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

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.

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.

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; P = 0.022), VI-TH (0.64 [0.47-0.86] vs. 0.77 [0.64-1.04]cm; P = 0.002), QF-TH (1.32 [0.96-1.63] vs. 1.48 [1.24-1.97]cm; P =0.005) and TA-PA (7.08[5.63-8.48] vs. 7.94[6.62-9.32]; P =0.002). Among them, TA-PA was identified as an independent predictor of 60-day mortality (β = -0.216, P = 0.035). TA-PA, in conjunction with APACHE II score and prealbumin level, constructed a predictive nomogram, with a consistency index (C-index) of 0.785 (95%CI: 0.728-0.834). Calibration assessed by Spiegelhalter Z-test showed P > 0.05 indicating adequate calibration for the predicted and observed models. Decision curve analysis (DCA) confirmed that the nomogram prediction model had good clinical benefits. Internal validation demonstrated stable model performance, with a concordance statistic (C-index) of 0.785. Compared to the ROC curve based solely on APACHE II score, the nomogram exhibited significantly higher area under the curve (AUC) (0.785, 95%CI: 0.728-0.834 vs. 0.726, 95%CI: 0.666-0.781, P = 0.023).

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.