Comparison of the Effects of SIMV-PC and SIMV-VC Ventilation Modes on Resting Energy Expenditure

Mechanical ventilation is a cornerstone of supportive therapy in critically ill patients. Although synchronized intermittent mandatory ventilation in pressure-controlled (SIMV-PC) and volume-controlled (SIMV-VC) modes are widely used in routine intensive care practice, limited data are available regarding their effects on patients' energy expenditure measured by indirect calorimetry.

Indirect calorimetry is recommended by the European Society for Clinical Nutrition and Metabolism (ESPEN) as the reference standard for determining energy expenditure in critically ill patients. Accurate estimation of energy expenditure is essential for optimizing nutritional therapy and avoiding both underfeeding and overfeeding. Overfeeding may increase carbon dioxide production, insulin requirements, respiratory workload, duration of mechanical ventilation, and intensive care unit length of stay.

This is a prospective, observational, repeated-measures study conducted in adult ICU patients receiving invasive mechanical ventilation. The study is non-interventional. Ventilation mode selection and all ventilator adjustments will be determined exclusively by the attending intensive care physician according to routine clinical practice. No study-specific ventilator changes, additional invasive procedures, or additional blood sampling will be performed.

Indirect calorimetry measurements will be performed during clinically stable periods under routine ICU monitoring. Measurements will be obtained only after at least 30 minutes of stabilization following any clinically indicated ventilation mode change. Patients receiving continuous enteral nutrition with no major changes in sedative or analgesic infusions during the preceding two hours will be eligible for metabolic measurements. Sedation level will be documented using the Richmond Agitation-Sedation Scale (RASS).

Metabolic parameters obtained by indirect calorimetry, including oxygen consumption (VO₂), carbon dioxide production (VCO₂), resting energy expenditure (REE), and respiratory quotient (RQ), will be compared between SIMV-PC and SIMV-VC ventilation modes. Ventilator parameters, respiratory mechanics, mechanical power, arterial blood gas findings, hemodynamic variables, and patient-ventilator synchrony will also be evaluated.

Because the study is observational and based entirely on routine clinical care, no additional risks beyond standard intensive care management are anticipated. If hemodynamic or respiratory instability develops during indirect calorimetry measurements, the measurement will be discontinued immediately and routine patient management will continue according to the attending physician's clinical judgment.

Inclusion criteria

• Adults aged 18 to 80 years.
• Receiving invasive mechanical ventilation in the intensive care unit.
• Receiving continuous enteral nutrition during indirect calorimetry measurements.
• Hemodynamically stable (mean arterial pressure ≥65 mmHg without significant increase in vasopressor requirement).
• Technically suitable for indirect calorimetry measurements.
• Written informed consent obtained from the patient or the legally authorized representative.

Exclusion criteria

• Age <18 years or >80 years.
• Pregnancy.
• Hemodynamic instability.
• High vasopressor requirement or marked increase in vasopressor dosage.
• Requirement for frequent or major changes in ventilator settings or FiO₂.
• FiO₂ >0.60.
• Bronchopleural fistula, significant air leak, or other pulmonary conditions interfering with indirect calorimetry.
• Extracorporeal membrane oxygenation (ECMO).
• Continuous renal replacement therapy (CRRT).
• Significant body temperature fluctuations.
• Active agitation or severe patient-ventilator dyssynchrony.
• Acute metabolic changes that may interfere with indirect calorimetry measurements.