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Inspiratory muscle training for intensive care patients: A multidisciplinary practical guide for clinicians

Open AccessPublished:July 11, 2018DOI:https://doi.org/10.1016/j.aucc.2018.06.001

      Abstract

      Objectives

      To describe a multidisciplinary approach to inspiratory muscle training (IMT) for patients in the intensive care unit (ICU).

      Background

      Inspiratory muscle weakness is a known consequence of prolonged mechanical ventilation, and there is emerging evidence that specific IMT can ameliorate this weakness. However, IMT is not yet standard practice in many ICUs, possibly because of the wide variety of methods reported and a lack of published practical guidelines. While the optimal parameters for IMT are yet to be established, we share our detailed methodology which has been shown to be safe in selected ventilator-dependent patients and is the only approach which has been shown to increase quality of life in ICU patients.

      Methods

      Patients who have experienced invasive mechanical ventilation for at least 7 days can commence IMT in either the ventilator-dependent phase or when weaned from mechanical ventilation. Intensity should be prescribed based on maximum inspiratory pressure, which is measurable through the tracheostomy or endotracheal tube via the ventilator or a respiratory pressure meter. Using a removable threshold device, we recommend high-intensity training (5 sets of 6 breaths at a minimum of 50% of maximum inspiratory pressure) performed once per day, supervised by the physiotherapist, with intensity increased daily such that patients can only just complete the 6th breath in each set.

      Results

      Using this high-intensity approach, IMT is likely to improve not only inspiratory muscle strength but also quality of life in patients recently weaned from mechanical ventilation of 7 days' duration or longer. Effective IMT requires a multidisciplinary approach to maximise feasibility, with doctors, nurses, and therapists working closely to optimise conditions for successful IMT.

      Conclusions

      This multidisciplinary approach to implement IMT in ICU patients should assist clinicians in translating best-available evidence into practice, with the potential to enhance patient recovery.

      Keywords

      1. Introduction

      Invasive mechanical ventilation (MV) is an essential and life-saving intervention in the intensive care unit (ICU); however, prolonged MV often leads to impaired inspiratory muscle strength and endurance.
      • Bissett B.
      • Leditschke I.A.
      • Neeman T.
      • Boots R.
      • Paratz J.
      Weaned but weary: one third of adult intensive care patients mechanically ventilated for 7 days or more have impaired inspiratory muscle endurance after successful weaning.
      • Chang A.T.
      • Boots R.J.
      • Brown M.G.
      • Paratz J.
      • Hodges P.W.
      Reduced inspiratory muscle endurance following successful weaning from prolonged mechanical ventilation.
      • De Jonghe B.
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      • Durand M.C.
      • Malissin I.
      • Rodrigues P.
      • Cerf C.
      • et al.
      Respiratory weakness is associated with limb weakness and delayed weaning in critical illness.
      • Goligher E.C.
      • Ferguson N.D.
      • Brochard L.J.
      Clinical challenges in mechanical ventilation.
      • Dres M.
      • Dubé B.P.
      • Mayaux J.
      • Delemazure J.
      • Reuter D.
      • Brochard L.
      • et al.
      Coexistence and impact of limb muscle and diaphragm weakness at time of liberation from mechanical ventilation in medical intensive care unit patients.
      Diaphragmatic atrophy occurs rapidly and is detectable in patients after just 18–69 h of controlled MV.
      • Levine S.
      • Nguyen T.
      • Taylor N.
      • Friscia M.E.
      • Budak M.T.
      • Rothenberg P.
      • et al.
      Rapid disuse atrophy of diaphragm fibers in mechanically ventilated humans.
      In patients mechanically ventilated for more than 24 h, diaphragmatic dysfunction is associated with difficult and prolonged weaning.
      • De Jonghe B.
      • Bastuji-Garin S.
      • Durand M.C.
      • Malissin I.
      • Rodrigues P.
      • Cerf C.
      • et al.
      Respiratory weakness is associated with limb weakness and delayed weaning in critical illness.
      • Dres M.
      • Dubé B.P.
      • Mayaux J.
      • Delemazure J.
      • Reuter D.
      • Brochard L.
      • et al.
      Coexistence and impact of limb muscle and diaphragm weakness at time of liberation from mechanical ventilation in medical intensive care unit patients.
      In a study of 124 patients ventilated for more than 24 h, 54% had detectable inspiratory muscle weakness before ventilatory weaning, and inspiratory muscle weakness was independently associated with 1-year mortality.
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      • et al.
      Respiratory weakness after mechanical ventilation is associated with one-year mortality – a prospective study.
      Diaphragmatic atrophy tends to be worse in patients ventilated with controlled modes compared to spontaneous modes and appears to be related to a patient's inspiratory effort whilst ventilator-dependent.
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      • Fan E.
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      Evolution of diaphragm thickness during mechanical ventilation. Impact of inspiratory effort.
      While many Australian ICUs preferentially use more spontaneous modes of ventilation (i.e. pressure support), impaired inspiratory muscle strength remains a persistent problem in a third of patients largely ventilated through spontaneous modes.
      • Bissett B.
      • Leditschke I.A.
      • Neeman T.
      • Boots R.
      • Paratz J.
      Weaned but weary: one third of adult intensive care patients mechanically ventilated for 7 days or more have impaired inspiratory muscle endurance after successful weaning.
      Furthermore, inspiratory muscle weakness is likely to contribute to the elevated dyspnoea observed in one-third of patients recently weaned from prolonged MV.
      • Bissett B.
      • Leditschke I.A.
      • Neeman T.
      • Boots R.
      • Paratz J.
      Weaned but weary: one third of adult intensive care patients mechanically ventilated for 7 days or more have impaired inspiratory muscle endurance after successful weaning.
      Thus inspiratory muscle impairment may negatively affect recovery for many ICU patients.
      Over the past 15 years, inspiratory muscle training (IMT) (i.e. specific strengthening of the inspiratory muscles) has emerged as a promising treatment for ICU patients. A recent systematic review
      • Vorona S.
      • Sabatini U.
      • Al-Maqbali S.
      • Bertoni M.
      • Dres M.
      • Bissett B.
      • et al.
      Inspiratory muscle rehabilitation in critically ill adults: a systematic review and meta-analysis.
      of 28 studies (n = 1184) revealed that IMT improves inspiratory muscle strength in ICU patients and is associated with acceleration of ventilatory weaning. Furthermore, in patients who have failed to wean from MV, IMT may enhance weaning success.
      • Martin A.D.
      • Smith B.K.
      • Davenport P.D.
      • Harman E.
      • Gonzalez-Rothi R.J.
      • Baz M.
      • et al.
      Inspiratory muscle strength training improves weaning outcome in failure to wean patients: a randomized trial.
      Improvements in inspiratory muscle strength, as a result of IMT, have also been associated with improvements in quality of life following just 2 weeks of training in patients recently weaned from MV.
      • Bissett B.M.
      • Leditschke I.A.
      • Neeman T.
      • Boots R.
      • Paratz J.
      Inspiratory muscle training to enhance recovery from mechanical ventilation: a randomised trial.
      Given the reversibility of inspiratory muscle weakness and the potential benefits to patients' quality of life, ICU clinicians are now seeking to translate this evidence into practice.
      Despite this emerging evidence, IMT is not standard practice in most ICUs around the world, as reflected in a recent survey of French physiotherapists.
      • Bonnevie T.
      • Villiot-Dangerd J.
      • Graviera F.
      • Dupuisf J.
      • Prieurc G.
      • Médrinalc C.
      Inspiratory muscle training is used in some intensive care units, but many training methods have uncertain efficacy: a survey of French physiotherapists.
      The challenges of implementing IMT in ICU include the wide variety of training methods reported in the literature
      • Vorona S.
      • Sabatini U.
      • Al-Maqbali S.
      • Bertoni M.
      • Dres M.
      • Bissett B.
      • et al.
      Inspiratory muscle rehabilitation in critically ill adults: a systematic review and meta-analysis.
      • Bonnevie T.
      • Villiot-Dangerd J.
      • Graviera F.
      • Dupuisf J.
      • Prieurc G.
      • Médrinalc C.
      Inspiratory muscle training is used in some intensive care units, but many training methods have uncertain efficacy: a survey of French physiotherapists.
      and a lack of published guidelines regarding implementation in a real-world context. This article aims to describe a practical multidisciplinary approach to IMT in ICU, drawing on the available evidence and more than 10 years' clinical experience with IMT in both ventilator-dependent and recently weaned patients. It is hoped that a detailed description of this approach, which requires medical, nursing, and physiotherapy input, will prove useful to clinicians seeking to implement IMT in their own ICU.

