Effects of rate and amplitude of breathing on respiratory system elastance and resistance during growth of healthy children

Intrinsic properties of lung and chest wall tissues can lead to breathing rate (frequency [f]) and amplitude (tidal volume [VT]) dependence of respiratory system resistance (R) and elastance (E). To explore these dependencies on R and E within physiological limits of tidal volume and breathing frequencies during early childhood. we measured airway opening pressure (Pao) and flow (V′ao) in 15 anesthetized, paralyzed, intubated, and mechanically ventilated healthy children (age 1 day to 72 months; weight 2.5−21 kg) at multiple combinations of VT (6, 10, and 14 mL/kg) and frequency (10, 20, and 30 breaths/min). In each instance, R and E were estimated by multiple linear regression applied to the tracheal pressure, flow, and volume (V), assuming a simple series R‐E model. R decreased substantially with increasing frequency and weight (Wt), but was unaffected by changes in VT (R = 764Wt−0.91 · f−0.57). E decreased sharply with increasing Wt, was lower at higher VT, and was slightly, yet significantly, increased at higher frequency (E = 2,905Wt−1.38 · V T −0.18 · f 0.11). Such frequency dependence of R and E is consistent with stress adaptive, or viscoelastic, properties of respiratory tissues. The small VT dependence of E is similar to that observed in other species under healthy conditions and presumably reflects the combined nonlinear pressure‐volume relationships of the healthy parenchymal and chest wall tissues. Lack of VT dependence of R at high inspiratory flow rates suggests that turbulent flows are either not an important form of energy dissipation in the lower airways of children or they are counterbalanced by a decrease in tissue damping at high VT. The above regression models represent the first attempt to quantify simultaneously the separate effects of lung growth as well as rate and amplitude of breathing on R and E. Similar equations based on a larger sample of healthy subjects can provide normative R and E values for comparison with mechanically ventilated children with lung disease. Pediatr Pulmonol. 1998; 25:270–277. © 1998 Wiley‐Liss, Inc.