Application of continuous positive airway pressure to trace static pressure-volume curves of the respiratory system

Objective: To evaluate a new technique for pressure-volume curve tracing. Design: Prospective experimental study. Setting: Animal research laboratory. Subjects. Six anesthetized rats. Interventions: Two pressure-volume curves were obtained by means of the super-syringe method (gold standard) and the continuous positive airway pressure (CPAP) method. For the CPAP method, the ventilator was switched to CPAP and the pressure level was raised from 0 to 50 cm H2O in 5 cm H2O steps and then decreased, while we measured lung volume using respiratory inductive plethysmography. Thereafter, lung injury was induced using very high-volume ventilation. Following injury, two further pressure-volume curves were traced. Pressure-volume pairs were fitted to a mathematical model. Measurements and Main Results. Pressure-volume curves were equivalent for each method, with intraclass correlation coefficients being higher than .75 for each pressure level measured. Bias and precision for volume values were 0.46 +/- 0.875 mL in basal measurements and 0.31 +/- 0.67 mL in postinjury conditions. Lower and upper inflection points on the inspiratory limb and maximum curvature point on the deflation limb obtained using both methods and measured by regression analysis also were correlated, with intraclass correlation coefficients (95% confidence interval) being .97 (.58, .99), .85 (.55, .95), and .94 (.81, .98) (p < .001 for each one). When inflection points were estimated by observers, the correlation coefficient between methods was .90 (.67, .98) for lower inflection points (p < .001). However, estimations for upper inflection points and maximum curvature point were significantly different. Conclusions: The CPAP method for tracing pressure-volume curves is equivalent to the super-syringe method. It is easily applicable at the bedside, avoids disconnection from the ventilator, and can be used to obtain both the inspiratory and the deflation limbs of the pressure-volume curve. Use of regression techniques improves determination of inflection points.