      2. Inspiratory muscle training methods

      2.1 General principles of IMT in ICU patients

      Inspiratory muscle training is a targeted strengthening of the inspiratory muscles through the application of resistance during inspiration. A recent systematic review
      • Vorona S.
      • Sabatini U.
      • Al-Maqbali S.
      • Bertoni M.
      • Dres M.
      • Bissett B.
      • et al.
      Inspiratory muscle rehabilitation in critically ill adults: a systematic review and meta-analysis.
      has shown that IMT in ICU patients has been conducted using whole-body training, resistive loading, or threshold loading. There are no studies that have compared these approaches directly. An important difference between resistive and threshold loading is that resistive loading depends on the flow generated by the patient, making training intensity variable, depending on effort. In contrast, with threshold loading, patients must generate a preset pressure to allow airflow for each breath, and once that threshold is achieved, inspiratory flow is not dependent on patient effort. This means that threshold loading is the easiest of these IMT methods to standardise and prescribe for ICU patients.
      The greatest benefits of IMT in ICU patients have been observed in studies which use a threshold device to provide the loading stimulus
      • Vorona S.
      • Sabatini U.
      • Al-Maqbali S.
      • Bertoni M.
      • Dres M.
      • Bissett B.
      • et al.
      Inspiratory muscle rehabilitation in critically ill adults: a systematic review and meta-analysis.
      • Martin A.D.
      • Smith B.K.
      • Davenport P.D.
      • Harman E.
      • Gonzalez-Rothi R.J.
      • Baz M.
      • et al.
      Inspiratory muscle strength training improves weaning outcome in failure to wean patients: a randomized trial.
      • Cader S.A.
      • Vale R.G.
      • Castro J.C.
      • Bacelar S.C.
      • Biehl C.
      • Gomes M.C.
      • et al.
      Inspiratory muscle training improves maximal inspiratory pressure and may assist weaning in older intubated patients: a randomised trial.
      • Condessa R.L.
      • Brauner J.S.
      • Saul A.L.
      • Baptista M.
      • Silva A.C.
      • Vieira S.R.
      Inspiratory muscle training did not accelerate weaning from mechanical ventilation but did improve tidal volume and maximal respiratory pressures: a randomised trial.
      (Fig. 1), as opposed to ventilator manipulations to provide resistance.
      • Caruso P.
      • Denari S.D.
      • Ruiz S.A.
      • Bernal K.G.
      • Manfrin G.M.
      • Friedrich C.
      • et al.
      Inspiratory muscle training is ineffective in mechanically ventilated critically ill patients.
      Through the spring-loaded threshold mechanism, the prescribing clinician can be sure of the reproducibility of the patient's efforts, facilitating accurate titration of IMT pressures to achieve a strengthening benefit.
      • Vorona S.
      • Sabatini U.
      • Al-Maqbali S.
      • Bertoni M.
      • Dres M.
      • Bissett B.
      • et al.
      Inspiratory muscle rehabilitation in critically ill adults: a systematic review and meta-analysis.
      Figure 1
      Fig. 1Spring-loaded inspiratory muscle training device.
      Training regimes for IMT vary widely across different patient populations (e.g. patients with chronic obstructive pulmonary disease and
      • Hill K.
      • Cecins N.M.
      • Eastwood P.R.
      • Jenkins S.C.
      Inspiratory muscle training for patients with chronic obstructive pulmonary disease: a practical guide for clinicians.
      chronic heart failure
      • Dall'Ago P.
      • Chiappa G.R.
      • Guths H.
      • Stein R.
      • Ribeiro J.P.
      Inspiratory muscle training in patients with heart failure and inspiratory muscle weakness: a randomized trial.
      ) and athletes
      • Johnson M.A.
      • Sharpe G.R.
      • Brown P.I.
      Inspiratory muscle training improves cycling time-trial performance and anaerobic work capacity but not critical power.
      • Volianitis S.
      • McConnell A.K.
      • Koutedakis Y.
      • McNaughton L.
      • Backx K.
      • Jones D.A.
      Inspiratory muscle training improves rowing performance.
      , including both high-intensity interval training
      • Martin A.D.
      • Smith B.K.
      • Davenport P.D.
      • Harman E.
      • Gonzalez-Rothi R.J.
      • Baz M.
      • et al.
      Inspiratory muscle strength training improves weaning outcome in failure to wean patients: a randomized trial.
      and endurance-focused training (e.g. breathing against lower resistance for 20–30 min
      • Dall'Ago P.
      • Chiappa G.R.
      • Guths H.
      • Stein R.
      • Ribeiro J.P.
      Inspiratory muscle training in patients with heart failure and inspiratory muscle weakness: a randomized trial.
      ). Many ICU patients are not capable of maintaining resisted inspiration over several minutes, as described in IMT protocols for other patient groups,
      • Hill K.
      • Cecins N.M.
      • Eastwood P.R.
      • Jenkins S.C.
      Inspiratory muscle training for patients with chronic obstructive pulmonary disease: a practical guide for clinicians.
      • Dall'Ago P.
      • Chiappa G.R.
      • Guths H.
      • Stein R.
      • Ribeiro J.P.
      Inspiratory muscle training in patients with heart failure and inspiratory muscle weakness: a randomized trial.
      and in our experience, the high-intensity interval approach
      • Martin A.D.
      • Smith B.K.
      • Davenport P.D.
      • Harman E.
      • Gonzalez-Rothi R.J.
      • Baz M.
      • et al.
      Inspiratory muscle strength training improves weaning outcome in failure to wean patients: a randomized trial.
      • Bissett B.M.
      • Leditschke I.A.
      • Neeman T.
      • Boots R.
      • Paratz J.
      Inspiratory muscle training to enhance recovery from mechanical ventilation: a randomised trial.
      • Cader S.A.
      • Vale R.G.
      • Castro J.C.
      • Bacelar S.C.
      • Biehl C.
      • Gomes M.C.
      • et al.
      Inspiratory muscle training improves maximal inspiratory pressure and may assist weaning in older intubated patients: a randomised trial.
      • Condessa R.L.
      • Brauner J.S.
      • Saul A.L.
      • Baptista M.
      • Silva A.C.
      • Vieira S.R.
      Inspiratory muscle training did not accelerate weaning from mechanical ventilation but did improve tidal volume and maximal respiratory pressures: a randomised trial.
      is ideal. This approach uses few repetitions at the highest manageable resistance to briefly load the inspiratory muscles, while also allowing rests and recovery between sets to maximise tolerability. Of all the approaches described in the recent systematic review
      • Vorona S.
      • Sabatini U.
      • Al-Maqbali S.
      • Bertoni M.
      • Dres M.
      • Bissett B.
      • et al.
      Inspiratory muscle rehabilitation in critically ill adults: a systematic review and meta-analysis.
      of IMT in ICU patients, our approach uses the highest intensity (i.e. minimum of 50% of maximum inspiratory pressure), and this is safe and well-tolerated in carefully selected ICU patients.
      Inspiratory muscle training can be performed in both ventilator-dependent and independently breathing ICU patients and ideally is conducted with the patient in an upright sitting position. As the technicalities of application differ between these two groups, they will be described separately.

