Inclusion Criteria:  18 ~ 80 years old; Under deep sedation (RASS score ≤ -3) with mechanical ventilation at the time of enrollment; Acute Physiology and Chronic Health Evaluation II (APACHE-II) score ≥8.  Exclusion Criteria:  Pregnancy; Recent gastrointestinal surgery; History of acute or chronic gastrointestinal dysfunction before ICU admission; Primary or secondary vascular malformation of SMA; Any contraindication to the use of ultrasound evaluation (e.g., abdominal incision).