The Role of Nutritional Support in the Physical and Functional Recovery of Critically Ill Patients

A Narrative Review

Danielle E. Bear; Liesl Wandrag; Judith L. Merriweather; Bronwen Connolly; Nicholas Hart; Michael P. W. Grocott

Disclosures

Crit Care. 2017;21(226) 

In This Article

Background

Many basic questions about the provision of nutritional support to critically ill patients remain unanswered.[1] Outcome from critical illness has previously been measured using relatively blunt outcome measure tools such as mortality, days on mechanical ventilation and rates of acquired infection. Recent randomised controlled trials (RCTs) have not shown any mortality benefit when specific nutritional interventions have been investigated over the first week of critical illness[2,3,4] and other studies have reported harm.[5,6,7] This has led to useful debate regarding the most appropriate timing, type and amount of nutrition support that should be delivered to critically ill patients.

It is generally accepted that providing some enteral nutrition (EN) early (within 48–72 hours of admission) modulates the immune response and reduces oxidative stress and infections by limiting bacterial translocation via the gut.[1,8,9] In addition, provision of nutrition over the course of critical illness may alter the composition and function of the host microbiome,[10] further influencing the immune response. However, the clinical impact of this physiological observation is unknown. In contrast, it has been hypothesised that early feeding blunts autophagy, preventing adequate clearance of damaged cells and resulting in muscle wasting and increased muscle weakness.[1,7]

As the number of patients surviving critical illness has risen, there has been an increase in reported physical and functional disability as well as impairment of quality of life following discharge from the intensive care unit (ICU).[11,12,13] Body composition and physical and functional impairment have been measured using a variety of tools at different time points over the course of critical illness and the patient's recovery trajectory.[14,15] At least 33 different measures of skeletal muscle mass, strength and function have been identified for use in critically ill patients.[14] These measures include both volitional and non-volitional tools, with varying reliability and validity. Currently, the most appropriate measure to use at each time point and for each intervention is unknown. Whichever measure is used, it is clear that recovery in survivors from critical illness is poor, even up to 5 years post discharge from the ICU.[13]

Skeletal muscle weakness, termed intensive care unit-acquired weakness (ICU-AW), contributes significantly to the physical and functional disability observed in these patients. Skeletal muscle wasting, both early in critical illness[16] and potentially ongoing as a result of persistent inflammatory catabolic syndrome (PICS),[17] has been identified as a contributing factor to ICU-AW.[18] Furthermore, low skeletal muscle mass on admission to the ICU has been shown to be a predictor of poor outcome.[19] It is here, in the reduction of skeletal muscle wasting and the recovery of survivors of critical illness, that nutrition support may prove the most beneficial, but there is remarkably little nutrition research specifically focusing on these outcomes.

The aims of early nutrition support in critically ill patients are often cited as the reduction of catabolism, attenuation of muscle wasting and maintenance of nutritional status.[8,9] However, to date there has been limited focus on muscle wasting and functional performance as outcomes in critical care nutrition trials. This narrative review will discuss clinical trials which have evaluated the physical and functional impact of critical care nutrition interventions, either as secondary outcomes, sub-studies or post-hoc analyses, as well as the effect of timing, type and amount of nutrition support on recovery following critical illness. Particular attention will be given to factors that should be considered in the design of future RCTs of nutrition in this patient group, which will probably benefit from adoption of a translational science approach.

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