      2.2 Ventilator-dependent patients

      2.2.1 Patient selection

      There are several factors to consider in determining whether IMT is appropriate for an ICU patient, and these factors are summarised in Fig. 2.
      Figure 2
      Fig. 2Patient selection for inspiratory muscle training in ICU. ICU = intensive care unit; PEEP = positive end expiratory pressure; FiO2 = fraction of inspired oxygen; RR = respiratory rate. * Recently weaned means independently breathing for 24 h per day without any invasive ventilatory support.
      As patients need to actively participate in IMT, alertness and cooperation are essential. Patients need to be awake enough to comprehend the purpose of the intermittent loading so that they perceive it as a temporary training stimulus, as opposed to a sign of deterioration or a source of distress. Thus, minimisation of sedation is a crucial component of the multidisciplinary approach to IMT in ICU, and based on our experience,
      • Bissett B.M.
      • Leditschke I.A.
      • Neeman T.
      • Boots R.
      • Paratz J.
      Inspiratory muscle training to enhance recovery from mechanical ventilation: a randomised trial.
      • Bissett B.
      • Leditschke I.A.
      • Green M.
      Specific inspiratory muscle training is safe in selected patients who are ventilator-dependent: a case series.
      • Bissett B.
      • Leditschke I.A.
      Inspiratory muscle training to enhance weaning from mechanical ventilation.
      alertness should be titrated to a Riker Sedation Agitation Score
      • Riker R.R.
      • Picard J.T.
      • Fraser G.L.
      Prospective evaluation of the sedation-agitation scale for adult critically ill patients.
      of 4 (or equivalent).
      Artificial airways do not preclude IMT as training is feasible and safe in selected patients with either an endotracheal tube (ETT) or tracheostomy in situ. As threshold-based IMT necessitates disconnection from MV, patients must not be dependent on high levels of positive end expiratory pressure (PEEP) as disconnection is highly likely to result in de-recruitment and atelectasis. However, for medically stable patients with PEEP levels ≤10 cmH2O and a fraction of inspired oxygen <0.60, our approach to IMT does not result in clinically or statistically significant changes in oxygen saturation or respiratory rate,
      • Bissett B.
      • Leditschke I.A.
      • Green M.
      Specific inspiratory muscle training is safe in selected patients who are ventilator-dependent: a case series.
       suggesting that atelectasis is unlikely in this group of patients. Owing to other risks of disconnection, IMT is not feasible for patients undergoing nitric oxide therapy, nebulised prostacyclin, or high frequency oscillation. However, in our experience, IMT is safe with patients admitted with a primary diagnosis of chronic obstructive pulmonary disease (COPD) (including those with a degree of auto-PEEP), as long as they also satisfy the criteria described in Fig. 2.
      The patient's baseline respiratory rate should be stable at less than 25 breaths per minute, as we have found that patients will struggle to maintain a respiratory rate faster than this during loaded breathing (where the inspiratory valve is open for at least 1 s
      • Bissett B.
      • Leditschke I.A.
      Inspiratory muscle training to enhance weaning from mechanical ventilation.
      • Sprague S.S.
      • Hopkins P.D.
      Use of inspiratory strength training to wean six patients who were ventilator-dependent.
      ). Haemodynamic parameters should also be stable, as IMT could theoretically cause transient changes to intrathoracic pressure which may affect venous return, although there are no data to directly support the clinical impact of this risk. In patients with stable blood pressure and heart rate, IMT does not cause statistically or clinically significant changes in either parameters.
      • Bissett B.
      • Leditschke I.A.
      • Green M.
      Specific inspiratory muscle training is safe in selected patients who are ventilator-dependent: a case series.
      In terms of success with weaning from MV, patients most likely to benefit from IMT are those who have failed to wean via usual methods such as progressively longer t-piece trials over a period of days or weeks.
      • Martin A.D.
      • Smith B.K.
      • Davenport P.D.
      • Harman E.
      • Gonzalez-Rothi R.J.
      • Baz M.
      • et al.
      Inspiratory muscle strength training improves weaning outcome in failure to wean patients: a randomized trial.
      • Elbouhy M.
      • AbdelHalim H.
      • Hashem A.
      Effect of respiratory muscles training in weaning of mechanically ventilated COPD patients.
      • Jubran A.
      • Grant B.J.
      • Duffner L.A.
      • Collins E.G.
      • Lanuza D.M.
      • Hoffman L.A.
      • et al.
      Effect of pressure support vs unassisted breathing through a tracheostomy collar on weaning duration in patients requiring prolonged mechanical ventilation: a randomized trial.
      Although inspiratory muscle weakness may be evident as early as 24–48 h following commencement of MV,
      • Dres M.
      • Dubé B.P.
      • Mayaux J.
      • Delemazure J.
      • Reuter D.
      • Brochard L.
      • et al.
      Coexistence and impact of limb muscle and diaphragm weakness at time of liberation from mechanical ventilation in medical intensive care unit patients.
      • Levine S.
      • Nguyen T.
      • Taylor N.
      • Friscia M.E.
      • Budak M.T.
      • Rothenberg P.
      • et al.
