Indirect Calorimetry

Indirect calorimetry is used in intensive care to measure how much oxygen a patient consumes and how much carbon dioxide is produced. These measurements are used to estimate energy expenditure and help guide nutritional treatment. In mechanically ventilated patients, however, ventilator settings may potentially influence the accuracy of indirect calorimetry. This study will investigate whether changes in positive end-expiratory pressure (PEEP), a routinely used ventilator setting, affect measurements obtained by indirect calorimetry in critically ill mechanically ventilated adults. Each participant will undergo indirect calorimetry at three PEEP levels: 8, 12, and 16 cmH2O. Each PEEP level will be maintained for a stabilization period before measurements are obtained. Other ventilator settings will be kept unchanged during the measurement sequence whenever clinically feasible. The study will compare oxygen consumption (VO2), carbon dioxide production (VCO2), respiratory quotient (RQ), and resting energy expenditure (REE) across the three PEEP levels. The aim is to determine whether clinically relevant changes in PEEP systematically influence indirect calorimetry measurements and their interpretation.

The primary objective of this prospective observational study is to compare the effects of synchronized intermittent mandatory ventilation in pressure-controlled mode (SIMV-PC) and volume-controlled mode (SIMV-VC) on resting energy expenditure (REE) measured by indirect calorimetry in adult intensive care unit (ICU) patients receiving invasive mechanical ventilation. The primary metabolic parameters to be evaluated are oxygen consumption (VO₂), carbon dioxide production (VCO₂), resting energy expenditure (REE), and respiratory quotient (RQ). Secondary objectives include comparison of respiratory rate, tidal volume, minute ventilation,arterial blood gas parameters, mechanical power, ventilator mechanics, hemodynamic variables, sedation level, and patient-ventilator synchrony between the two ventilation modes. The results of this study may contribute to identifying ventilation strategies associated with lower energy expenditure while maintaining adequate ventilatory support in critically ill patients and may help optimize individualized ventilator management and nutritional therapy.

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.