Patient-ventilator partitioning of the work of breathing during weaning

Abstract

The work of breathing and its division between the patient and the mechanical ventilator were studied during weaning of 5 post-operative surgical patients from Synchronized Intermittent Mandatory Ventilation. Work by the patient$(\dot W_p )$ was estimated by integrating the product of flow and pressure over time during intervals when waveforms indicated patient effort; ventilator work$(\dot W_V )$ was similarly estimated during positive pressure inspirations. The ratio of$\dot W_p$ to the rate of work on the lungs$(\dot W_L )$ increased progressively during weaning from 0.14±0.04 to 1.2±0.15 while$\dot W_V /\dot W_L$ dropped from 1.31±0.08 to 0.13±0.11. Work on the lungs decreased during weaning. This was due in part to significant improvements in lung mechanics: resistance decreased from 9.9±0.9 to 6.1±1.6 cmH₂O/1/s and compliance increased from 58±17 to 102±30 ml/cmH₂O. The patient and ventilator work ratios, and the work of breathing quantify factors which may be directly useful to the clinician and to future systems to automate weaning.