124 FLOW DEPENDENCE OF RESPIRATORY MECHANICS IN MECHANICALLY VENTILATED NEONATES

Abstract

To investigate the effects of inspiratory flow and/or inspiratory time on the different components of respiratory mechanics, 15 mechanically ventilated neonates for respiratory failure, were studied (gestational age : 26 to 40 weeks; birth weight : 750 to 5900 gm). All neonates were ventilated during volume controlled ventilation with a Servo 900C Siemens ventilator. Dynamic and static elasticity (E_dyn,rs and E_st,rs), airway and tissue resistance (R_aw and R_vis,rs) ^ intrinsic positive-end-expiratory pressure (PEEPi) were calculated by using interrupter technique (1) at various inspiratory times (20, 25, 33, 50 and 67 per cent of the respiratory cycle), without changing of the tidal volume and the respiratory rate. R_aw increased whereas R_visc,rs decreased with increasing flow, provoking decreasing of total respiratory system resistance (R_tot,rs = R_aw + R_visc,rs). R_visc,rs represented an important part of R_tot,rs (40 - 70% as a function of flow). E_st,rs remained constant but E_dyn,rs increased with ncreasing flow. Time constant of the respiratory system decreased with decreasing inspiratory time below 0.65 seconds. It could traduct an adaptatation of respiratory mechanics with inspiratory time variations to maintain tidal volume constant. These results demonstrate that the respiratory mechanics are flow and/or time dependent in mechanically ventilated newborn infants. This dependence is mainly related to viscoelastic properties of the respiratory system and should be considered to optimize ventilator settings. (1) Similowski T et al. J.Appl.Physiol. 1989;67:2219-2229.

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