What is the work of breathing on a Servo-i (servo ventilator)?

Work of Breathing on the Servo-i Ventilator

The work of breathing (WOB) on a Servo-i ventilator is measured in joules per liter (J/L) and can be calculated as the difference in area between the actual versus relaxed airway pressure contour plotted against either time or volume. 1

Understanding Work of Breathing Measurement

• 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 (estimated via esophageal pressure) 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

Measurement Methodology on Servo-i

• WOB is quantified as the difference in area between the actual versus relaxed airway pressure contour plotted against either time or volume 1
• 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 1
• 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

Clinical Applications and Interpretation

• Normal WOB values on the Servo-i during pressure support ventilation range from approximately 0.3-0.6 J/L for patients with high respiratory drive and 0.6-0.9 J/L for those with low respiratory drive 2
• Higher WOB values indicate increased patient effort, which may suggest inadequate ventilatory support 1
• A patient actively contributes to total WOB whenever ventilatory demands exceed the inspiratory flow rate setting - a phenomenon called "flow deprivation" 1

Factors Affecting WOB Measurements

• Patient-ventilator asynchrony can significantly affect WOB measurements 3
• Intrinsic PEEP (PEEPi) increases WOB by requiring additional effort to trigger the ventilator 1
• Application of external PEEP can counterbalance PEEPi and reduce WOB 1
• Inspiratory rise time settings affect WOB - shorter rise times can reduce WOB by increasing initial inspiratory flow 2

Clinical Significance

• WOB assessment helps clinicians optimize ventilator settings to reduce respiratory muscle fatigue 1
• 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

Common Pitfalls in WOB Measurement

• 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 1
• Patient-ventilator asynchrony can lead to inaccurate readings - ensure the patient is not actively breathing against the ventilator during measurement 3
• The presence of leaks in the ventilator circuit could lead to falsely low WOB readings 3