Spirodynamics. New methods for continuous monitoring of respiratory mechanics in ventilator-treated patients

Abstract

Introduction: Ventilator treatment is often life-saving but has the inherent risk of causing damage to lung tissues. Overdistension and repetitive collapsing/opening of alveoli should be avoided. Monitoring of respiratory mechanics has a central role in accomplishing this. Methods used today to identify pressure/volume (P/V) curves are based on static/semistatic methods that necessitate a change of ventilator settings and have mainly been used as research tools. The aim of this thesis was to develop clinically applicable methods for continuous and thorough monitoring of respiratory mechanics during on-going ventilator treatment.Methods: Studies were performed in a lung model and in patients. The use of catheters for measurement of oesophageal and tracheal pressures was evaluated. The dynostatic algorithm was created and validated for calculation of alveolar P/V-curves during dynamic conditions. The algorithm analyses pressure and flow at isovolume levels on the inspiratory and expiratory limbs of a tracheal P/V-loop, for every sample during the breath, assuming that the inspiratory and expiratory resistances are equal. Respiratory mechanics in 10 patients with acute lung injury were studied at different PEEP and tidal volume levels using this method.Results: A double-lumen, liquid-filled stomach tube measures oesophageal pressure reliably when positioned accurately. Direct measurements of tracheal pressures are a necessity for monitoring of respiratory mechanics and can be achieved by inserting an end-hole catheter through the endotracheal tube lumen, positioning its tip within 2 cm from the tip of the tube. The dynostatic method is highly reliable when the ratio between inspiratory and expiratory resistances is between 2.3:1 and 1:2.3. Respiratory mechanics during on-going ventilator treatment showed a high individual variability but good reproducibility. Within each breath, volume-dependent compliance decreased successively through the initial, middle and final parts of the P/V-curve. This pattern became more prominent with increased PEEP and tidal volume levels, indicating increased overdistension.Conclusions: The monitoring concept presented provides a safe, accurate and continuous method of monitoring of respiratory mechanics during on-going ventilator treatment.