Magnetic twitch assessment of diaphragm and quadriceps weakness in critically ill mechanically ventilated patients
Introduction
Critically ill patients develop significant muscle weakness (Goddard and Adhikari, 2016; Jolley et al., 2016; Supinski and Callahan, 2013; Supinski et al., 2016). This syndrome, termed intensive care unit acquired weakness (ICUAW), affects the diaphragm, the major muscle of breathing, and limb muscles. Diaphragm muscle weakness is now known to influence intensive care unit (ICU) outcomes such that patients with weak diaphragms have a much higher mortality and a longer duration of mechanical ventilation than patients with stronger diaphragms (Supinski and Callahan, 2013; Supinski et al., 2016). In addition, limb muscle weakness persists long after ICU survival, reduces long term exercise performance, and impacts the ability of these patients to return to productive employment (Herridge et al., 2014, 2011).
There are, however, important issues regarding ICUAW that are not fully elucidated (Dres et al., 2017; O’Neill et al., 2017; Supinski et al., 2018). In particular, recent work has suggested that diaphragm muscle weakness is far more severe and common in these patients than limb muscle weakness. Specifically, diaphragm weakness was found to be present in 63 % of ICU patients, while limb muscle weakness was present in only 34 % in one recent study (Dres et al., 2017). This previous study, however, assessed limb muscle strength using Medical Research Council (MRC) scoring, which is a volitional test requiring a high level of patient cooperation. In addition, measurements of strength using MRC scoring requires a subjective assessment of patient force generation by investigators and the scores generated are categorical rather than linearly quantitative. Because of the limitations of MRC testing, one recent study concluded this method of assessment is not a reliable determination of limb muscle strength (O’Neill et al., 2017).
The purpose of the present study was to overcome the limitations of assessing muscle strength with volitional testing by utilizing magnetic stimulation techniques to assess both diaphragm and quadriceps muscle strength in a cohort of MICU patients. To accomplish this task, we measured diaphragm strength by determining the transdiaphragmatic pressure generated in response to supramaximal magnetic stimulation of the phrenic nerves and we measured quadriceps strength by determining the quadriceps force generated in response to supramaximal magnetic stimulation of the femoral nerve in mechanically ventilated ICU patients. In addition, we used ultrasound to assess the thickness of the diaphragm and the quadriceps (i.e. the rectus femoris and vastus intermedius) in these patients. We compared the diaphragm pressure and quadriceps force elicited in this patient population to previous values reported for normal, healthy patients. We also assessed the relationship of muscle strength to muscle thickness for each of these two muscles (diaphragm and quadriceps) by correlating transdiaphragmatic twitch pressure generation to diaphragm thickness and by correlating quadriceps twitch pressure generation to quadriceps thickness. Finally, we employed regression analysis to determine the relationship of diaphragm and quadriceps strength to several common indices of patient illness (severity of organ failure score, chronic illness comorbidity score, respiratory system static compliance, airway resistance, age, duration of mechanical ventilation).
Section snippets
Patient selection
Approval to conduct this research was obtained from the University of Kentucky Institutional Review Board. Consent for inclusion in these studies was obtained from all subjects and/or their surrogates. Inclusion into the study was considered for all adult patients requiring mechanical ventilation for more than 48 h for respiratory failure in one of the University of Kentucky adult medical ICUs. Subjects were included regardless of sex, race, or age. Patients were excluded: (a) if the physician
Patient characteristics
A total of 83 patients were recruited into the study. Not every parameter could be successfully obtained in each subject. Ten patients either withdrew before measurements could be made or there were technical issues (e.g. massive obesity) that precluded measurement of both diaphragm and quadriceps strength. Technically adequate (i.e. achieving twitch supramaximality) diaphragm strength measurements (PdiTw) were obtained in 64 patients and technically adequate quadriceps strength (QuadTw) were
Summary of major findings
The current report utilized non-volitional magnetic stimulation techniques to compare leg muscle and diaphragm muscle strength in critically ill intensive care unit (ICU) patients. The present results clearly demonstrate that ICU patients have both profound limb and diaphragm muscle weakness. For the quadriceps, we found that the average twitch force was only 6% of that previously reported for normal healthy individuals. For the diaphragm, average twitch pressure was only 19 % of previously
Limitations
There are several limitations to the techniques used and measurements made in this report. For one thing, it would have been ideal to make serial measurements of diaphragm and quadriceps strength beginning with the day of ICU admission. While we were interested in recruiting patients at early time points during their ICU stay, the families and clinicians caring for these patients preferred not to assent to early inclusion in a research study.
A second limitation is that we assessed quadriceps
Conclusions
The present study indicates that ICU patients manifest profound limb and diaphragm muscle weakness. For the quadriceps, the average twitch force of ICU patients was only 6 % of healthy individuals and, for the diaphragm, average twitch pressure was only 19 % of normal values. We also found that assessments of muscle thickness greatly underestimated muscle weakness, with severe muscle weakness often present despite muscle thickness in the normal range. In addition, our findings suggest that the
Grants
This work was supported by the National Institutes of HealthR01HL113494 and R01HL141356 and the Department of Veterans Affairs5I01BX002132.
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