Evaluation of a CO2 Partial Rebreathing-Based Functional Residual Capacity Measurement Method for Mechanically Ventilated Patients

There is a need for an automated bedside functional residual capacity (FRC) measurement method that can continually monitor both the size and a change in size of a patient’s lung volume during mechanical ventilation without the use of bulky equipment, expensive tracer gases or step increases in inspired oxygen fraction. We developed a CO2 rebreathing method for FRC measurement that simply requires data from a volumetric capnometer (partial pressure of end-tidal carbon dioxide (PetCO2) and volume of CO2 eliminated (VCO2) for the measurement. This study was designed to assess the accuracy, precision and repeatability of the proposed FRC measurement system during stable ventilation. Methods: Accuracy and precision of measurements were assessed by comparing the CO2 rebreathing FRC values to the gold standard, body plethysmography, in nine spontaneously breathing volunteers. Repeatability was assessed by comparing subsequent measurements in nine intensive care patients whose lungs were under mechanical ventilation. The accuracy and precision of the CO2 FRC measurement during mechanical ventilation were then compared to the reference method, modified multiple breath nitrogen washout, in the same ICU patients. Results: Compared to body plethysmography, the accuracy (mean bias) of the CO2 method was -0.085 L and precision (1 standard deviation) was 0.033 L (-2.3 ± 9.2% of body plethysmography). The accuracy in the mechanically ventilated patients was -0.055 L and precision was 0.336 L (-2.6% ± 17.5% of nitrogen washout). The difference between repeated FRC measurements in the ICU patients was 0.020 ± 0.42 L (mean ± standard deviation) (1.1 ± 23.4 %). Conclusions: The CO2 rebreathing method for FRC measurement provides acceptable accuracy and repeatability compared to existing methods during ventilation with mechanical ventilation. Further study of the CO2 rebreathing method is needed. INTRODUCTION Monitoring functional residual capacity (FRC) is an important means of assessing the pulmonary status and the effect of ventilator setting in patients with acute respiratory failure requiring mechanical ventilation 1 . FRC has been used to size the mechanically ventilated lung in acute lung injury (ALI) since the injured lung volume is smaller than predicted for a given patient height 2 . Once the lung has been sized, the tidal volume can be scaled appropriately so as to not provoke additional volutrauma during mechanical ventilation. An automated bedside method is needed for continual monitoring of the FRC so that the mechanical ventilator can be set appropriately in response to the progression of and recovery from ALI. An automated bedside FRC monitor employed during mechanical ventilation should not be bulky, rely on expensive tracer gases or require a step increase in inspired oxygen fraction. We have developed an automated bedside FRC measurement system that is based on the partial rebreathing signals obtained from the NICO2 cardiopulmonary monitor (model 7300, Philips-Respironics, Wallingford, CT). The signal resulting from the partial rebreathing period provides the largest single-breath step change during the transition from the last breath of rebreathing to the first breath of nonrebreathing, and this single-breath transition signal can be used to measure FRC. The FRC measurement signal is comprised of the change in excreted CO2 (VCO2) and the change in partial pressure of endtidal CO2 (PetCO2) during the transition. Although the signal differences obtained during the transition are somewhat small due to the limited degree of rebreathing achieved by partial rebreathing, it appears possible to measure FRC for patients whose lungs are mechanically ventilated under controlled mechanical ventilation.