Flow and volume dependence of respiratory mechanics in anesthetized children

With the use of constant flow, end-inspiratory airway occlusion, respirator y system resistance (R-rs) can be partitioned into a flow resistive compone nt (R-int) and an additional component (Delta R), reflecting viscoelasticit y and time constant inequality. Similarly, respiratory system elastance (E- dyn) can be partitioned into static elastance (E-st) and elastance due to v iscoelasticity and time constant inequality (Delta E). We measured R-rs and Ed(yn) and their subdivisions (R-int and Delta R, E-st and Delta E, respec tively) and studied their flow and volume dependence in eight otherwise hea lthy children (median age 3.6 y; range 1.9-5.2 y) undergoing general anesth esia for oral rehabilitation. With a constant inspiratory flow ((V) over do t(I)) of approximately 15 mL/s/kg and tidal volume of 12 mL/kg, the mean va lues of R-rs, R-int, and Delta R were: 0.20, 0.11, and 0.10 cmH(2)O/mL/s . kg. Under the same conditions, the mean E-st and Delta E were: 1.04 and 0.1 2 cmH(2)O/mL/kg. With increasing (V) over dot(I) and under constant V-T, De lta R decreased (p < 0.001) progressively. R-int also decreased paradoxical ly (p < 0.001). Hence, R-rs decreased (p < 0.001) with increasing (V) over dot(I). E-st decreased (p < 0.001) with increasing (V) over dot(I), whereas Delta E increased (p < 0.005). With increasing V-T and under constant (V) over dot(I), R-int decreased (p < 0.001) and Delta R tended to increase (p = 0.058); R-rs did not change. With increasing V-T under constant (V) over dot(I), both E-st and Delta E decreased (p < 0.001 and p = 0.001, respectiv ely). Thus, in contrast to the findings in adults, R-int and E-st decreased in children with increasing how and under constant tidal volume, probably reflecting decreased functional residual capacity in anesthetized children, compared with adults. The flow and volume dependence of Delta R and Delta E were similar to those in adults, whereas R-rs did not necessarily follow the direction of changes of Delta R.