By Richard Kallet, MS, RRT, FCCM

Director of Quality Assurance, Respiratory Care Services, Department of Anesthesia, San Francisco General Hospital

Mr. Kallet reports he is a major stockholder in the Asthma & Allergy Prevention Company, is a consultant for Getinge Group, and receives grant/research support from Nihon-Kohden.

SYNOPSIS: Mechanically ventilated patients undergoing inspiratory muscle training demonstrated significant increases in both maximum inspiratory and expiratory muscle pressures; the training was associated with reduced weaning duration of 2.3 days.

SOURCE: Vorona S, Sabatini U, Al-Maqbali S, et al. Inspiratory muscle rehabilitation in critically ill adults. A systematic review and meta-analysis. Ann Am Thorac Soc 2018;15:735-744.

This systematic review included 28 studies, of which 71% were randomized, controlled trials, and which included 1,185 adult patients. At baseline, patients exhibited substantial loss of inspiratory muscle strength (IMT), with a mean maximum inspiratory pressure (MIP) of 15-54 cm H2O, which is approximately 10-35% of normal laboratory values.1 Numerous IMT techniques were used, including strength vs. endurance conditioning (i.e., < 200 vs. > 200 repetitions/session) or physical therapy aimed at increasing minute ventilation demand and, therefore, respiratory muscle power output (work per minute). Most studies used threshold loading rather than resistive loading techniques. Commencement of IMT varied widely (ranging from 24 hours after intubation to only after initial weaning trial failure), as did the duration of therapy (ranging from three days to six weeks). However, these differences appeared to not affect therapeutic efficacy.

In the randomized, controlled trials, mean MIP increased by 40% in the IMT arm vs. 18% in controls. Mean maximum expiratory pressure (MEP), which represents abdominal muscle strength, increased by 63% vs. 17% in controls. MEP is a signifier for cough strength and is an indirect accessory muscle of inspiration during loaded breathing. However, both findings represented only a modest increase in strength at 6 cm H2O (95% confidence interval [CI], 5-8 cm H2O) and 9 cm H2O (95% CI, 5-14 cm H2O), respectively. Nevertheless, after excluding studies with serious risk for bias, there was a clinically significant reduction in weaning duration by 2.3 days. All studies concluded that IMT in critically ill patients is feasible with only a very rare incidence of serious adverse effects (e.g., bradycardia).

COMMENTARY

The authors are to be commended for drawing our attention to a peculiar paradox in pulmonary and critical care medicine. For decades, researchers have been preoccupied with weaning strategies and ventilator dependence. However, we largely have ignored a crucial aspect of skeletal muscle functionality — namely, improving strength and endurance in debilitated patients requires systematic retraining. Although we reflexively accept this in terms of physical therapy for other skeletal muscle groups, the critical care community has never embraced this concept seriously.

Interestingly, this perplexing situation has coincided with decades-long misperceptions regarding respiratory muscle function in the critically ill, despite the revolution in our understanding of respiratory muscle physiology that began in the mid-1970s. Perhaps our greatest advancement in clinical practice was the return to spontaneous breathing trials, which irrefutably confirmed what previously was evident, but generally has not translated into clinical practice: Most mechanically ventilated patients (approximately 70%) exhibit little if any problem resuming unassisted breathing.2 However, this still leaves a substantial percentage of patients requiring a structured program of respiratory muscle rehabilitation to improve patient-centered clinical outcomes.

In essence, most patients with substantial abnormalities in chest mechanics should be able to resume unassisted breathing when their minute ventilation demand comes down to a threshold whereby their respiratory muscles can handle imposed resistive and elastic loading. In other words, when the minute ventilation approaches or falls below 10 L/minute (suggesting reasonable respiratory muscle power output demands), failure to resume unassisted breathing represents an imbalance between work-per-breath (load) and muscle strength (capacity). When other correctable factors (positive fluid balance, pleural effusion, ascites, nutritional status, etc.) are addressed, then improving strength becomes the most obvious focus for treatment. Most mechanically ventilated patients suffer not only from slowly resolving airway and parenchymal inflammation, but also from respiratory muscle dysfunction.

The most salient finding by Vorona et al is that despite the diverse approaches to IMT, the mean reductions in weaning duration approximates those found in mechanical ventilation duration in randomized, controlled trials whose authors examined spontaneous breathing studies and daily sedation interruptions (i.e., 1.5- to two-day reductions). In addition, the Vorona et al study underscores the fact that no consensus exists on how respiratory muscle rehabilitation should be approached.

Areas that need to be explored include: the timing of IMT initiation, frequency and duration of treatments, how quickly to accelerate muscle loading, and whether and what combinations of strength vs. endurance conditioning should be used. Of paramount importance in both the acute and recovery phases of respiratory failure is what constitutes an appropriate balance between maintaining and improving respiratory muscle strength and providing adequate respiratory muscle rest and recovery? This becomes particularly important during rehabilitation as weakened muscles are highly susceptible to fatigue. Chronic exposure to excessive workloads produces respiratory muscle inflammation and damage, contributing to ventilator dependence.1 This last point is something that often eludes clinicians’ consideration and a point Vorona et al emphasized. All these issues should be addressed in a systematic fashion to continue the progress already made in weaning and sedation practices that have produced substantial reductions in both the duration of mechanical ventilation and length of stay in the intensive care setting.

REFERENCES

  1. Kallet RH. Patient-ventilator interaction during acute lung injury, and the role spontaneous breathing: Part 1: Respiratory muscle function during critical illness. Respir Care 2011;56:181-189.
  2. Lemaire F. Difficult weaning. Intensive Care Med 1993;19:S69-S73.