Environment of CareMuscle strength assessment in critically ill patients with handheld dynamometry: An investigation of reliability, minimal detectable change, and time to peak force generation☆
Introduction
Muscle weakness may contribute to both short- and long-term morbidity in survivors of a critical illness, but the clinical estimation of strength with manual muscle tests can be challenging. Intensive care unit–acquired weakness (ICUAW) [1] may be present at awakening in 25% of patients who have required mechanical ventilation for at least 5 days [2] and has been associated with both increased hospital mortality [2] and delayed successful extubation [3]. Although tests of volitional muscle strength are used at the bedside in clinical practice and in both observational studies and randomized controlled trials of early exercise in patients from the time of awakening to the post intensive care unit (ICU) period [2], [3], [4], [5], [6], [7], [8], [9], they may be affected by patients' effort, alertness, and motivation. Thus, during the initial stages of the recovery spectrum where these confounders can be particularly pronounced, the reliability, measurement error, and sensitivity of strength tests are key to objective quantification and reevaluation of ICUAW [10].
The Medical Research Council (MRC) 0-to-5 scale [11] and summated score [1], [12] may afford reliable grading of manual muscle tests in patients with Guillain-Barre syndrome [12] and several other populations who have required intensive care [2], [4], [5]. However, the MRC scale is an ordinal measure limited by its sensitivity at the higher grades [13]. It was originally designed for the examination of peripheral nerve lesions [11], and as polyneuropathy can be a component of ICUAW, the 0-to-3 MRC scale grades may provide adequate scoring options for strength assessment in patients with profound weakness. However, as demonstrated in a sample of critically ill patients, the higher MRC grades of average muscle strength encompass a large range of hand grip forces when measured with dynamometry [2], indicating a lack of differentiation between grades that may undersell strength graduations. Moreover, the difficulty of differentiating between MRC scale grades 4 and 5 has been suggested [14] as a reason for variable MRC score interrater reliability, in patients recovering from a critical illness [4]. While there is a paucity of data linking muscle strength to objective measures of physical function in the critically ill, it is possible that people with mild or no weakness according to the MRC scale could still have either weakness relative to their baseline or limited function because of impairments in associated elements of neuromuscular control. This may include the timing of force generation, which has not been previously investigated in the critically ill.
Dynamometry is the standard method of volitional muscle force measurement and may overcome some limitations of the MRC scale. While dynamometry is often regarded to have inadequate dynamic range for use in patients with very weak muscles [10], [15], [16], handheld devices are most applicable in the strength range of MRC grades 4 and 5. Nonetheless, with the exception of hand grip gauges, strength assessment with portable devices may be limited by examiners' ability to provide sufficient resistive force to stronger subjects or weaker subjects' inability to stabilize nontested joints within the same limb. Still, portable/handheld force gauges have been successfully used to measure both hand grip and knee extension in studies of critically ill patients [2], [6], [7], [8], [9]. However, both the change in strength required to overcome the measurement error associated with dynamometry and the test-retest reliability to ensure adequate consistency remain unknown. Indeed, the study of the reliability of dynamometry in the ICU has been limited to 1 interrater investigation [17].
Therefore, we aimed to investigate both the test-retest and interrater reliability of a muscle strength assessment with portable dynamometry in survivors of a critical illness. We also aimed to examine the minimal detectable difference (MDD) in force required to mitigate measurement error. To ascertain if the pattern of force production, as measured by the time of peak force generation, was altered in critically ill patients, healthy controls were additionally sampled. Finally, to enable a comparison with existing literature, we sought to depict forces according to MRC scale.
Section snippets
Materials and methods
This research conformed to the principles of the Declaration of Helsinki and was approved by the Southern Adelaide Health Service/Flinders University Human Research Ethics Committee (no. 277/09), including procedures for informed consent. A repeated-measures methodology was applied to assess both the interrater and test-retest reliability of peripheral muscle strength. Interrater reliability was assessed using 2 physiotherapists (examiner A and B), who completed the protocol 2 to 4 hours apart
Subject characteristics
One hundred eighty nine critically ill patients were screened (Fig. 1). The presence of a neurologic condition (n = 34), contraindication to strength testing (n = 18), inability to assess more than 2 limbs (n = 6), or a cognitive/intellectual/psychologic impairment (n = 16) accounted for 58% of the primary reasons for the 127 patient exclusions. Of the 22 patients from whom consent was sought, 17 participated in the study. However, interrater testing was omitted in 2 subjects because of the
Discussion
This is one of the first reports of the reliability of dynamometry in critical illness [17], [30], and the ICCs we obtained are analogous to the range of values reported in both other early data [17] and stable clinical conditions that may also display muscle weakness [31], [32], [33], [34]. In our sample of mostly alert and calm patients, with at least antigravity muscle strength, we found excellent (ICCs ≥0.75) [35] interrater and test-retest reliability of both the bilateral hand grip and
Acknowledgments for research support
The authors wish to acknowledge the Flinders Medical Centre Physiotherapy Department for the staffing support required to complete this study.
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Sources of funding: Flinders University research student maintenance funds were used to cover the reasonable cost of travel associated with healthy participants' attendance at data collection sessions. Flinders Medical Centre Intensive and Critical Care Unit research funds were used to purchase the dynamometry equipment used in this study. This study was not sponsored. There is no conflict of interest to declare for any of the authors in relation to this work.