Tibialis anterior pennation angle at ICU admission and 60-day mortality in critically ill patients: a prospective observational study.

Background: Skeletal muscle is an important organ strongly associated with prognosis in critically ill patients. 60-day mortality represents a key endpoint for evaluating the transitional phase from ICU survival to functional recovery, yet the association between ultrasound-derive muscle indices at ICU admission and 60-day mortality remains unclear. Methods: This dual-center, prospective observational study was performed from January to December 2024. Four ultrasound indices of muscle quantity [thickness and cross-sectional area of the rectus femoris (RF-TH and RF-CSA), thickness of the vastus intermedius (VI-TH), thickness of the quadriceps femoris (QF-TH)], and one index of quality (tibialis anterior pennation angle, TA-PA) were assessed within 24 h of ICU admission. The primary endpoint was 60-day all-cause mortality. To identify predictive factors, multivariable logistic regression analysis was employed. Additionally, a predictive nomogram model was developed. Results: A total of 247 critically ill adult patients were included, with a median age of 61.0 years (51.0-72.0), including 171 males (69.20%). During 60-day follow-up, 53 patients (21.50%) died. Compared with the survivors, the non-survivors exhibited significantly lower RF-TH (0.54 [0.43-0.70] vs. 0.64 [0.48-0.85]cm; Conclusions: Lower TA-PA at ICU admission is an independent predictor of 60-day mortality. Incorporating this ultrasound-derived muscle quality index with APACHE II score and prealbumin level may improve early risk stratification in critically ill patients.