Transpulmonary pressure and phrenic activity in early inspiration

The direct impact of changes in end-tidal transpulmonary pressure (P'L) on phrenic (Phr) activity has been investigated in 6 cats anaesthetized with pentobarbital or ketamine. Rapid changes in P'L were accomplished by step-like changes in tracheal pressure (Ptr), i.e., from Ptr = -12, -6, +6 and +9 cmH2O to Ptr = 0, and reversely. Phr activity was determined 0.2 and 0.4 s after the onset of Phr activity. This analysis was carried out for a few breaths immediately preceding and following the Ptr change, and the results for Ptr = 0 were used as a reference. Oppositely directed steps in Ptr, e.g., from Ptr = 0 to Ptr = +6 and, reversely, from Ptr = +6 to Ptr = 0, caused equal but opposite changes in Phr activity. Phr activity decreased with increasing P'L, and this decrease amounted to 15% and 12% for each cmH2O increase in P'L at 0.2 and 0.4 s respectively. These results imply that changes in P'L especially affect Phr activity early in inspiration. The results also show that Phr activity in the initial phase of inspiration strongly depends on P'L. Inhibition of inspiration facilitatory neurons by slowly adapting stretch receptors may be the kernel of the underlying mechanism of our findings.