A single computer-controlled mechanical insufflation allows determination of the pressure-volume relationship of the respiratory system.

Abstract

To evaluate and further develop a method for determination and mathematical characterisation of the elastic pressure-volume (Pel-V) relationship in mechanically ventilated human subjects during one single modified insufflation with simultaneous determination of resistance of the respiratory system. Eight adult non-smoking human subjects without heart, lung, or thoracic cage disease scheduled for non-thoracic surgery. The study was performed in anaesthetised and muscle-relaxed subjects. The Pel-V curve was determined with a computer-controlled Servo Ventilator 900C during a modified insufflation with either constant or sinusoidally varying flow. Pressure and flow were measured with the built-in sensors of the ventilator. Tracheal pressure (Ptr) was calculated by subtracting the pressure drop over the tracheal tube. The elastic recoil pressure in the peripheral lung, Pel, was obtained from the calculated Ptr by subtracting the pressure drop over the airways. Ptr was also directly measured through a catheter. The calculated Ptr gave similar results as the directly measured Ptr, thus indicating the reliability of the signal originating from the ventilator sensor for computation of downstream pressures. The inflection points of the sigmoidal Pel-V curve and the compliance of the linear segment were determined with high reproducibility. Using one single modified insufflation allows a fast and accurate determination of respiratory mechanics. The Pel-V curves were determined with high reproducibility and were adequately described by a three-segment model of the curve incorporating a linear segment between two asymmetrical non-linear segments.