Control system for automated titration of positive end-expiratory pressure and tidal volume using dynamic nonlinear compliance as the setpoint

Introduction

An automated respiratory mechanics control was developed to individually adapt the energy transfer from the ventilator to the respiratory system. The controller titrates the positive end-expiratory pressure (PEEP) and tidal volume (VT) to ventilate the lung at its maximal compliance in order to avoid excessive lung overinflation as well as underinflation.

Methods

The mechanics controller consists of a software program to set PEEP and VT and a user interface to observe the compliance and the controller state. The program has following structure: (1) Dynamic compliance is calculated breath by breath using the implemented slice function [1]. This function divides VT into six consecutive volume slices of equal size. (2) For each volume slice, one value of dynamic compliance (CSLICE) is determined by least-squares fit using the linear resistance and compliance model within each slice [2]. The six CSLICE values are plotted over the corresponding volume, giving the compliance–volume curve. (3) The shape-compliance function of the controller identifies one out of six shape categories [3]. (4) The PEEP and VT-change function calculates the PEEP and VT titration depending on the shape category and sends a command to the ventilator for setting the new PEEP and VT automatically.

Results

The system was tested with previously recorded patient data (McRem) [4]. The compliance controller retrospectively analysed the respiratory data and determined the shape category depending on the course of CSLICE. For shapes representing an intratidal increase of CSLICE, the controller increased the PEEP. A reduction of PEEP occurred when CSLICE decreased intratidally. PEEP was maintained when CSLICE was maximal and constant. Furthermore, for hybrid shape categories (one part in the linear region and one part in the increasing and/or decreasing region) the VT was reduced.

Conclusion

The automated respiratory mechanics control system titrates PEEP and VT automatically until intratidal compliance reaches its maximal value within an appropriate VT.