Structure to function: muscle failure in critically ill patients

Abstract

Impaired physical function and reduced physical activity are common findings in intensive care unit (ICU) survivors. More importantly, reduced muscle strength during critical illness is an independent predictor of survival. Skeletal muscle wasting as a direct consequence of critical illness has been suggested as the cause. However, data on the physiological processes regulating muscle mass, and function, in these critically ill patients are limited as this is not only a technically challenging research area, but also the heterogeneity of the patient group adds complexity to the interpretation of results. Despite this, clinical and research interest in this area is growing. This article highlights the issues involved in measurement of muscle function and mass in critically ill patients and the physiological complexities involved in studying these patients. Although the data are limited, this article reviews the animal and healthy human data providing a rational approach to the potential pathophysiological mechanisms involved in muscle mass regulation in critically ill patients, including the established muscle wasting 'risk factors' such as ageing, immobility and systemic inflammation, all of which are common findings in the general critical care population.

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Zudin Puthucheary (left) is a National Institute of Health Research Clinical Training Fellow in Respiratory and Critical Care jointly appointed at University College London and Kings College London. Nicholas Hart (right), Dr Puthucheary's supervisor, is a Clinical Research Consultant in Respiratory and Critical Care Medicine within Guy's and St Thomas' NHS Foundation Trust and Kings College London National Institute of Health Research Comprehensive Biomedical Research Centre. The current research programme is focused on prevention of respiratory morbidity with three particular themes of: muscle wasting, repair and rehabilitation; advanced physiological monitoring; and chronic respiratory failure.

Figure 1. Schematic representation of the factors regulating muscle mass and function in critically ill patients

MPS, muscle protein synthesis; MPB, muscle protein breakdown