Critically ill patients who require mechanical ventilation frequently experience rapid loss of muscle mass and physical function during their stay in the intensive care unit (ICU). As part of standard care, physiotherapy and limb mobilization exercises are commonly provided, even in patients who are sedated and confined to bed in a supine position. However, the actual physiological and metabolic intensity of these routine physiotherapy interventions is poorly understood, and current prescriptions are largely based on clinical judgment rather than objective measures of patient effort or tolerance.

Oxygen consumption (VO₂) is a direct indicator of metabolic demand and physiological workload. In mechanically ventilated patients, indirect calorimetry integrated into the ventilator allows continuous measurement of VO₂ and carbon dioxide production without adding invasive procedures. While indirect calorimetry is routinely used to individualize nutritional support in the ICU, its potential role in quantifying the metabolic cost of physiotherapy interventions has been scarcely explored.

The aim of this prospective observational study is to quantify the acute metabolic response to a standardized session of supine physiotherapy in adult critically ill patients receiving invasive mechanical ventilation. During routine physiotherapy sessions performed as part of usual care, oxygen consumption will be continuously measured using indirect calorimetry integrated into the ventilator circuit. Each session will include a baseline resting period, the physiotherapy intervention itself, and a post-intervention recovery period, allowing patients to act as their own controls.

Physiotherapy sessions will consist of passive limb mobilization in deeply sedated patients and passive or active-assisted mobilization in patients who are awake or lightly sedated and able to cooperate. The primary outcome will be the change in oxygen consumption during physiotherapy compared with baseline rest. Secondary analyses will describe the total metabolic load of the session, peak oxygen consumption, and the time required for oxygen consumption to return toward baseline levels after the intervention. Basic cardiorespiratory variables, such as heart rate, blood pressure, oxygen saturation, and ventilatory parameters, will also be recorded to assess physiological stability and tolerance.

By objectively characterizing the metabolic cost of common supine physiotherapy interventions, this study aims to improve understanding of the physiological demands imposed on mechanically ventilated ICU patients. The results may help inform safer and more individualized physiotherapy prescriptions in critical care, supporting a more objective approach to dosing rehabilitation based on patients' real metabolic responses rather than solely on activity type or sedation level.

Inclusion Criteria:

• Adults aged 18 years or older.
• Admission to the intensive care unit.
• Invasive mechanical ventilation via endotracheal tube or tracheostomy.
• Ability to remain in the supine position during the measurement period.
• Clinical indication for supine physiotherapy as part of usual ICU care.
• Richmond Agitation-Sedation Scale (RASS) score between -5 and 0 at the time of the session.
• Hemodynamic stability, defined as mean arterial pressure ≥65 mmHg with or without vasopressor support and no initiation or increase >20% in vasopressor dose within the previous 60 minutes.
• Respiratory stability, defined as no changes in ventilator settings within the previous 30 minutes and oxygen saturation ≥90% with FiO₂ ≤0.60 and PEEP ≤12 cmH₂O.
• Technical conditions allowing valid indirect calorimetry measurement (absence of significant air leaks and no planned circuit disconnections).

Exclusion Criteria:

• Use of extracorporeal membrane oxygenation (ECMO) or other extracorporeal gas exchange systems.
• Continuous renal replacement therapy during the measurement period when it may interfere with metabolic measurements.
• Significant air leaks in the airway or ventilator circuit that invalidate indirect calorimetry measurements.
• Planned procedures or interventions during the measurement period that could interfere with gas exchange (e.g., bronchoscopy, recruitment maneuvers, ventilator setting changes).
• Severe respiratory instability requiring FiO₂ >0.60 or PEEP >12 cmH₂O.
• Ongoing neuromuscular blockade infusion.
• Clinical contraindications to limb mobilization (e.g., unstable fractures, active bleeding, uncontrolled intracranial hypertension).
• Agitation or risk of unplanned device removal (RASS >0).
• Refusal of participation by the patient or legally authorized representative, when applicable.

Adult critically ill patients admitted to the intensive care unit who require invasive mechanical ventilation and receive routine supine physiotherapy as part of standard care. The study population includes both deeply sedated patients undergoing passive mobilization and awake or lightly sedated patients able to perform active-assisted mobilization, allowing characterization of metabolic responses across different levels of consciousness and participation.