Effect of PEEP on Indirect Calorimetry in Critically Ill Patients

Indirect calorimetry (IC) is considered a reference method for assessment of energy expenditure in critically ill patients. It measures oxygen consumption (VO2) and carbon dioxide production (VCO2), from which respiratory quotient (RQ) and resting energy expenditure (REE) are calculated. Reliable measurements are important when IC is used to guide nutritional therapy in mechanically ventilated intensive care patients.

Measurement accuracy may potentially be influenced by ventilator settings and changes in respiratory mechanics. Positive end-expiratory pressure (PEEP) is frequently adjusted during intensive care, but there is limited clinical evidence regarding whether changes in PEEP within commonly used ranges systematically affect contemporary indirect calorimetry measurements. The approved research project therefore includes a physiological substudy specifically examining the influence of PEEP on VO2, VCO2, RQ, and REE.

This is a prospective, single-center, repeated-measures physiological study conducted in mechanically ventilated adult intensive care patients at Sahlgrenska University Hospital. Participants undergo indirect calorimetry during a standardized sequence of three PEEP levels: 8, 12, and 16 cmH2O. Each participant therefore serves as their own control.

At each PEEP level, the patient is allowed to reach a stable condition before indirect calorimetry measurements are obtained. The ethics protocol specifies a stabilization period before measurement and that other ventilator settings should remain unchanged during the PEEP sequence. The final operational study protocol uses PEEP levels of 8, 12, and 16 cmH2O.

Indirect calorimetry measurements will include VO2, VCO2, RQ, and REE. Measurements will be performed under standardized conditions, with attention to measurement stability and ventilator leakage. In the underlying research protocol, acceptable IC measurement periods are defined using steady-state criteria and controlled ventilator conditions.

The primary objective is to determine whether VO2, VCO2, RQ, and REE change systematically across PEEP levels of 8, 12, and 16 cmH2O. Repeated measurements will be analyzed using mixed-effects models accounting for clustering of observations within participants.

PEEP changes are performed within clinically accepted ranges under continuous intensive care monitoring. The measurement sequence will be interrupted if clinically relevant deterioration in hemodynamics, oxygenation, or other physiological parameters occurs. The ethics protocol specifically requires PEEP changes to remain within clinically acceptable levels and to be discontinued if adverse physiological effects are observed.

A relative difference of 10% in indirect calorimetry-derived measurements between PEEP levels was considered clinically meaningful. Assuming a within-participant standard deviation of approximately 12.5%, corresponding to a standardized paired effect size of 0.8, approximately 15 evaluable participants are required to achieve 80% power at a two-sided alpha level of 0.05. To account for incomplete measurement sequences and technically inadequate indirect calorimetry measurements, 20 participants are planned for enrollment.

Inclusion criteria

• Age 18 years or older.
• Admitted to CIVA or NIVA at Sahlgrenska University Hospital.
• Receiving invasive mechanical ventilation.
• Expected intensive care stay longer than 72 hours.
• Arterial catheter in place.
• Clinical condition considered sufficiently stable by the responsible physician and research nurse to undergo the study-specific PEEP sequence.
• PEEP levels of 8, 12, and 16 cmH2O considered clinically acceptable for the participant during the study measurement period.

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 where metabolic measurements may be difficult to interpret.
• Terminal illness or decision for end-of-life care.
• Previous participation in the study during the same ICU admission.
• Clinical instability or other condition judged by the responsible physician to make the PEEP measurement sequence unsafe or inappropriate.