This prospective observational study aims to evaluate the ability of tissue perfusion pressure (TPP) to predict fluid responsiveness in critically ill patients with shock and tissue hypoperfusion. Eligible patients requiring fluid resuscitation will undergo a standardized fluid challenge test. Hemodynamic parameters, including cardiac output and TPP, will be recorded before and after fluid administration. Fluid responsiveness will be defined according to the change in cardiac output following fluid loading. The study will assess the correlation between changes in TPP and cardiac output and evaluate the diagnostic performance of TPP for predicting fluid responsiveness. The results may support the use of TPP as a practical and physiologically relevant parameter for individualized fluid management in patients with shock.

Inclusion Criteria:

• Age ≥18 years
• Admitted to the Intensive Care Unit (ICU)
• Diagnosed with shock and evidence of tissue hypoperfusion
• Judged by the treating physician to require fluid resuscitation
• Receiving invasive hemodynamic monitoring
• Written informed consent obtained from the patient or legally authorized representative

Exclusion Criteria:

• Pregnancy
• Immediate requirement for extracorporeal life support, including: VV-ECMO, VA-ECMO, Ventricular assist devices
• Expected survival less than 72 hours
• Absence of invasive arterial pressure monitoring or arterial waveform availability <25%
• Any condition deemed unsuitable for study participation by the investigator.

The study population will consist of critically ill adult patients with shock and tissue hypoperfusion admitted to the Intensive Care Unit of Zhongda Hospital, Southeast University. Patients requiring fluid resuscitation and invasive hemodynamic monitoring as part of routine clinical care will be screened for eligibility and consecutively enrolled in this prospective observational study.