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 (EEVCW) 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 EEVCW changes measured by OEP resulted very close to the identity line (slope 1.06, intercept -0.02 L, r(2) = 0.89) and their difference was not related to their absolute magnitude. After PEEP increase, the new steady state of EEVCW was reached approximately in 15 breaths; and, after PEEP decrease, in 3-4 breaths. The slow increase in EEVCW was mainly because of the abdominal compartment. Conclusion: OEP measurements of EEVCW accurately reflect the changes of EELV. Furthermore, OEP allows a continuous compartmental analysis, even during unsteady conditions.