1997 OPEN FORUM Abstracts
THE EFFECT OF VENTILATION MODE ON THE C20 /Cdyn INDEX OF LUNG OVERDISTENTION
Robert L. Chatburn RRT, Teresa Volsko RRT. Rainbow Babies & Childrens Hospital Cleveland, OH, MetroHealth Medical Center, Cleveland, OH.
Pulmonary volutrauma is a risk of mechanical ventilation that may be reduced if an appropriate index can be monitored. Fisher et al (Pediat Pulmonol 1988;5:10-14) have described an index of lung overdistention that is the ratio of compliance during the last 20% of inspiration to total dynamic compliance. A value of < 0.80 is consistent with lung overdistention. This index has been incorporated in at least one commercial monitor. Kano et al (J Appl Physiol 1944;77(3):1185-1197) found that the C20/Cdyn index was affected by the mode of ventilation. The purpose of this study was to test the hypothesis that the C20/Cdyn index changes with the mode of ventilation, independent of actual changes in lung overdistention. METHOD: We used a lung model consisting of an elastic bellows (compliance = 1 mL/cm H2O) attached to a 3.0 ET tube. The bellows was inside a rigid walled cylinder with a piston. The position of the piston could be changed to contact the expanding bellows at different points in inspiration. There was one compliance before the bellows touched the piston and a smaller compliance after, simulating overdistention. A PV curve for the model was created with a calibrated syringe and a water manometer. The model was ventilated with a neonatal ventilator (Bear BP200) in either the volume controlled (VCV) mode (time triggered, flow limited, time cycled) or pressure controlled (PCV) mode (time triggered, pressure limited, time cycled) at the same tidal volume. Two tidal volumes were examined, one just below and one just above that required for the bellows to touch the piston (le, with and without simulated lung overdistention as demonstrated on pressure-volume loops). Three sets of data were collected for each tidal volume using a BICORE C-100 neonatal monitor. Mean values for C20/Cdyn were compared with paired t-tests. Results: C20/Cdyn data is mean ± SD in figure below.
Static Curve for Lung Model (See original for figure)
Effect of Mode on Index (See original for figure)
The C20/Cdyn index increased by switching to PCV even though volume and overdistention were constant. CONCLUSION: Presence of a pressure plateau during pressure controlled ventilation masks overdistention. The index is most accurate with constant flow inspiration.