Skeletal muscle weakness affecting the respiratory and peripheral muscles is common in critically ill patients and can lead to difficulties in weaning, prolonged ICU admission, and significant morbidity in survivors. A number of techniques can be used to assess muscle strength. In the peripheral muscles, volitional techniques employing scoring systems or portable hand dynamometers are relatively simple and quick to use, requiring little or no specialist equipment. Such techniques can, however, only be applied to conscious and cooperative patients, preventing assessment of muscle weakness in many ICU patients. The volitional requirement also limits the ability to distinguish poor motivation and impaired cognition from true loss of muscle function. Non-volitional techniques involving motor nerve stimulation provide measures of muscle force production in non-cooperative patients but require specialist equipment. Normative data for comparative purposes are limited. Also, it is not clear which peripheral muscle best reflects generalized muscle weakness. Measurements of maximal inspiratory and expiratory pressures are widely used to assess respiratory muscle strength in ICU patients and are applicable to patients who can make some respiratory effort. As with all tests requiring patient cooperation, reliability is limited. Phrenic nerve stimulation allows direct, non-volitional assessment of diaphragm and phrenic nerve function, and normative values for comparative purposes are available. Magnetic phrenic nerve stimulation is well tolerated, can be performed in the presence of vascular catheters, and is used to document respiratory muscle weakness and track progression in critically ill patients.
Muscle strength assessment of critically ill patients is associated with functional ability and quality of life at hospital discharge
