Flow and Volume Dependence of Respiratory Mechanics in Anesthetized Children

Abstract

With the use of constant flow, end-inspiratory airway occlusion, respiratory system resistance (Rrs) can be partitioned into a flow resistive component (Rint) and an additional component (deltaR), reflecting viscoelasticity and time constant inequality. Similarly, respiratory system elastance (Edyn) can be partitioned into static elastance (Est) and elastance due to viscoelasticity and time constant inequality (deltaE). We measured Rrs and Edyn and their subdivisions (Rint and deltaR, Est and deltaE, respectively) and studied their flow and volume dependence in eight otherwise healthy children (median age 3.6 y; range 1.9-5.2 y) undergoing general anesthesia for oral rehabilitation. With a constant inspiratory flow (VI) of approximately 15 mL/s/kg and tidal volume of 12 mL/kg, the mean values of Rrs, Rint, and deltaR were: 0.20, 0.11, and 0.10 cmH2O/mL/s.kg. Under the same conditions, the mean Est and deltaE were: 1.04 and 0.12 cmH2O/mL/kg. With increasing VI and under constant VT, deltaR decreased (p < 0.001) progressively. Rint also decreased paradoxically (p < 0.001). Hence, Rrs decreased (p < 0.001) with increasing VI. Est decreased (p < 0.001) with increasing VI, whereas delta E increased (p < 0.005). With increasing VT and under constant VI, Rint decreased (p < 0.001) and deltaR tended to increase (p = 0.058); Rrs did not change. With increasing VT under constant VI, both Est and deltaE decreased (p < 0.001 and p = 0.001, respectively). Thus, in contrast to the findings in adults, Rint and Est decreased in children with increasing flow and under constant tidal volume, probably reflecting decreased functional residual capacity in anesthetized children, compared with adults. The flow and volume dependence of deltaR and deltaE were similar to those in adults, whereas Rrs did not necessarily follow the direction of changes of deltaR.

Airway resistance variability and response to bronchodilator in children with asthma

“…Rrs changes with dilation and re-narrowing associated with breathing, but also with alterations in upper airway geometry associated with the glottis [9,36,37]. Rrs also changes with flow associated with turbulence and other effects and in anaesthetised children can change two-fold with a three-fold change in flow under isovolumetric conditions [38]. Agonists can modulate the volume dependence of Rrs [39,40], which may have contributed to the changes in Rrs and Rrs SD in asthma and with BD.…”

The transient relationship between pressure and volume in the pediatric pulmonary system

“…The parameters in these models have been estimated using pressure response to volumetric step inputs and constant flow inputs. For respiratory response in anesthetized children, Kaditis et al (1999) found different estimates of model parameters than those seen in adults (D'Angelo et al, 1991). Further, this model has been used to characterize the simple viscoelastic response of young and adult pigs (De Robertis et al, 2001) and static to slow dynamic response in pigs (Bitzen et al, 2004).…”

“…The linear model of the current study can be expressed as a discrete lumped-parameter model with constant coefficients with a form as shown in Fig. 1 and compared with the pediatric model of Kaditis et al (1999) (Table 3). One characteristic of the Kaditis model, and the models of Bates et al (1985), is that the presence of the isolated dashpot introduces a Dirac delta function into the model's transfer function.…”

“…First, the Kaditis model predicts less rate sensitivity. Over the range from 0.01 to 2 Hz, the pressure at peak volume increases by only approximately 20% in the Kaditis Table 2, restated in terms of a lumped parameter model and scaled by subject mass, as proposed by D' Angelo et al (1991) and Kaditis et al (1999). b IF: isoflow conditions; IV: isovolume conditions.…”

“…where P(t) is the transient change in pressure measured by the tracheal transducer, V(t) is the volume of air injected into the pulmonary system after passive exhalation, and P e (V) is the Angelo et al, 1991;Milic-Emili et al, 1990;Kaditis et al 1999). The independent variable is lung volume, and the dependent variable is lung pressure.…”