The aim of the study is to monitor changes in muscle mass in critically ill patients and to verify whether a decrease in muscle mass of ≥10% during the first 7 days of hospitalization is associated with increased 28-day mortality. We will be used ultrasoud for measurement the rectus femoris cross sectional area.

All patients will be admitted to participating ICU centers from December 2025 to December 2026 will be screened and enrolled based on the inclusion and exclusion criteria. Informed consent will be requested. Ultrasound measurements will be performed by the same investigator within the first 24h after ICU admssion and again on day 7 of the patient´s ICU stay. All patient data will be entered into case report form by a study physician. In the follow-up will be determined 28 mortality.

Inclusion Criteria:

• age ≥18 years
• mechanical ventilation expected for at least 48 hours

Exclusion Criteria:

• Clinical Frailty Score above 7 prior admission

• Neuromuscular disease in previous medical history

• Amputated lower extremities

• Trauma of lower extremities involving tights

• Conditions affecting the nervous system with involvement of the thigh muscle

• Inability to cooperate during ultrasound examination

• Inability to perform ultrasound examination on day 7

  9. Inability to perform ultrasound examination on day 7

Critically ill adult patients will be admitted to intensive care unit with suspect length of artificial ventilation more than 48 hours.