The dynostatic algorithm accurately calculates alveolar pressure on-line during ventilator treatment in children

Abstract

Background: Monitoring of respiratory mechanics during ventilator treatment in paediatric intensive care is currently based on pressure and flow measurements in the ventilator or at the Y-piece. The characteristics of the tracheal tube will modify the pressures affecting the airways and alveoli in an unpredictable manner. The dynostatic algorithm (DSA), based on a one-compartment lung model, calculates the alveolar pressure during on-going ventilation. The DSA is based on accurate measurement of tracheal pressure. The purpose of this study was to test the validity of the DSA in a paediatric lung model and to apply the concept in an observational clinical study in children.

Methods: We validated the DSA in a paediatric lung model with linear, nonlinear pressure flow and frequency-dependent characteristics by comparing calculated dynostatic (alveolar) pressures with directly measured alveolar pressures in the model and proximal plateau pressure with maximum alveolar pressure. Sixty combinations of ventilation modes, positive end expiratory pressures, inspiratory : expiratory ratios, volumes and frequencies were studied. A 0.25-mm fibreoptic pressure transducer in the tube lumen was used in combination with volume and flow from ventilator signals. Clinical measurements were performed in eight patients during anaesthesia and postoperative ventilator treatment.

Results: In the lung model we found a correlation coefficient between calculated and measured alveolar pressure of 0.93-0.99 with root mean square median values of 1 cm H2O. Distal plateau pressure agreed well with maximum alveolar pressure. In the clinical situation, the algorithm provided a breath-by-breath display of the volume-dependent lung compliance and the temporal course of alveolar pressure during uninterrupted ventilation.

Conclusions: Fibreoptic measurement of tracheal pressure in combination with the dynostatic calculation of alveolar pressure provides an on-line monitoring of the effects of ventilatory mode in terms of volume-dependent compliance, tracheal peak pressure and true positive end expiratory pressure.