Tissue Penetration of Antibiotics in CriTically Ill Children

This clinical study evaluates the extent to which three commonly administered beta-lactam antibiotics (piperacillin-tazobactam, meropenem, and amoxicillin-clavulanic acid) penetrate tissue in critically ill pediatric patients. The primary objectives of this study are to: * Determine whether tissue penetration of beta-lactam antibiotics is impaired in critically ill pediatric patients during initial dosing and steady-state conditions. * Evaluate whether antibiotic concentrations within interstitial tissue fluid achieve defined pharmacokinetic and pharmacodynamic targets. * Assess the impact of patient demographics and clinical characteristics on antibiotic disposition in muscle tissue. Investigators will measure unbound antibiotic concentrations in tissue fluid using microdialysis, a technique involving the placement of a microdialysis catheter into the muscle tissue. Participating patients will undergo the following procedures: * Insertion of a flexible microdialysis catheter into the vastus lateralis (thigh) muscle under continuous sedation. * Administration of the prescribed antibiotic therapy in accordance with standard medical care. * Collection of microdialysate and blood samples across specified dosing intervals to quantify tissue and plasma drug concentrations.

Inclusion Criteria: * Patient admitted to the Pediatric Intensive Care Unit (PICU). * Age: 1 month to 15 years. * Receiving intravenous antibiotic treatment with piperacillin-tazobactam, meropenem, or amoxicillin-clavulanic acid. * Receiving continuous analgosedation (required for microdialysis catheter insertion; if analgosedation is stopped after insertion, the catheter may remain in place). * Intra-arterial (preferred) or intravenous access available for blood sampling. Exclusion Criteria: * Presence of coagulopathy and/or thrombocytopenia: * Platelet count \< 50,000/µL * International Normalized Ratio (INR) \> 1.5 * Activated partial thromboplastin time (aPTT) \> 1.5 x upper limit of normal value (age-specific thresholds: \> 50 sec for 30-90 days; \> 42 sec for 90-180 days; \> 38 sec for \> 180 days) * Personal or family history of excessive bleeding. * Current treatment with coumarin derivative anticoagulants. * Known hypersensitivity to beta-lactam antibiotics. * Pregnancy. * Absence of parental (or legal guardian) informed consent. * Absence of suitable catheter for blood sampling.