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.