Expiratory Muscles, Neglected No More

In this issue of Anesthesiology, Shi et al.1 report on the reproducibility expiratory muscle thickness measured with ultrasound imaging in 30 patients ventilated intensive care unit (ICU). They also assess changes during first week mechanical ventilation 77 ICU patients. conclude that measurements are reproducible, and decreases 2 10 increases 1 ventilation.Should we concern ourselves thickness? Yes, for several reasons. Forceful contractions muscles necessary to achieve dynamic airway compression, an essential element effective cough.2 Expiratory recruitment improves diaphragmatic neuromechanical coupling loaded breathing,3 including a failed weaning trial.4 Indirect weakness have been associated need reinstitution after initial extubation.5Two crucial aspects any transducer, transducers, validity, or how well measurement represents variable, reliability, consistency measuring test. assume is valid tool measure thickness. Is assumption acceptable? Likely so, considering similar technique, Wait al.6 reported close correlation between cadavers using ruler. As compared interrater reproducibility7 obtained by two investigators at same anatomical location about 5 min apart intrarater repeatability7 each investigator. The intraclass coefficient was high (0.994). 95% limits agreement ranged from –13.1 6.8%. Similarly, (0.991 0.998), corresponding –11.4 13.8%.Based arm study sensitivity analysis, used 15% 20% change as thresholds identify “likely be attributable biologic processes such atrophy hypertrophy, opposed variance [alone].”1 Using these thresholds, 17 22% experienced decrease ventilation. These figures half purported prevalence patients.8That diaphragm more susceptible than other skeletal not surprising. For example, less 3 days controlled immobilization, cross-sectional area fibers while pectoralis major does not.9 At functional level, rat model long-term conditions, Corpeno al.10 specific force before development atrophy. posttranslational modifications myosin. Salah al.11 reversed post-translational almost completely restored diaphragm’s day rats treated BGP-15, heat shock protein 72 co-inducer. contrast, 8 ventilation, BGP-15 has no effect soleus muscle, which severe loss function starts later, it parallels myosin loss.12 observations suggest different pathways implicated peripheral dysfunction critically ill patients.13 raise possibility distinct interventions may required address groups.In group who thickness, recorded 11.5% within 24 h enrollment. This double observed limb patients.14 If differences confirmed future studies, must inaccurate expect response critical illness groups lower even if those fiber-type distribution (Type I fiber predominance)15 share some role (posture balance).16,17Except longer hospital length stay, were worse clinical outcomes duration mortality. Several mechanisms explain results. First, powered outcomes. Indeed, themselves caution against overinterpretation their explorative outcome data. Second, thickness—and conceivably function—do affect respiratory physiology. difficult reconcile important when faced increased loads,4 unless alone accurately reflect function.10 latter supported observations. patients, underestimate dimension protein-to-DNA ratio.14 pilot study,18 electrical stimulation abdominal wall reduced stay despite leading thickness.Finally, proposed threshold(s) classify having (or increase) distribution-based anchor-based.19 That variability technique its quantification mean crossing threshold ipso facto leads impairment. It impairment (if present) necessarily translates into clinically significant deficit. overall strength normal cause hypercapnia (in patients).20 Maximal pressure one third cough ineffective.2 Distribution-based imply limited time frame reflects over periods time. To limit influence day-to-day variance, categorized obtaining regression line available patient. Whether strategy sufficient unclear marred “large intra- inter-participant variability” 27.3% sessions experiment had excluded due disagreements.18 whether large interparticipant variabilities previous fluid overload, edema, intra-abdominal pressures. factors could lead architecture unrelated atrophy.In conclusion, elegant investigation, make strong case use Challenges remain. Does independently contribute Are mechanistically linked outcomes, they indirect marker disease severity? Can strategies designed restore impact patients’ outcomes?Dr. Laghi received research grants National Institutes Health (Bethesda, Maryland), Veterans Administration Research Service (Washington, D.C.), Liberate Medical, LLC (Crestwood, Kentucky), Science Foundation (Alexandria, Virginia). Dr. Cacciani institutional funding Karolinska Institute (Stockholm, Sweden).The authors by, nor maintain financial interest in, commercial activity topic article.