      Rapid disuse atrophy of diaphragm fibers in mechanically ventilated humans.
      very early IMT (i.e. from 48 h) has only shown minimal benefit in terms of weaning success, despite modest improvements in inspiratory muscle strength.
      • Cader S.A.
      • Vale R.G.
      • Castro J.C.
      • Bacelar S.C.
      • Biehl C.
      • Gomes M.C.
      • et al.
      Inspiratory muscle training improves maximal inspiratory pressure and may assist weaning in older intubated patients: a randomised trial.
      • Condessa R.L.
      • Brauner J.S.
      • Saul A.L.
      • Baptista M.
      • Silva A.C.
      • Vieira S.R.
      Inspiratory muscle training did not accelerate weaning from mechanical ventilation but did improve tidal volume and maximal respiratory pressures: a randomised trial.
      Clearly some patients will wean successfully from MV without any need for IMT. However, many ICU patients who successfully wean from MV of ≥7 days have ongoing deficits in inspiratory muscle strength and endurance
      • Bissett B.
      • Leditschke I.A.
      • Neeman T.
      • Boots R.
      • Paratz J.
      Weaned but weary: one third of adult intensive care patients mechanically ventilated for 7 days or more have impaired inspiratory muscle endurance after successful weaning.
      • Chang A.T.
      • Boots R.J.
      • Brown M.G.
      • Paratz J.
      • Hodges P.W.
      Reduced inspiratory muscle endurance following successful weaning from prolonged mechanical ventilation.
      which can manifest as elevated dyspnoea both at rest and during exercise.
      • Bissett B.
      • Leditschke I.A.
      • Neeman T.
      • Boots R.
      • Paratz J.
      Weaned but weary: one third of adult intensive care patients mechanically ventilated for 7 days or more have impaired inspiratory muscle endurance after successful weaning.
      From this perspective, our practice is to consider IMT for any ICU patient from Day 7 of MV, with reassessment every day thereafter if not initially suitable.
      There are several conditions where IMT would not be appropriate for ICU patients, for example, in those who are acutely deteriorating, experiencing severe pain or dyspnoea, or who have shifted to a palliative treatment approach.
      • Bissett B.M.
      • Leditschke I.A.
      • Paratz J.D.
      • Boots R.J.
      Protocol: inspiratory muscle training for promoting recovery and outcomes in ventilated patients (IMPROVe): a randomised controlled trial.
      As studies of IMT to date have not included pregnant women, it is not possible to comment on the suitability of IMT for this subgroup of ICU patients, and the appropriateness of IMT would need to be discussed by the multidisciplinary team in terms of potential benefits and risks for the individual patient and fetus.

      2.2.2 Equipment

      A commercially available spring-loaded threshold IMT device can usually provide an appropriate training resistance for ICU patients, ranging between 9 and 41 cmH2O (Fig. 1). This IMT device can be connected to an ETT or tracheostomy via a flexible or rigid connector, either directly or via an inline suction circuit (Fig. 3). For patients with an elevated sputum load, it is advisable to maintain the suction circuit in situ, as suctioning may be required between IMT sets due to rapid pressure changes and sputum dislodgement.
      Figure 3
      Fig. 3IMT device connected to endotracheal tube with inline suction in situ. IMT = inspiratory muscle training.
      The spring-based simple IMT device is single-patient use and does not require maintenance and specific cleaning beyond rinsing and air-drying the mouthpiece. In our experience, the only risk to device integrity is that if the intensity is inadvertently dialled above 41 cmH2O, the spring is liable to break, and the whole device will need to be replaced.
      If available, a respiratory pressure meter can be used to determine the patient's baseline maximal inspiratory pressure (MIP). Using a flexible connector, the device can be attached directly to the ETT or tracheostomy (Fig. 4), and the patient can inhale maximally from residual volume. The instruction “empty your lungs” may be helpful in achieving an accurate measurement, ensuring the device is attached to the ETT or tracheostomy just after the lungs have been “emptied”. The patient should be coached to inhale as forcefully as possible, and in line with established best practice, this MIP measurement should be repeated three times to ensure consistency.
      • Green M.
      • Road J.
      • Sieck G.
      • Similowski T.
      Tests of respiratory muscle strength. ATS/ERS statement on respiratory muscle testing.
      In ICU patients, MIP measurement may be a fatiguing process, and several minutes' rest on MV may be required between attempts.
      Figure 4
      Fig. 4Respiratory pressure meter connected to endotracheal tube via connector tube.
      If a respiratory pressure meter is not available, another option is to measure the “negative inspiratory force” (NIF) through the ventilator settings menu (often available under “special procedures” or “lung mechanics”) (Fig. 5). Similarly, this measurement should be from as close to residual volume as possible, but note that this will not be truly possible for patients with PEEP >0 cmH2O. Nonetheless, if the patient can produce their maximal effort, the NIF score may provide a reproducible surrogate value of inspiratory muscle strength which will be useful in titrating IMT intensity.
      Figure 5
      Fig. 5Negative inspiratory force (NIF) measurement through the ventilator.

