Respiratory Quotient and Phosphate Changes During Early Critical Illness

Critically ill patients often experience major changes in metabolism during the first days of intensive care. When nutrition is started or increased after a period of reduced intake, some patients may develop refeeding syndrome, a potentially serious condition associated with shifts in electrolytes such as phosphate, potassium, and magnesium. This study will investigate whether changes in respiratory quotient (RQ), measured by indirect calorimetry, are associated with changes in serum phosphate during the first intensive care week. RQ reflects the balance between oxygen consumption and carbon dioxide production and provides information about which energy substrates are being used by the body. A rising RQ may indicate increasing carbohydrate metabolism and could potentially provide an early physiological marker of metabolic changes associated with refeeding. Mechanically ventilated adult intensive care patients will undergo daily indirect calorimetry during the first five days of the study period, with an additional measurement on day 7 when feasible. Serum phosphate and other electrolytes, nutritional intake, insulin treatment, ventilator parameters, and relevant physiological variables will be recorded at corresponding time points. The study will also evaluate how well established screening criteria for refeeding risk, including the NICE criteria, identify patients who subsequently develop metabolic changes consistent with refeeding. No study-specific changes to nutrition or other treatment will be made. This accurately reflects the observational nature of the study: routine IC, laboratory and nutritional data are collected without changing treatment.

Inclusion Criteria: * Age 18 years or older. * Admitted to the central intensive care unit (CIVA) or neurointensive care unit (NIVA) at Sahlgrenska University Hospital. * Receiving invasive mechanical ventilation. * Expected intensive care stay longer than 72 hours. * Enteral or parenteral nutritional therapy started or planned. * Arterial catheter in place for clinically indicated blood sampling. * Participation considered clinically safe by the responsible physician and research nurse. Exclusion Criteria: * Ongoing or planned continuous renal replacement therapy or intermittent - hemodialysis (CRRT/iHD). * Extracorporeal membrane oxygenation (ECMO). * Fraction of inspired oxygen (FiO2) \>0.70. * Severe renal failure or rapidly increasing creatinine without dialysis, where phosphate levels may not adequately reflect metabolic changes. * Terminal illness or decision for end-of-life care. * Previous participation in the study during the same ICU admission.