Critical Care Commentary: Mobilize mechanically ventilated patients

Survival after critical illness is common, with numerous evidence-based strategies showing impressive outcomes in recent years.^1-3 Patients requiring mechanical ventilation are at risk for developing neuromuscular weakness – a syndrome referred to as "ICU-acquired weakness" (ICU-AW). Those with sepsis seem to be at particularly high risk of developing ICU-AW.⁴ Because patients supported by mechanical ventilation in the ICU may be immobile for prolonged time periods, the idea of early mobilization is attractive. Traditional management strategies during mechanical ventilation incorporated a mindset that imposed a state of passivity on patients. Such an approach seems largely driven by the notion that the ventilator should "take over" breathing and that any patient interaction would be detrimental to this goal. Deep sedation and suspension of physical and mental activity is typically the end result. It is remarkable that harm from complete bed rest was described generations ago,⁵ but complete bed rest in patients supported by mechanical ventilation in the ICU remains the norm. It appears that the improved survival of patients with critical illness, which leads to more survivors with persistent physical and mental impairments, has heightened awareness of the problem among clinicians. This has been the springboard from which the notion of mobilization during critical illness has emerged.

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Early mobilization patients demonstrated better maximal walking distances while in the hospital (33.4 vs 0 meters) and more ventilator-free days (23.5 vs 21.1).

Persistent physical weakness and functional impairment is one of the greatest burdens on ICU survivors.^6-9 Margaret Herridge and colleagues⁶ published a sentinel paper describing the 1-year outcomes in patients recovering from ARDS. One hundred percent noted loss of muscle bulk, proximal muscle weakness, and fatigue; half of this relatively young group of patients was unemployed 1 year after their ICU experience. At the 5-year mark after discharge, the remaining survivors continued to have physical dysfunction. Their scores on the physical portion of the SF-36 score were markedly lower than the normal population. The 6-minute walk distances were significantly reduced.⁷

ICU-acquired weakness

Critical illness polyneuropathy, first described by Charles Bolton and colleagues in the 1980s,¹⁰ is characterized by a primary axonal degeneration, typically affecting motor nerves more than sensory nerves. These investigators noted severe motor and sensory polyneuropathy at the peak of critical illness, along with difficulty liberating patients from mechanical ventilation. Critical illness polyneuropathy is often seen in those with sepsis or systemic inflammatory response syndrome (SIRS) and is a common finding in those with ICU-AW).¹¹ Because electrophysiologic nerve studies are necessary to diagnose critical illness polyneuropathy (and such tests are rarely done), its true incidence is not known.

A separate entity known as critical illness myopathy (CIM) is another important contributor to ICU-AW. Patients with this syndrome have generalized muscle weakness but preserved sensory function.¹² The myopathy is thought to be driven by muscle injury from systemic inflammation^13,14 and skeletal muscle proteolysis from the catabolic state of critical illness. Prolonged physical immobility with resulting deconditioning is another important contributor.¹⁵ When electrophysiologic and histopathologic analyses are performed, many patients with ICU-AW manifest both neuropathic and myopathic physiologic findings. However, since few patients receive such testing, it is not clear if this observation applies to the general population of patients with ICU-AW.

The extent to which ICU-AW is the result of immobility is not known, since all studies describing this problem have been performed in a population of patients who are immobile. Since this state is a constant, it remains speculative whether early mobility can attenuate or eliminate the specific pathologic and pathophysiologic conditions of peripheral nerves and skeletal muscle that are described in the literature. Indeed, altering the natural history of ICU-AW is a major objective of early mobilization. Even if the neural and muscular pathologic condition cannot be fully eliminated, the ability to have improved mental awareness during critical illness (ie, reduced delirium) can lend itself to a chance for better outcomes. For example, if a patient with some degree of ICU-AW can understand the nature of his/her handicap and adapt to this limitation, functional outcomes may improve. Such early activity allows therapists to teach patients strategies that will work optimally in spite of physical limitations. Such an approach offers the opportunity for patients to accommodate to their physical weaknesses and maintain independence.

Early mobility in the ICU

In 2007, the first report of early mobilization in mechanically ventilated ICU patients was reported by Bailey et al.¹⁶ This descriptive study took place in the respiratory ICU at LDS Hospital in Salt Lake City, Utah. Patients were supported by mechanical ventilation for more than 4 days in this ICU and were transferred there after being cared for in another ICU in the hospital for an average of 10 days. Targeted activities included sitting on the edge of the bed, sitting in a chair after bed transfer, and ambulation. Ventilator settings required were FIO₂ < 0.6 and PEEP < 10 cm H₂O. Patients could not have orthostatic hypotension and were required to be off catecholamine infusions. More than half of the activity events were ambulation – with nearly half of these occurring in intubated patients! The median ambulation distance was 200 feet and over 80% of patients survived to hospital discharge. There were very few adverse events and no unplanned extubations. The same group reported a follow-up study to determine the change in mobilization frequency when patients were transferred from other ICUs to their respiratory ICU where a culture of early ambulation had been well established.¹⁷ In this ICU where mobilization was an expected part of care, patients were more than two times as likely to receive mobility therapy (OR 2.47; 95% CI 1.9-3.4, P < .0001). Avoidance of sedation also increased the chances of mobilization (OR 1.90; 95% CI 1.2-3.2, P < .009).