Measurements of PEEP-induced changes in lung volume: Evaluation of a laser monitor

Study objective: To evaluate a newly developed laser monitor in the measurement of positive end-expiratory pressure (PEEP)-induced changes in end-expiratory lung volume (EELV) in spontaneously breathing subjects. Design: An open comparison between two different methods to assess breathing parameters. Setting: A respiratory research unit. Participants: Six spontaneously breathing, healthy volunteers. Interventions: Stepwise increases of PEEP from 0 to 0.8 to 1.6 kPa during spontaneous breathing; repeated validation of the laser monitor in each subject. Measurements and results: Pressure and flow were recorded at the airway opening. Abdominal wall displacement (AWD) measured by the laser sensor was recorded simultaneously. The time lag between the volume and the laser signals during baseline was 0.068 ± 0.052 s and during maximal PEEP, 0.1084 ± 0.093 s. There was no baseline drift in either the PEEP or the AWD signals. Mean EELV decreased by 290 mL to 1,157 mL after the PEEP valve was removed. Within each individual, the ratio between EELV and AWD showed only small variations. Measurements of tidal volume (V(T)) and AWD showed good agreement at all PEEP levels. Mean V(T) decreased in all but one subject during PEEP. With the increase in PEEP, the end-expiratory abdominal baseline increased linearly over a large range of end-expiratory pressures with a flattening of the curve at high PEEP levels in all subjects. Conclusions: The laser monitor is sufficiently accurate for measuring PEEP-induced changes in EELV during spontaneous breathing in healthy adult subjects. Monitors incorporating multiple laser sensors may have considerable clinical promise.