      2.2.3 Training parameters/prescription

      As ICU patients often have very weak inspiratory muscles (e.g. MIP of approximately 30–35 cm H2O when ventilated 7 days or longer
      • Bissett B.
      • Leditschke I.A.
      • Neeman T.
      • Boots R.
      • Paratz J.
      Weaned but weary: one third of adult intensive care patients mechanically ventilated for 7 days or more have impaired inspiratory muscle endurance after successful weaning.
      • De Jonghe B.
      • Bastuji-Garin S.
      • Durand M.C.
      • Malissin I.
      • Rodrigues P.
      • Cerf C.
      • et al.
      Respiratory weakness is associated with limb weakness and delayed weaning in critical illness.
      ), it is unlikely that they will tolerate sustained loading of the respiratory muscles without distressing fatigue. To maximise benefits and minimise distress, a high-intensity low-repetition approach has been used successfully in numerous trials.
      • Martin A.D.
      • Smith B.K.
      • Davenport P.D.
      • Harman E.
      • Gonzalez-Rothi R.J.
      • Baz M.
      • et al.
      Inspiratory muscle strength training improves weaning outcome in failure to wean patients: a randomized trial.
      • Cader S.A.
      • Vale R.G.
      • Castro J.C.
      • Bacelar S.C.
      • Biehl C.
      • Gomes M.C.
      • et al.
      Inspiratory muscle training improves maximal inspiratory pressure and may assist weaning in older intubated patients: a randomised trial.
      • Condessa R.L.
      • Brauner J.S.
      • Saul A.L.
      • Baptista M.
      • Silva A.C.
      • Vieira S.R.
      Inspiratory muscle training did not accelerate weaning from mechanical ventilation but did improve tidal volume and maximal respiratory pressures: a randomised trial.
      An intensity of at least 50% of MIP is well-tolerated, although in practice this should be considered the minimum training threshold, with intensity titrated upwards to the highest tolerated level where the patient can just complete the 6th breath in a set of six breaths.
      • Martin A.D.
      • Smith B.K.
      • Davenport P.D.
      • Harman E.
      • Gonzalez-Rothi R.J.
      • Baz M.
      • et al.
      Inspiratory muscle strength training improves weaning outcome in failure to wean patients: a randomized trial.
      • Bissett B.M.
      • Leditschke I.A.
      • Paratz J.D.
      • Boots R.J.
      Protocol: inspiratory muscle training for promoting recovery and outcomes in ventilated patients (IMPROVe): a randomised controlled trial.
      Given this high level of intensity, only few repetitions are required, and a set of six breaths is achievable. A total package of five sets of six breaths daily (with rest days on weekends) is well-tolerated, and in carefully selected patients, it is unlikely to result in any short-term deterioration in oxygen saturation, respiratory rate, heart rate, or blood pressure, even without supplemental oxygen during training. It is important that the clinician supervising IMT returns the patient to normal ventilatory support between sets and allows adequate rest and recovery before attempting the next set. In our experience, most patients only require 1–2 min; however, this rest period may be longer in very weak or highly anxious patients.
      If baseline MIP (or NIF) is not readily available, it is still possible to titrate IMT intensity via a trial-and-error method, commencing at a relatively low intensity (e.g. 9–15 cm H2O) and gradually increasing intensity until the patient can just complete the 6th breath in each set. In this case, the first IMT session is likely to expend all the patient's energy in establishing the correct intensity, and training on the subsequent day should commence at the pre-established intensity.
      As the patient's strength improves over time, the intensity should be increased incrementally to continue to provide the maximum tolerable training load. In our experience, this is typically in the magnitude of 1–2 cmH2O every 1–2 days. Thus if a patient commences IMT at 15 cmH2O, they may be training at 25 cmH2O within 1 week. If the patient experiences a complication such as ventilator-associated pneumonia, where lung function is compromised by sputum load, it may be necessary to reduce the intensity or cease training for a few days and re-establish the appropriate threshold for training once the complication has resolved and the eligibility criteria are met again.

      2.2.4 Trouble-shooting

      Not surprisingly, IMT may be particularly challenging for extremely weak patients, particularly those whose MIP is less than 18 cmH2O. For these patients, commercially available threshold devices can be difficult if not impossible to use, as they are incapable of generating the pressure required to open the valve at the lowest setting for six breaths. For these patients, we have had some success with commencing with a sham IMT device of sorts, where they breathe for six breaths through just the flexible connector tube attached to their ETT or tracheostomy, providing only the slightest resistance. Once patients can manage five sets of six breaths at this level for several days, we can commence with just a few breaths at the lowest setting on the IMT device (i.e. 9 cmH2O) and gradually build up to a full 6-breath set.
      Timing of IMT is crucial with respect to ventilatory weaning. For patients undergoing spontaneous breathing trials (SBT), we have found that IMT is best scheduled either during the ventilator-dependent period, or at the end of the SBT, such that the patient can return to the ventilator for a complete rest at the end of the 5th set. It is essential that if the patient is breathing with a deflated tracheostomy cuff during the SBT, this cuff is reinflated during IMT to avoid air leaks and ensure accurate training intensity. Furthermore, if the tracheostomised patient is using a speaking valve during their SBT, this valve must be removed, and the tracheostomy inner cannula must be changed to a non-fenestrated tube, and the tracheostomy tube cuff must be reinflated, otherwise there will be excessive air leak around the tracheostomy tube and training pressures will not be reliably achieved. Clearly, close collaboration between the clinician supervising IMT and the bedside nurse in ICU is essential.
      Another challenge of introducing IMT concerns the timing of other rehabilitation interventions. If patients are also completing whole-body exercise activities, we suggest scheduling IMT earlier in the day, allowing a rest period before the next exercise session. If patients are exhausted by whole-body exercise, they are unlikely to have the stamina and concentration required for optimal high-intensity IMT.
      Very occasionally, for patients breathing through a humidified ventilator circuit, the valve in the IMT device can become water-logged and stuck within the chamber. If this is the case, the device should be replaced as it will not be providing the preset threshold pressure expected.
      In accordance with previous recommendations regarding safety of physiotherapy interventions in ICU,
      • Zeppos L.
      • Patman S.
      • Berney S.
      • Adsett J.A.
      • Bridson J.M.
      • Paratz J.D.
      Physiotherapy in intensive care is safe: an observational study.
      IMT should be ceased should any of the following occur: alteration in blood pressure > or < or >20% resting, new arrhythmia, oxygen desaturation >10%, pulmonary artery pressure (systolic) >60 mmHg, suspected pneumothorax, and agitation risking detachment of equipment or lines or requiring increased sedation.
      • Bissett B.M.
      • Leditschke I.A.
      • Paratz J.D.
      • Boots R.J.
      Protocol: inspiratory muscle training for promoting recovery and outcomes in ventilated patients (IMPROVe): a randomised controlled trial.
      However, observational data have demonstrated that these events are extremely unlikely in ICU patients who meet the eligibility criteria described in Fig. 2.
      • Bissett B.
      • Leditschke I.A.
      • Green M.
      Specific inspiratory muscle training is safe in selected patients who are ventilator-dependent: a case series.

