Acute gastrointestinal injury (AGI) is related to poor outcomes of critically ill patients [1] through many underlying mechanisms [2]. It is also a part of the process of multiple organ dysfunction syndrome (MODS). However, the morbidity of acute gastrointestinal dysfunction in critically ill patients is highly underestimated due to the scarcity of accurate measurement and thus the causes are still unclear.

In this study, investigators are going to apply the technique of point-of-care ultrasound (POCUS) evaluation on the bowel diameters, wall thickness and movement combined with intra-abdominal pressure to determine the occurrence of AGI. The superior mesenteric artery (SMA) blood flow is also evaluated by POCUS to find out the association between SMA blood flow and AGI.

1. Patients are recruited within 24 hours of our surgical ICU admission. The patients are mostly admitted for post-surgery monitor in case of complications due to their baseline health conditions. In some cases, patients are admitted from the emergency room because of severe sickness. Those who are with deep sedation and mechanical ventilation during the first ultrasound evaluation are eligible for the study.
2. Written informed consent was obtained from all patients or next of kin before the recruitment.
3. Information will be collected at enrollment, including demographic characteristics, diagnosis, and so on.
4. There will be two phases of ultrasound evaluation. In Phase One, investigators perform ultrasound evaluation within 24 hours after ICU admission to investigate the SMA blood flow in supine position using POCUS. In Phase Two, starting after the enteral feeding has been initiated, usually within 3 days after the enrollment, ultrasound evaluation on GI function will be performed according to gastrointestinal and urinary tract sonography ultrasound (GUTS) protocol using POCUS. GUTS protocol includes the ultrasound measurement for bowel diameter, bowel wall thickness, peristalsis combined with intra-abdominal pressure(IAP) measured through ureteral catheterization. All measurements would be used to calculate GUTS score to grade the AGI [4].

Inclusion Criteria:

1. 18 ~ 80 years old;
2. Under deep sedation (RASS score ≤ -3) with mechanical ventilation at the time of enrollment;
3. Acute Physiology and Chronic Health Evaluation II (APACHE-II) score ≥8.

Exclusion Criteria:

1. Pregnancy;
2. Recent gastrointestinal surgery;
3. History of acute or chronic gastrointestinal dysfunction before ICU admission;
4. Primary or secondary vascular malformation of SMA;
5. Any contraindication to the use of ultrasound evaluation (e.g., abdominal incision).

Patients admitted into the Department of Critical Care Medicine of Peking Union Medical College Hospital, who were eligible of inclusion criteria, were included in the study.