Work of Breathing on the Servo-i Ventilator

Understanding Work of Breathing Measurement

The American Thoracic Society recommends that work of breathing represents the energy expenditure required for a patient to breathe and is a key parameter for assessing respiratory muscle activity during mechanical ventilation 1
On the Servo-i ventilator, WOB is derived from pleural pressure changes and knowledge of the passive properties of the chest wall 1
Two main variables can be derived for respiratory effort assessment: work of breathing (WOB) and pressure-time product (PTP) 1

The American College of Chest Physicians suggests that WOB is quantified as the difference in area between the actual versus relaxed airway pressure contour plotted against either time or volume 2
Simple inspection of the airway pressure contour can provide useful information on respiratory muscle activity - any deviation from the relaxed configuration indicates active contraction of inspiratory muscles 2
The measurement requires knowledge of the compliance curve of the chest wall, which can be assumed to be approximately 4% of predicted vital capacity per cm H₂O 1

Higher WOB values indicate increased patient effort, which may suggest inadequate ventilatory support 2
A patient actively contributes to total WOB whenever ventilatory demands exceed the inspiratory flow rate setting - a phenomenon called "flow deprivation" 2

Intrinsic PEEP (PEEPi) increases WOB by requiring additional effort to trigger the ventilator 4
Application of external PEEP can counterbalance PEEPi and reduce WOB 4

WOB assessment helps clinicians optimize ventilator settings to reduce respiratory muscle fatigue 2
High WOB values may predict weaning failure, while appropriately low values suggest readiness for ventilator discontinuation 1
Monitoring WOB allows for comparison of different modes of assisted ventilation and quantification of patient effort 1
The Servo-i ventilator's ability to measure WOB helps clinicians set appropriate levels of pressure support to minimize respiratory muscle work while maintaining adequate ventilation 1

Expiratory muscle activity may increase measurement variability and generate unreliable values 1
Failure to account for PEEPi when measuring WOB can lead to underestimation of respiratory effort 4