Estimation of end-expiratory lung volume variations by optoelectronic plethysmography

Objective: To test the capability of optoelectronic plethysmography (OEP) t o monitor positive end-expiratory pressure (PEEP)induced changes of end-exp iratory lung volume (EELV) changes in mechanically ventilated paralyzed pat ients. 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 deg rees, sedated and paralyzed. Interventions. PEEP variations (+/-5 cm H2O) r elative to the baseline PEEP of 10 cm H2O. Measurements and Main Results: In the model protocol, we tested the reprodu cibility of the OEP by repeating volume measurements of a plastic torso mod el over a 21-hr period, every 30 mins. The variations of OEP measurements o f 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 th e PEEP increase/decrease and we compared its variations with the correspond ing variations of EELV measured by helium dilution technique. The regressio n line between EELV changes measured by helium and EEVCW changes measured b y 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 mag nitude. After PEEP increase, the new steady state of EEVCW was reached appr oximately 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 EEL V. Furthermore, OEP allows a continuous compartmental analysis, even during unsteady conditions.