinuation of mechanical ventilation. In addition, the physiologic basis for a weaning strategy in mechanical ventilation is also coming into better focus. Two recent trials of weaning suggest different optimal modes, one favoring T-piece trials and the other supporting the use of pressure support ventilation. In either case, the above weaning techniques appear to be superior to intermittent mandatory ventilation in separating patients from mechanical ventilatory support. Based on available clinical trials, pressure support ventilation or T-piece trials appear to be the preferred methods for discontinuation of mechanical ventilatory support. A method using a simple T-piece trial technique is described.

Weaning from ventilatory support

Resumption of spontaneous unassisted breathing after an episode of acute respiratory failure often is achieved without major difficulty. In a significant number of patients however, weaning from mechanical ventilation is a long and difficult process that markedly increases the duration of mechanical ventilation and consumes a significant fraction of critical care resources. Some criteria have been suggested to predict early and more accurately the moment the patient is ready to be separated from the ventilator. At the present time, the f/VT ratio (rapid shallow breathing index) appears to yield the best predictive power. None of these indices, however, is powerful enough to be relied on solely, and their use should be limited to that of aids to the critical care physician. The inability to sustain spontaneous ventilation usually is the consequence of an imbalance between respiratory demand and respiratory muscle capacity. Increased elastic workload, increased resistive workload, and increased VE are the main causes of excessive demand imposed on the respiratory system. Respiratory muscle pump failure usually relates to peripheral nerve dysfunction or muscular dysfunction. Left ventricular dysfunction also is an important cause of weaning failure. The usual methods of weaning from mechanical ventilation are T-piece trials with abrupt definitive discontinuation of mechanical ventilation if tolerated or with progressive intermittent trials, IMV, and PSV. All have their advantages and disadvantages, and the method of weaning per sé is not the only critical factor. Although their conclusions were different regarding the best method of weaning, however, two recent clinical trials suggest that ventilatory management has a major influence on the outcome of weaning from mechanical ventilation in difficult-to-wean patients. The global management of such patients requires a systematic approach with consideration of all factors involved in the process of separation from the ventilator. New computer-assisted systems already are tested and, in the future, may provide a significant advantage in the management of weaning from mechanical ventilation.

Comparison of the effects of pressure support ventilation delivered by three different ventilators during weaning from mechanical ventilation

OBJECTIVE

To compare the effects of pressure support ventilation (PSV) delivered at the same level by three different ventilators on patients' work of breathing (WOB), breathing pattern and gas exchange.

DESIGN

Prospective, self-controlled clinical study.

SETTING

Intensive care unit of a tertiary university hospital.

PATIENTS

Nine intubated adult patients during weaning from mechanical ventilation.

INTERVENTIONS

Patients were randomly connected to one of three ventilators: the Siemens Servo 900 C (SC), the Ohmeda CPU 1 (CPU), and the Engström Erica (EE) during both zero cmH2O PSV and 15 cmH2O PSV.

MEASUREMENTS AND RESULTS

During zero PSV, there was no significant difference in terms of WOB, VT, VE, or auto-PEEP among the three ventilators, although there was a trend towards higher levels of WOB with EE. During 15 cmH2O PSV, WOB was significantly less with SC than with EE or CPU (0.47 +/- 0.48 J/l for SC, 1.0 +/- 0.48 for EE and 0.78 +/- 0.51 for CPU1, p = 0.003). WOB was 64% less than at zero PSV with SC but only 38% less with EE. This was associated with a different pressurization shape, as assessed by the interior surface of Paw-VT loops (1.23 +/- 0.09 J/l for SC, 0.9 +/- 0.02 for EE, and 0.79 +/- 0.18 for CPU; p < 0.001). At 15 cmH2O PSV, auto-PEEP was significantly lower with SC than with EE (1.7 +/- 2.1 cmH2O for SC, 4.7 +/- 3.6 for EE, and 2.8 +/- 0.3 for CPU; p = 0.04). External expiratory resistances, in cmH2O/l/s, were significantly higher with EE than with CPU or SC (12.9 +/- 3.2 EE, 7.5 +/- 2.4 CPU, 5.9 +/- 0.5 SC; p < 0.001).

CONCLUSION

During PSV, the different working principles of different mechanical ventilators profoundly affect patient's WOB. Among the various factors, velocity of pressurization of PSV may play a role in its efficacy in unloading the respiratory muscles.

Relationship between expired capnogram and respiratory system resistance in critically ill patients during total ventilatory support

To examine the relationship of expired capnograms and respiratory system resistance (Rrs) in intubated critically ill patients, we consecutively studied 41 mechanically ventilated patients to (1) analyze the association between expired CO2 slope and auto-positive end-expiratory pressure (auto-PEEP), between Rrs and auto-PEEP, between Rrs and expired CO2 slope, and between Rrs and arterial minus end-tidal PCO2 gradient (PaCO2-PETCO2 gradient) and (2) to investigate the capacity of the expired CO2 slope and PaCO2-PETCO2 gradient to predict Rrs during mechanical ventilation. Regression analysis found a close correlation between Rrs and expired CO2 slope (r = 0.86; p < 0.001), between Rrs and auto-PEEP (r = 0.75; p < 0.001), and between auto-PEEP and expired CO2 slope (r = 0.74; p < 0.001). Weak correlation was found between Rrs and PaCO2-PETCO2 gradient (r = 0.48; p < 0.01). Prediction interval limits at 95 percent confidence level for Rrs are approximately +/- 7.39 cm H2O/L/s from the predicted value obtained by the regression equation, where Rrs = 11.42 + 2.28 expired CO2 slope. These observations suggest that CO2 elimination in critically ill patients is strongly modulated by lung, airway, endotracheal tube, and ventilator equipment resistances. Although continuous capnogram waveform monitoring at the bedside might be useful to assess Rrs, very accurate predictions could be done only in determinate patients.