      2.3 Independently breathing patients

      2.3.1 Patient selection

      Patients who have weaned from MV and are breathing independently may have commenced IMT while still ventilator-dependent or may have been unable to participate during MV. In some studies, more than 80% of ventilator-dependent patients cannot participate in IMT for various reasons, most commonly because of low levels of consciousness or delirium.
      • Bissett B.M.
      • Leditschke I.A.
      • Neeman T.
      • Boots R.
      • Paratz J.
      Inspiratory muscle training to enhance recovery from mechanical ventilation: a randomised trial.
      • Bissett B.M.
      • Leditschke I.A.
      • Paratz J.D.
      • Boots R.J.
      Protocol: inspiratory muscle training for promoting recovery and outcomes in ventilated patients (IMPROVe): a randomised controlled trial.
      Nonetheless, many patients who have successfully weaned from MV typically have residual inspiratory muscle weakness, which may manifest as dyspnoea at rest or during exercise.
      • Bissett B.
      • Leditschke I.A.
      • Neeman T.
      • Boots R.
      • Paratz J.
      Weaned but weary: one third of adult intensive care patients mechanically ventilated for 7 days or more have impaired inspiratory muscle endurance after successful weaning.
      This is particularly the case for patients who have experienced MV for 7 days or longer, where both inspiratory and limb weakness are known to coexist.
      • Bissett B.
      • Leditschke I.A.
      • Neeman T.
      • Boots R.
      • Paratz J.
      Weaned but weary: one third of adult intensive care patients mechanically ventilated for 7 days or more have impaired inspiratory muscle endurance after successful weaning.
      • De Jonghe B.
      • Bastuji-Garin S.
      • Durand M.C.
      • Malissin I.
      • Rodrigues P.
      • Cerf C.
      • et al.
      Respiratory weakness is associated with limb weakness and delayed weaning in critical illness.
      • Dres M.
      • Dubé B.P.
      • Mayaux J.
      • Delemazure J.
      • Reuter D.
      • Brochard L.
      • et al.
      Coexistence and impact of limb muscle and diaphragm weakness at time of liberation from mechanical ventilation in medical intensive care unit patients.
      A recent trial demonstrated that in patients who had recently weaned from MV, just 2 weeks of daily IMT improved not only inspiratory muscle strength but also quality of life.
      • Bissett B.M.
      • Leditschke I.A.
      • Neeman T.
      • Boots R.
      • Paratz J.
      Inspiratory muscle training to enhance recovery from mechanical ventilation: a randomised trial.
      Thus patients who have recently weaned from MV of 7 days or more are ideal candidates for IMT.
      As for ventilator-dependent patients, independently breathing patients need to be alert and able to participate during IMT and should be medically stable without excessive pain or dyspnoea interfering with their breathing capacity. Owing to the use of a mouthpiece in the independently breathing population, patients also need to be able to maintain an adequate lip seal, and thus patients with bulbar dysfunction (e.g. due to stroke) or severe burns around the lips and mouth would not be appropriate for IMT (unless there is a tracheostomy in situ). Consistent with other guidelines, IMT in independently breathing patients is also not recommended for patients with high risk of pneumothorax or spontaneous rib fractures or within 12 months of lung surgery because of the high intrathoracic pressures generated during training.
      • Hill K.
      • Cecins N.M.
      • Eastwood P.R.
      • Jenkins S.C.
      Inspiratory muscle training for patients with chronic obstructive pulmonary disease: a practical guide for clinicians.

      2.3.2 Equipment

      In the independently breathing patient, the same commercially available IMT device described previously can be used; however, in this case, the mouthpiece and nose clip are essential (unless the patient is breathing through a tracheostomy).
      A respiratory pressure meter can be used to determine the baseline MIP before commencing training, but rather than attaching the meter to an ETT or tracheostomy, the patient inhales through a disposable mouthpiece attached to the pressure meter. This MIP measurement should occur from as close to residual volume as possible and be repeated three times for reliability.
      • Green M.
      • Road J.
      • Sieck G.
      • Similowski T.
      Tests of respiratory muscle strength. ATS/ERS statement on respiratory muscle testing.

      2.3.3 Training parameters/prescription

      Based on the randomised trial showing benefits for both strength and quality of life, patients should commence training at an intensity of at least 50% of MIP and complete five sets of six breaths daily (with rest days on weekends).
      • Bissett B.M.
      • Leditschke I.A.
      • Neeman T.
      • Boots R.
      • Paratz J.
      Inspiratory muscle training to enhance recovery from mechanical ventilation: a randomised trial.
      Patients should train at the highest tolerable intensity where they can just complete the 6th breath in each set. Where MIP measurement is unavailable, an appropriate IMT intensity can still be titrated from the patient's ability to perform six breaths in a row.
      As strength improves, the intensity should be incrementally increased over days and weeks to ensure that training stimulus remains adequate. This is likely to be approximately 1–2 cm H2O every few days, but may be faster in some patients. Training should continue for at least 2 weeks to achieve benefits in terms of quality of life.
      • Bissett B.M.
      • Leditschke I.A.
      • Neeman T.
      • Boots R.
      • Paratz J.
      Inspiratory muscle training to enhance recovery from mechanical ventilation: a randomised trial.
      While there is currently no evidence to guide training cessation in ICU patients, we suggest continuing until the patient achieves normal MIP scores for their age and gender [males = 120 − (0.41 × age); females = 108 – (0.61 × age)].
      • Evans J.A.
      • Whitelaw W.A.
      The assessment of maximal respiratory mouth pressures in adults.
      Extrapolating from evidence of IMT in patients with chronic lung disease, benefits are likely to be optimal with 6–8 weeks of training,
      • Hill K.
      • Cecins N.M.
      • Eastwood P.R.
      • Jenkins S.C.
      Inspiratory muscle training for patients with chronic obstructive pulmonary disease: a practical guide for clinicians.
      thus ongoing IMT may be beneficial well beyond the ICU admission.

