A Method of Calculating Total Respiratory System Compliance from Resonant Frequency: Validity in a Rabbit Model

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Abstract
Ten anesthetized, tracheotomized, adult rabbits were used to test the validity of a method for calculation of total respiratory system compliance from resonant frequency (Cr). Reference values were obtained during constant flow inflation of the relaxed respiratory system by dividing the volume gain by the related difference in pressure at the airway opening (inflation method compliance, Ci). The animals were connected to a new type of servo-controlled infant ventilator. Besides volume-controlled mechanical ventilation at constant inspiratory flow rate and intermittent mandatory ventilation, there is a negative ventilator resistance mode integrated in this device for resistive unloading (Schulze A, Schaller P, Gehrhardt B, Mädler H-J, Gmyrek D: Pediatr Res 28:79-82,1990). To measure resonant frequency (fr), the respiratory system was totally unloaded for a short period by a negative ventilator resistance exceeding the combined resistances of the endotracheal tube and airways. This evoked a continuous oscillation at fr. By analogy with electrical circuit theory, Cr was calculated according to C=l/(4π2· I · fr 2) where C is compliance and I is inertance. The inertance of the endotracheal tube is given and that of the bronchial tree was ignored assuming a much greater total cross-sectional area and therefore much lower inertance when compared with the endotracheal tube. Three pairs of Ci – Cr values were obtained from each animal: 1) during intact respiratory muscle activity; 2) after pancuronium relaxation, and 3) after surfactant depletion by saline washout. There was a significant linear correlation between Ci and Cr values with the regression line lying close to the identity line (Cr=1.1 Ci – 0.74; r=0.97; pi increased from 45.3 ± 9.4 to 47 ± 7.1 mL/kPa after pancuronium relaxation and dropped to 25.5 ± 5.2 after surfactant depletion. The corresponding Cr values were 49.5 ± 11, 51 ± 8.2, and 27 ± 5.9 mL/kPa, respectively. The fr value decreased from 274 ± 33.2 min-1 before to 267 ± 21.4 after relaxation, and was 371 ± 39.5 min-1 after injury.