Intrinsic properties of lung and chest wall tissues can lead to breathing rate (frequency [f]) and amplitude (tidal volume [V T ]) 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 (P ao ) and flow (VΠ 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 V T (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 V T (R = 764Wt -0.91 ΠΈ f -0.57 ). E decreased sharply with increasing Wt, was lower at higher V T , and was slightly, yet significantly, increased at higher frequency (E = 2,905Wt -1.38 ΠΈ V T -0.18 ΠΈ f 0.11 ). Such frequency dependence of R and E is consistent with stress adaptive, or viscoelastic, properties of respiratory tissues. The small V T 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 V T 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 V T . 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.