Inspiratory Muscle Rehabilitation in Critically Ill Adults. A Systematic Review and Meta-Analysis

Abstract

Rationale: Respiratory muscle weakness is common in critically ill patients; the role of targeted inspiratory muscle training (IMT) in intensive care unit rehabilitation strategies remains poorly defined.

Objectives: The primary objective of the present study was to describe the range and tolerability of published methods for IMT. The secondary objectives were to determine whether IMT improves respiratory muscle strength and clinical outcomes in critically ill patients.

Methods: We conducted a systematic review to identify randomized and nonrandomized studies of physical rehabilitation interventions intended to strengthen the respiratory muscles in critically ill adults. We searched the MEDLINE, Embase, HealthSTAR, CINAHL, and CENTRAL databases (inception to September Week 3, 2017) and conference proceedings (2012 to 2017). Data were independently extracted by two authors and collected on a standardized report form.

Results: A total of 28 studies (N = 1,185 patients) were included. IMT was initiated during early mechanical ventilation (8 studies), after patients proved difficult to wean (14 studies), or after extubation (3 studies), and 3 other studies did not report exact timing. Threshold loading was the most common technique; 13 studies employed strength training regimens, 11 studies employed endurance training regimens, and 4 could not be classified. IMT was feasible, and there were few adverse events during IMT sessions (nine studies; median, 0%; interquartile range, 0-0%). In randomized trials (n = 20), IMT improved maximal inspiratory pressure compared with control (15 trials; mean increase, 6 cm H2O; 95% confidence interval [CI], 5-8 cm H2O; pooled relative ratio of means, 1.19; 95% CI, 1.14-1.25) and maximal expiratory pressure (4 trials; mean increase, 9 cm H2O; 95% CI, 5-14 cm H2O). IMT was associated with a shorter duration of ventilation (nine trials; mean difference, 4.1 d; 95% CI, 0.8-7.4 d) and a shorter duration of weaning (eight trials; mean difference, 2.3 d; 95% CI, 0.7-4.0 d), but confidence in these pooled estimates was low owing to methodological limitations, including substantial statistical and methodological heterogeneity.

Conclusions: Most studies of IMT in critically ill patients have employed inspiratory threshold loading. IMT is feasible and well tolerated in critically ill patients and improves both inspiratory and expiratory muscle strength. The impact of IMT on clinical outcomes requires future confirmation.

Keywords: artificial respiration; inspiratory muscle training; physical therapy; respiratory muscles; weaning.

Figures

Figure 1.

Inspiratory muscle training techniques. Two main types of inspiratory muscle training techniques have been applied in critically ill patients: resistive loading and threshold loading. Resistive loading involves the application of a resistor to the airway (right). The resistor increases the pressure required for the respiratory muscles to generate a given flow (the area shaded black). The required pressure is the product of resistance and flow and therefore depends on the inspiratory flow that the patient attempts to generate. Accordingly, the total training effect varies with the patient’s respiratory mechanics and respiratory drive, making it difficult to standardize. Threshold loading involves the application of a threshold valve to the airway (left). The valve is designed so that a certain level of training pressure (Ptr) must be generated by the patient’s respiratory muscles before it opens to permit inspiratory flow (analogous to the effect of intrinsic positive end-expiratory pressure). Consequently, the pressure required to maintain at least some inspiratory flow is independent of the flow and volume generated by the patient or ventilator. The total training effect (shaded in black) will therefore be independent of mechanics and respiratory drive and is easier to standardize. A threshold load may also be applied directly on the ventilator by setting a pressure trigger at a desired threshold pressure level. Threshold loading is typically performed by applying a Ptr anywhere between 20 and 50% of the patient’s maximal inspiratory pressure for a relatively brief period of time (a few repetitions or a few minutes) at regular intervals. Note: chest wall elastance and resistance are ignored on the diagram for simplicity. Paw = airway pressure; Pes = esophageal pressure.

Figure 2.

Effect of inspiratory muscle training (IMT) on the change in maximal inspiratory pressure from baseline to the completion of the treatment course. The effect of IMT did not significantly differ with strength training versus endurance training regimens. Weight refers to the contribution of each study to the meta-analysis estimate of effect. CI = confidence interval; MD = mean difference; SD = standard deviation.

Figure 3.

Impact of inspiratory muscle training (IMT) on the duration of ventilation in mechanically ventilated patients. After exclusion of studies at serious risk of bias, the treatment effect was no longer significant (mean difference, 4.6 d; 95% CI, −1.0 to 10.1 d; I2 = 94%). Weight refers to the contribution of each study to the meta-analysis estimate of effect. CI = confidence interval; MD = mean difference; SD = standard deviation.

Figure 4.

The impact of inspiratory muscle training (IMT) on the duration of weaning from mechanical ventilation. After exclusion of studies at serious risk of bias, the effect remained significant (3.2 d; 95% CI, 0.6–5.8 d; I2 = 95%). Weight refers to the contribution of each study to the meta-analysis estimate of effect. CI = confidence interval; MD = mean difference; SD = standard deviation.