Dependence of Intrapulmonary Pressure Amplitudes on Respiratory Mechanics during High-Frequency Oscillatory Ventilation in Preterm Lambs

摘要

In the healthy animal lung, high-frequency oscillatory ventilation (HFOV) achieves effective ventilation at tidal volumes (VT) less than or equal to dead space while generating very small pressure fluctuations in the alveolar spaces (ΔPA). We hypothesized that the respiratory mechanical parameters influence the magnitude of the intrapulmonary pressure fluctuations during HFOV. A computer model of the neonatal respiratory system was used to examine the independent effects of altering the compliance, nonlinear and linear resistance, and inertance of the respiratory system on VT, and cyclic intrapulmonary pressures under homogeneous and heterogeneous conditions. The impact of low compliance on the transmission of pressure from the airway opening to the trachea (ΔPtr/ΔPao) and alveolar compartment (ΔPA/ΔPao) during HFOV was determined in a preterm lamb lung model. In the computer model, an increase in flow-dependent resistance to simulate changing the internal diameter of the tracheal tube from 4.0 mm to 2.5 mm halved the transmission of the pressure waveform to both the carina and the alveolar compartment. Increased peripheral resistance was associated with an increased ΔPtr/ΔPao but a reduction in ΔPA/ΔPao. The ΔPA/ΔPao also decreased with increasing alveolar compartment compliance, a finding that was verified in the preterm lamb lung. There was an exponential decrease in the magnitude of ΔPA1 compared with ΔPA2 as the ratio of the time constants of the two parallel compartments (τ1/τ2) increased in the heterogeneous computer lung model. The transmission of driving pressure amplitude to both the proximal airways and lung tissue during HFOV is dependent on lung mechanics and may be greater in the poorly compliant lung than that observed previously in experiments on healthy animals.Abbreviations: ΔPA, amplitude of oscillatory pressure waveform in the alveolar compartment; ΔPao, amplitude of oscillatory pressure waveform in the trachea; ΔPtr, amplitude of oscillatory pressure waveform in the trachea; C, total compliance of parallel alveolar compartments; C1, C2, compliance of independent parallel compartments; Cg, compliance of central airways, including gas compressibility; Cti, tissue compliance in ovine lung model; G, coefficient of tissue damping; H, coefficient of tissue elasticity; HFOV, high-frequency oscillatory ventilation; I, inertance of central airways; ID, internal diameter of tracheal tube; MAwP, mean airway pressure (as measured and displayed by ventilator); R1, R2, resistance of first and second parallel compartments; Rc, resistance of central airways; Rp, total resistance of parallel compartments; RTT, tracheal tube resistance; TT, tracheal tube; VT, tidal volume; τ, time constant (τ = resistance × compliance)