      2.3.4 Trouble-shooting

      In patients who have weaned from MV and can continue training for several weeks, there is the risk of a ceiling effect as they may reach the training limit of the IMT device (i.e. 41 cmH2O). However, this training intensity is equivalent to 50% of 82 cmH2O, which would be within the normal predicted range for MIP in most patients.
      • Evans J.A.
      • Whitelaw W.A.
      The assessment of maximal respiratory mouth pressures in adults.
      In this situation, it is likely that the patient has returned to their baseline inspiratory muscle strength. Nonetheless, electronic IMT devices could provide higher training pressures, and research is needed to determine whether there are any advantages to increasing inspiratory muscle strength further in ICU survivors.

      2.4 Role of the medical team

      The success of IMT in ICU patients is critically dependent on the engagement of the medical team. As patients need to actively participate, the medical team will need to minimise sedation which interferes with the patient's ability to comprehend and perform the training. The medical team may also be valuable in recognising a patient who is failing to wean from MV or who has recently weaned from prolonged MV and referring these patients to physiotherapy or respiratory therapy staff for IMT.
      Furthermore, an appreciation of the nature of IMT as a potentially fatiguing stimulus will allow the medical team to prescribe ventilatory weaning plans which incorporate IMT at suitable time frames (e.g. towards the end of a SBT, rather than at the beginning). Clearly close collaboration between doctors and the rest of the multidisciplinary team is essential for the success of IMT in ICU patients.

      2.5 Role of the ICU nurse

      The beside nurse coordinates many therapies for the ICU patient, and the nurse's understanding of the value and importance of IMT will be critical to its success. The ICU nurse plays a pivotal role in optimising sedation levels to maximise alertness and facilitate IMT. Although an IMT treatment session only takes around 10–15 min, it should occur when a patient is well-positioned (i.e. high sitting), calm, and stable so that they can concentrate on training. Thus, the ICU nurse will need to liaise with the therapist as to the best time of day for IMT to occur, which may be after the patient has been hoisted into a chair or had a bed bath or procedure.
      The ICU nurse may also directly assist with sputum clearance in ventilator-dependent patients before IMT, as sputum plugging is likely to interfere with training. For patients with a large sputum load, the ICU nurse may work closely with the therapist during IMT to provide suction as required, as the rapid changes in pressure may shift secretions unexpectedly. Furthermore, the nurse can anticipate the additional alarms that are likely to sound during IMT if the patient is being removed from the ventilator and reconnected repeatedly and work closely with the therapist to minimise the disruptive impact of these.
      As the ICU nurse is largely responsible for the success of implementing a ventilatory weaning plan, their understanding of the role and effects of IMT is also crucial. As IMT is providing a high intensity stimulus, some patients may take several minutes to recover their stable breathing pattern following training.
      • Cader S.A.
      • Vale R.G.
      • Castro J.C.
      • Bacelar S.C.
      • Biehl C.
      • Gomes M.C.
      • et al.
      Inspiratory muscle training improves maximal inspiratory pressure and may assist weaning in older intubated patients: a randomised trial.
      In this context, ICU nurses are best placed to provide reassurance about the strengthening value of respiratory training, and this reassurance should reduce patient anxiety. The ICU nurse can also use their judgement about the need to return to ventilatory support should a patient's respiratory effort necessitate this. Thus, the ICU nurse and therapist work very closely to facilitate IMT in ICU patients, particularly with those who are ventilator-dependent.

      2.6 Role of the physiotherapist or respiratory therapist

      In studies of IMT which have demonstrated benefits in ICU patients, IMT has been performed directly under the supervision of a physiotherapist
      • Bissett B.M.
      • Leditschke I.A.
      • Neeman T.
      • Boots R.
      • Paratz J.
      Inspiratory muscle training to enhance recovery from mechanical ventilation: a randomised trial.
      • Cader S.A.
      • Vale R.G.
      • Castro J.C.
      • Bacelar S.C.
      • Biehl C.
      • Gomes M.C.
      • et al.
      Inspiratory muscle training improves maximal inspiratory pressure and may assist weaning in older intubated patients: a randomised trial.
      • Condessa R.L.
      • Brauner J.S.
      • Saul A.L.
      • Baptista M.
      • Silva A.C.
      • Vieira S.R.
      Inspiratory muscle training did not accelerate weaning from mechanical ventilation but did improve tidal volume and maximal respiratory pressures: a randomised trial.
      • Bissett B.
      • Leditschke I.A.
      Inspiratory muscle training to enhance weaning from mechanical ventilation.
      or physical or respiratory therapist.
      • Martin A.D.
      • Smith B.K.
      • Davenport P.D.
      • Harman E.
      • Gonzalez-Rothi R.J.
      • Baz M.
      • et al.
      Inspiratory muscle strength training improves weaning outcome in failure to wean patients: a randomized trial.
      • Sprague S.S.
      • Hopkins P.D.
      Use of inspiratory strength training to wean six patients who were ventilator-dependent.
      • Martin A.D.
      • Davenport P.D.
      • Franceschi A.C.
      • Harman E.
      Use of inspiratory muscle strength training to facilitate ventilator weaning: a series of 10 consecutive patients.
      With a strong understanding of both respiratory pathophysiology and exercise training principles, these health professionals are well-skilled to initiate and progress IMT in ICU patients. Furthermore, as IMT may cause rapid changes in intrathoracic pressure and potentially shift sputum within the airways, these therapists are also skilled in monitoring and treating any side-effects of IMT as they arise. Therapists can also consider the timing of IMT in relation to other whole-body exercise interventions and ensure that fatiguing workloads are spread across the day for an ICU patient.
      Although IMT will be initiated and performed by the physiotherapist or respiratory therapist, it is our experience that successful IMT requires a genuinely multidisciplinary approach. This is consistent with other rehabilitation interventions in ICU patients as described elsewhere.
      • Green M.
      • Marzano V.
      • Leditschke I.
      • Mitchell I.
      • Bissett B.
      Mobilization of intensive care patients: a multidisciplinary practical guide for clinicians.

