What is a normal work of breathing number on a Servo-i (Servo ventilator) for a patient on a set rate?

Normal Work of Breathing Values on Servo-i Ventilator with Set Rate

For patients on a Servo-i ventilator with a set rate, the normal work of breathing (WOB) value typically ranges between 0.3-0.7 joules per liter (J/L), with values above 0.8 J/L suggesting excessive patient effort that may require adjustment of ventilator settings. 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 and knowledge of the passive properties of the chest wall 2
• Two main variables can be derived for respiratory effort assessment: work of breathing (WOB) and pressure-time product (PTP) 2
• WOB is measured from pressure-volume loops using the Campbell diagram method 2

Interpreting WOB Values on Servo-i

• Values below 0.3 J/L typically indicate minimal patient effort, potentially suggesting over-support 1
• Values between 0.3-0.7 J/L represent normal work of breathing for most patients on a set rate 1
• Values between 0.7-1.0 J/L suggest increased work that may be appropriate during weaning trials but concerning if persistent 1
• Values above 1.0 J/L indicate excessive work that may lead to respiratory muscle fatigue 1

Factors Affecting WOB Measurements

• Intrinsic PEEP (PEEPi) increases WOB by requiring additional effort to trigger the ventilator 1
• Patient-ventilator asynchrony can significantly increase WOB values 1
• High inspiratory drive (P0.1 >4.2 cm H2O) is associated with lower ventilator-provided work (WOBv) and higher patient effort 3
• The presence of an endotracheal tube adds significant work of breathing, which varies with tube diameter and flow rates 4

Clinical Applications of WOB Monitoring

• High WOB values may predict weaning failure, while appropriately low values suggest readiness for ventilator discontinuation 2, 1
• WOB assessment helps clinicians optimize ventilator settings to reduce respiratory muscle fatigue 1
• Monitoring WOB allows for comparison of different modes of assisted ventilation and quantification of patient effort 1
• Different ventilator algorithms respond differently to increasing patient effort, with some allowing the patient to assume all WOB and others providing a minimum level regardless of effort 5

Optimizing Ventilator Settings Based on WOB

• Shortening inspiratory rise time can be more effective than raising pressure support to preserve inspiratory assistance at high inspiratory drive 3
• Increasing initial inspiratory flow can reduce pressure-time product more effectively than raising pressure support levels 3
• When comparing different ventilators at similar settings, the Servo ventilator has demonstrated comparable or better WOB values compared to other systems 6

Common Pitfalls in WOB Measurement

• Expiratory muscle activity may increase measurement variability and generate unreliable values 2
• Failure to account for PEEPi when measuring WOB can lead to underestimation of respiratory effort 1
• WOB measurement does not allow quantification of isometric efforts or inefficient inspiratory efforts that fail to trigger the ventilator 2
• Measurement of WOB is probably inadequate for comparing settings where part of the change in tidal volume is achieved by the ventilator 2