Changes in rectus femoris diameter: a cooperation-independent assessment method for ICU-acquired weakness

ICU-acquired weakness (ICUAW) affects 50-100% mechanically ventilated critically ill patients, prolonging ventilation and hospital stay and raising long-term mortality. Conventional assessments including MRC scale and handgrip test require alert, cooperative patients, whereas electromyography and muscle biopsy are invasive and unsuitable for repeated bedside use. Static muscle ultrasound only reflects muscle mass but cannot accurately reflect actual muscle function.

This study aimed to develop a cooperation-free evaluation method combining bedside ultrasound and electrical stimulation to induce rectus femoris contraction, and verify its diagnostic performance for ICUAW.

A total of 58 mechanically ventilated adult patients (aged ≥18 years, mechanical ventilation duration ≥5 days) were enrolled and divided into the ICUAW group (n = 32, mean MRC<4) and non-ICUAW group (n = 26). Under supine position, rectus femoris was stimulated (30 mA, 50 Hz, 500 μs pulse). Ultrasound captured relaxed and contraction images to alculate the rest-contraction transverse diameter difference of rectus femoris (ΔRF-d, resting diameter minus contracted diameter), change in rectus femoris cross-sectional area (ΔCSA), resting thickness and CSA. Hierarchical backward stepwise multiple linear regression was performed to identify independent predictors of the mean MRC score, with 12 pre-defined clinical covariates allocated into three blocks (4 variables per block) and the within-block variable removal threshold set at P > 0.10. Variance inflation factor (VIF) was calculated to test multicollinearity, and receiver operating characteristic (ROC) curves were constructed to compare diagnostic performance of indicators.

All static and dynamic muscle indicators were significantly lower in the ICUAW group (n = 32) compared with non-ICUAW group (n = 26). ΔRF-d and corticosteroid use independently predicted MRC scores. The AUC of ΔRF-d was 0.909 (sensitivity 81.3%, specificity 92.9%), which was numerically higher than the AUC values of APACHE II (0.844), NRS-2002 (0.828), and ΔRF-CSA (0.829), indicating its relatively good discriminative capacity.

Ultrasound-measured rest-contraction transverse diameter difference of rectus femoris (ΔRF-d, resting diameter minus contraction diameter) induced by electrical stimulation serves as a promising cooperation-independent marker to assess muscle strength and screen ICUAW in critical care.