Estimation of respiratory parameters during mechanical ventilation: A simple method taking account of tubing compliance

A lumped parameter non linear model of a typical ventilator-patient circuit was used to simulate flow and pressure signals in a wide variety of respiratory disorders. Patient lungs were described by the series connection of a compliance (CL = constant) and a resistance proportional to the airflow (KL, = proportionality constant). Aim of this work is to present an estimation algorithm which allows to reduce the error on the estimate of the patient respiratory parameters when using pressure and flow signals at the ventilator side instead of the patient side. The proposed algorithm is based on a simplified model of the ventilator-patient circuit, which incorporates the tubing compliance (Ctube) and neglects the pressure losses along the tubes during flow. Ctube CL and KL are recursively estimated by minimizing a weighted pressure RMSE. The patient parameter estimates, thus obtained, are compared with the "true" values and those provided by the same algorithm when Ctube. is omitted in the simplified model. Simulation results show that the respiratory circuit may significantly affect the estimate of CL and KL when using airflow and pressure at the ventilator side. The proposed estimation approach appears capable to greatly improve the estimate, in particular in the presence of an increased respiratory resistance.