PRESSURE TRANSMISSION ACROSS THE RESPIRATORY SYSTEM AT RAISED LUNG-VOLUMES IN INFANTS

Forced expiratory flow-volume (FEFV) curves can be generated from end- tidal inspiration in infants with use of an inflatable jacket. We have developed a technique to raise lung volume in the infant before gener ation of FEFV curves. Measurements of pressure transmission to the air way opening by use of static maneuvers have shown no change with incre asing lung volume above end-tidal inspiration. The aim of this study w as to determine, under dynamic conditions (i.e., during rapid thoracic compression), whether the efficiency of pressure transmission across the chest wall is altered by raising lung volume above the tidal range . Dynamic pressure transmission (Ptx,dyn) was measured in five infants (age 6-17 mo). Jacket pressure (Pj), esophageal pressure, and volume were measured throughout passive and FEFV curves at lung volumes set b y 10, 15, and 20 cmH(2)O preset pressure. The group mean Ptx,dyn was 3 7 +/- 6% (SE) of Pj at end-tidal inspiration, and no change was seen w ith further increases in lung volume. However, a mean decrease in Ptx, dyn of 42% was evident throughout the tidal volume range (i.e., from e nd-tidal inspiration to end expiration). Isovolume static pressure tra nsmission (Ptx,st) was measured in three of the five infants by inflat ion of the jacket in a stepwise manner with the airway closed. Measure ments were made at end-tidal inspiration and lung volumes at 10, 15, a nd 20 cmH(2)O preset pressure. Resulting changes in Pj, esophageal pre ssure, and airway opening pressure were compared using linear regressi ons to determine Ptx,st. Mean Ptx,st to the airway opening was 56 +/- 4% (SE) at end-tidal inspiration and remained constant with increasing lung volume. We conclude that the transmission of Pj across the chest wall, during rapid thoracic compression, decreases throughout the tid al volume range but does not change with increased lung volume above e nd-tidal inspiration.