Respiratory mechanics during mechanical ventilation in health and in disease

University dissertation from Dept of Clinical Physiology, University Hospital, SE-221 85 Lund, Sweden

Abstract: The elastic pressure-volume (Pel-V) curve of the respiratory system can be used as a guide for improved ventilator management. It may indicate risks of lung damage associated with collapse or overdistension of lung units. The work described in this thesis was aimed at developing methods for the determination and characterisation of the Pel-V curve under static and dynamic conditions and at a better understanding of the Pel-V relationship in health and disease. A Servo Ventilator 900C, controlled by a computer, allowed standardised and accurate recording of extended Pel-V curves. The flow interruption technique was used to determine static Pel-V (Pel,st-V) curves and viscoelastic pressure-volume loops. A method for the determination of the dynamic Pel-V curve during a single prolonged insufflation, which can be used in clinical routines, was developed. An iterative technique was applied for the estimation of parameters characterising the elastic and viscoelastic behaviour of the respiratory system according to linear and non-linear models. During ventilation with small tidal volumes, the Pel,st-V curve was essentially linear in health and disease. At larger tidal volumes it was non-linear. Hysteresis in the Pel,st-V curve was very small in healthy humans and rabbits and in patients with critical lung disease of various kinds. The viscoelastic behaviour was best described by a non-linear model. The high viscoelastic recoil pressure observed at high volumes may contribute to lung damage. The influence of collapse and recruitment on the Pel-V curve in healthy subjects is not limited to the lower part of the curve, but extends up to high volumes and pressures. The conceptual and technical development, as described in this thesis, will hopefully pave the way for a better general understanding of the physiology behind the features of the Pel-V curve and for its use in ventilator management in clinical routines.

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