      3. Discussion

      This is the first practical guide for clinicians which specifically focuses on IMT in ICU patients. This guide is informed by evidence which has emerged over the past 15 years,
      • Vorona S.
      • Sabatini U.
      • Al-Maqbali S.
      • Bertoni M.
      • Dres M.
      • Bissett B.
      • et al.
      Inspiratory muscle rehabilitation in critically ill adults: a systematic review and meta-analysis.
      including randomised trials of IMT in both ventilator-dependent and independently breathing ICU patients. It is also underpinned by more than 10 years of experience with using IMT as a multidisciplinary team in a mixed ICU in a large teaching hospital,
      • Bissett B.
      • Leditschke I.A.
      • Neeman T.
      • Boots R.
      • Paratz J.
      Weaned but weary: one third of adult intensive care patients mechanically ventilated for 7 days or more have impaired inspiratory muscle endurance after successful weaning.
      • Bissett B.M.
      • Leditschke I.A.
      • Neeman T.
      • Boots R.
      • Paratz J.
      Inspiratory muscle training to enhance recovery from mechanical ventilation: a randomised trial.
      • Bissett B.
      • Leditschke I.A.
      Inspiratory muscle training to enhance weaning from mechanical ventilation.
      • Bissett B.
      • Leditschke I.
      • Paratz J.
      • Boots R.
      Respiratory dysfunction in ventilated patients: can inspiratory muscle training help?.
      and the importance of multidisciplinary collaboration in the success of this intervention cannot be emphasised enough.
      A limitation of this guide is that the simple device suggested only provides training pressures from 9 to 41 cmH2O. For some very weak patients, 9 cmH2O will be unachievable, and although some alternative strategies have been suggested in this case, these problems may be overcome with electronic devices which can provide a much broader range of training pressures. These would also be advantageous for overcoming the ceiling effect of the simple device, as patients could also train at much higher pressures. While a single pilot study reports favourable outcomes in tracheostomised patients,
      • Tonella R.M.
      • Ratti L.D.S.R.
      • Delazari L.E.B.
      • Junior C.F.
      • Da Silva P.L.
      • Herran A.R.D.S.
      • et al.
      Inspiratory muscle training in the intensive care unit: a new perspective.
      future studies should also explore the utility and feasibility of electronic IMT devices through ETTs and in ICU patients following successful ventilatory weaning, although the cost and infection control requirements of such devices would need to be considered.
      Another limitation of this guide is that there is very little evidence to inform clinicians about the long-term effects of IMT in ICU patients. There is some evidence that long-term IMT can reduce the risk of hospitalisation in patients with chronic lung disease,
      • Beckerman M.
      • Magadle R.
      • Weiner M.
      • Weiner P.
      The effects of 1 year of specific inspiratory muscle training in patients with COPD.
      although the pattern of training employed in this study is very different from the high intensity interval training recommended for ICU patients. Future studies should explore whether IMT has any effects on hospital or ICU readmission rates and whether the quality of life benefits reported in the short term (i.e. within 2 weeks)
      • Bissett B.M.
      • Leditschke I.A.
      • Neeman T.
      • Boots R.
      • Paratz J.
      Inspiratory muscle training to enhance recovery from mechanical ventilation: a randomised trial.
      have any impact on outcomes beyond the hospital stay. Furthermore, the relationship between IMT and other rehabilitation strategies (e.g. early mobilisation) is not well-understood in ICU patients. This relationship deserves further investigation as the balance between specific and whole-body training might be important in optimising benefits or avoiding counterproductive training in the longer term.
      Although all the evidence to date has examined the effect of supervised IMT in ICU patients, it is also possible that patients could benefit from continuing IMT independently following discharge from hospital, particularly if they are discharged before 6 weeks. Most IMT guidelines in other populations recommend at least 6–8 weeks of IMT to maximise benefits, with some recommending a maintenance program of two sessions per week for life to maintain benefits in the longer term (e.g. for patients with chronic obstructive pulmonary disease).
      • Hill K.
      • Cecins N.M.
      • Eastwood P.R.
      • Jenkins S.C.
      Inspiratory muscle training for patients with chronic obstructive pulmonary disease: a practical guide for clinicians.
      While future research should explore the feasibility of unsupervised IMT in ICU survivors, it seems reasonable to encourage patients who have made good progress to continue their training beyond hospital discharge.
      We have presented only one approach (high-intensity IMT using a threshold device), and the ideal training parameters for IMT are yet to be established. However, we have learned much from the evidence of IMT in patients with chronic lung disease, as well as endurance athletes, and the approach for ICU patients is informed by those principles (i.e. high-intensity interval training). It is hoped that this guide will assist ICU clinicians in translating the best-available evidence into practice by implementing IMT in their own ICUs, and if more clinicians are using a similar approach, this will enable future multicentre research to more robustly ascertain the impact of IMT on patient-centred outcomes, such as quality of life. In the short term, alongside other early and proactive rehabilitation strategies, IMT could be considered an important element of the rehabilitation toolkit to optimise recovery of critically ill patients.

      4. Conclusions

      Inspiratory muscle training is safe and feasible in selected ventilator-dependent ICU patients and those recently weaned from MV. Based on the available evidence, IMT is likely to enhance weaning and improve both inspiratory muscle strength and quality of life. The practical recommendations suggested in this guide should enable multidisciplinary ICU teams to offer IMT as an adjunct to rehabilitation for ICU patients, particularly those who experience MV for 7 days or longer.

      Acknowledgements

      The authors gratefully acknowledge the financial support of the Australian Capital Territory Chief Allied Health Office in the publication of this article. They also thank Mr Paul Bissett for his assistance with photography.

      Appendix ASupplementary data

      The following are the supplementary data related to this article:

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      Linked Article

      • Inspiratory muscle training can be monitored by electrical impedance tomography
        Australian Critical CareVol. 32Issue 2
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          We read with interest the article by Bissett et al. using the high-intensity approach for inspiratory muscle training (IMT) in patients with prolonged mechanical ventilation.1 Because IMT does not necessarily improve clinical outcomes,2 Bissett et al. proposed a list of indications when to apply IMT including patients awareness, fraction of inspired oxygen, and respiratory rate, which are not direct measures of muscle activities. The training intensity can be monitored with maximum inspiratory pressure, which needs to be measured invasively through the tracheostomy or using an endotracheal tube and not suitable for patients who are recently weaned.
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