A systematic method for validation of gas exchange measurements

Area of Science:

• Critical Care Medicine
• Respiratory Physiology
• Biomedical Engineering

Background:

• Accurate measurement of gas exchange is crucial for managing critically ill patients on mechanical ventilation.
• Current methods for assessing gas exchange during ventilation are often impractical in clinical settings.

Purpose of the Study:

• To validate two new commercial instruments for measuring oxygen consumption (VO2) and carbon dioxide production (VCO2) during mechanical ventilation.
• To assess the accuracy and reliability of these instruments using a lung model simulating physiological conditions.

Main Methods:

• A lung model was constructed using a gas jar and anesthesia bag, with precise delivery of CO2 and N2 to simulate metabolic gas exchange.
• The model was ventilated at various tidal volumes and frequencies, mimicking adult mechanical ventilation parameters.
• Two commercial instruments, Siemens-Elema Servo Ventilator 900B and Beckman Metabolic Cart, were used to measure VO2 and VCO2.
• The measured respiratory quotient (RQ) was compared against the known RQ of burning methanol (0.67).

Main Results:

• Both instruments accurately measured VO2 within 5-13% of predicted values at adult-relevant tidal volumes, unaffected by varying fraction of inspired oxygen (FiO2).
• Measurement discrepancies for VCO2 increased at tidal volumes below 350 ml.
• The Siemens-Elema system showed a 5% difference in measured vs. actual RQ, while the Beckman system demonstrated a 1.5% difference.

Conclusions:

• The validated instruments demonstrate potential for practical gas exchange measurement in mechanically ventilated patients.
• The Beckman Metabolic Cart exhibited higher accuracy in RQ measurement compared to the Siemens-Elema Servo Ventilator 900B.
• Further evaluation is needed, particularly regarding VCO2 accuracy at lower tidal volumes.