Estimation of end-expiratory lung volume variations by optoelectronic plethysmography

Objective: To test the capability of optoelectronic plethysmography (OEP) to monitor positive end-expiratory pressure (PEEP)-induced changes of end-expiratory lung volume (EELV) changes in mechanically ventilated paralyzed patients. Design: Laboratory and clinical investigation. Setting: Intensive care unit of the Ospedale Maggiore Policlinico di Milano. Patients: A total of eight patients with respiratory failure of various degrees, sedated and paralyzed. Interventions: PEEP variations (±5 cm H₂O) relative to the baseline PEEP of 10 cm H₂O. Measurements and Main Results: In the model protocol, we tested the reproducibility of the OEP by repeating volume measurements of a plastic torso model over a 21-hr period, every 30 mins. The variations of OEP measurements of the torso model (9337 mL value) were encountered in a range of 16 mL (SD = 4 mL). In the patient protocol, we measured the end-expiratory volume of the chest wall (EEV_cw) breath-by-breath by OEP before, during, and after the PEEP increase/decrease and we compared its variations with the corresponding variations of EELV measured by helium dilution technique. The regression line between EELV changes measured by helium and EEV_cw changes measured by OEP resulted very close to the identity line (slope 1.06, intercept -0.02 L, r² = 0.89) and their difference was not related to their absolute magnitude. After PEEP increase, the new steady state of EEV_cw was reached approximately in 15 breaths; and, after PEEP decrease, in 3-4 breaths. The slow increase in EEV_cw was mainly because of the abdominal compartment. Conclusion: OEP measurements of EEV_cw accurately reflect the changes of EELV. Furthermore, OEP allows a continuous compartmental analysis, even during unsteady conditions.