Bicarbonate Variation Monitoring With Indirect Calorimetry

Patients admitted to the intensive care unit frequently exhibit abnormal blood bicarbonate levels. These abnormalities are associated with patient outcomes and often prompt clinicians to modify management, either diagnostically or therapeutically (e.g., initiation or adjustment of mechanical ventilation, initiation or modification of extracorporeal renal replacement therapy, administration of specific diuretics, or sodium bicarbonate infusion). Currently, the only way to determine blood bicarbonate concentration is through repeated blood sampling for laboratory analysis. However, this method is both discontinuous and invasive. We have generated an in silico hypothesis suggesting that the mismatch between cellular CO₂ production/elimination and pulmonary CO₂ elimination reflects changes in blood bicarbonate levels. We propose to test this hypothesis in a cohort of intensive care unit patients. If confirmed, indirect calorimetry could be positioned as a continuous, non-invasive monitoring tool for blood bicarbonate.

Inclusion Criteria: * Adult patient admitted to the ICU within the past 72 hours * Receiving invasive mechanical ventilation * For whom the treating clinician plans to perform at least two blood bicarbonate measurements within the next 6 hours * No objection from the participant or their next of kin Exclusion Criteria: * Planned weaning from invasive mechanical ventilation or ventilator change before the second bicarbonate measurement * Conditions interfering with the interpretation of indirect calorimetry data: * Inspiratory oxygen fraction \> 60% * Administration of inhaled medications, including nitric oxide * Positive end-expiratory pressure (PEEP) \> 15 cmH₂O * Excessive leaks in the ventilator circuit, defined as expired volume \< 95% of inspired volume * Patient on mechanical circulatory support with a gas exchanger, or planned initiation before the second blood gas analysis * Patient on extracorporeal renal replacement therapy, or planned initiation before the second bicarbonate measurement * Patient who received sodium bicarbonate within 2 hours prior to inclusion, or planned administration before the second bicarbonate measurement * Frequent disconnections of the ventilator circuit, or planned bronchoscopy before the second bicarbonate measurement