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Can I measure work of breathing for patient and ventilator?

Article

Author: Munir Karjaghli

Date of first publication: 09.04.2020

Work of breathing (WOB) represents the integral of the product of volume and pressure. It represents the energy associated with a given VT at a given pressure (spontaneous, mechanical, or both).
Can I measure work of breathing for patient and ventilator?

Respiratory muscles or ventilator?

The airway pressure is the pressure of the whole respiratory system (lungs plus chest wall), the transpulmonary pressure is the pressure needed to distend the lung parenchyma, and the pleural pressure is the pressure needed to distend the chest wall.

In paralyzed patients with mechanical ventilation, plots of airway pressure versus VT indicate the total amount of work needed to inflate the respiratory system. This represents the work done by the ventilator on the whole respiratory system and the ETT, NOT the amount of work performed by the respiratory muscles.

Patient's work of breathing

Calculation of a patient’s WOB may be useful for understanding weaning failure, for helping in titrating ventilator support, and for evaluating the effects of different ventilation modes, the effects of therapeutic interventions, and the influence of ventilator performance (triggering, flow delivery, etc.).

For healthy people, the average total WOB ranges from 0.3 to 0.6 J/L. Patients with severe obstructive or restrictive lung disease “work” at levels two to three times this normal value at rest, with marked increases in work at higher minute ventilation (Banner MJ, Kirby RR, Gabrielli A, Blanch PB, Layon AJ. Partially and totally unloading respiratory muscles based on real-time measurements of work of breathing. A clinical approach. Chest. 1994;106(6):1835-1842. doi:10.1378/chest.106.6.18351​). How much work a patient can tolerate before the respiratory muscles tire is unclear. Maintaining adequate spontaneous ventilation is impossible in many patients when the workload exceeds 1.5 J/L.

Pressure-time product

The pressure-time product is the integral of the pressure performed by the respiratory muscles during inspiration. It is an alternative to WOB and has some theoretical and practical advantages over WOB calculations. The pressure-time product (PTP) is the area encompassed by the esophageal or airway pressure time tracing during inspiration.

In principle, the HAMILTON-G5/S1 ventilator measures the WOB for the patient only; this is valid for Paw and Paux (whichever is selected). WOB is airway pressure integrated over inspiratory volume until the pressure exceeds the PEEP/CPAP level. In the dynamic pressure/volume loop, WOB is the area to the left of PEEP/CPAP (as shown in Figure 1).

The PTP is the measured pressure drop required to trigger the breath multiplied by the time interval until the PEEP/CPAP level is reached at the beginning of inspiration. The PTP on Hamilton Medical ventilators is the area between PEEP and Paw or Paux/Pes (whichever is selected; Paux/Pes only for HAMILTON-G5/S1 and HAMILTON-C6, respectively), as long as Paw/Paux (Pes) is below PEEP as shown in Figure 2. Normal values for the pressure-time product range between 60 and 150 cmH2O/second per minute.  

Relevant ventilators: All
Relevant software: HAMILTON-C1/T1/MR1 SW v2.2.4 and lower; HAMILTON-C2 SW v 2.2.5 and lower; HAMILTON-C3 SW v 2.0.5 and lower; HAMILTON-G5/S1 SW v 2.81 and lower

WOB pressure-volume loop
Figure 1
WOB pressure-volume loop
Figure 1
Pressure-time product
Figure 2
Pressure-time product
Figure 2

Footnotes

References

  1. 1. Banner MJ, Kirby RR, Gabrielli A, Blanch PB, Layon AJ. Partially and totally unloading respiratory muscles based on real-time measurements of work of breathing. A clinical approach. Chest. 1994;106(6):1835-1842. doi:10.1378/chest.106.6.1835

Partially and totally unloading respiratory muscles based on real-time measurements of work of breathing. A clinical approach.

Banner MJ, Kirby RR, Gabrielli A, Blanch PB, Layon AJ. Partially and totally unloading respiratory muscles based on real-time measurements of work of breathing. A clinical approach. Chest. 1994;106(6):1835-1842. doi:10.1378/chest.106.6.1835



OBJECTIVE

To evaluate the clinical feasibility of using real-time measurements of work of breathing obtained at the bedside with a portable, commercially available respiratory monitor as an objective and quantifiable guideline for appropriately setting pressure support ventilation (PSV) to partially and totally unload the respiratory muscles in patients with respiratory failure.

DESIGN

In vivo measurements of work of breathing were performed on a consecutive series of patients after applying incremental levels of PSV.

SETTING

University teaching hospital in a surgical ICU.

PATIENTS

Thirty adults (18 men and 12 women, ages 20 to 77 years) who had acute respiratory failure were studied. All patients had an endotracheal or a tracheostomy tube in place and were breathing spontaneously, receiving continuous positive airway pressure and PSV.

INTERVENTIONS

Intraesophageal pressure (indirect measurement of intrapleural pressure) was measured with an esophageal balloon catheter positioned in the mid- to lower-third of the esophagus. Tidal volume was obtained by positioning a flow sensor between the "Y" piece of the breathing circuit and the endotracheal or tracheostomy tube. Airway pressure was measured from a catheter attached to the flow sensor. Data from these measurements were directed to the respiratory monitor (CP-100, Bicore Monitoring Systems) which calculates work of breathing performed by the patient using the Campbell diagram. Work of breathing performed by the ventilator to inflate the respiratory system was calculated by the monitor by integrating the change in airway pressure and tidal volume. Initially, the level of PSV was set to 0 cm H2O and work measurements were obtained. Pressure support ventilation was then increased until the work performed by the patient decreased to a range of 0.3 to 0.6 J/L, which corresponds to a normal range for physiologic work of breathing (ie, partial respiratory muscle unloading), and then until the work decreased to 0 J/L (ie, total respiratory muscle unloading).

RESULTS

Work performed by the patient varied inversely (r = -0.83; p < 0.001) and work performed by the ventilator varied directly with the level of PSV (r = 0.94; p < 0.001). Work performed by the patient was 1.5 +/- 0.3 J/L at zero pressure support ventilation and decreased significantly to 0.50 +/- 0.1 J/L (p < 0.05) as the level of PSV was increased to 18 +/- 7 cm H2O. The respiratory muscles were partially unloaded under these conditions. Patient work decreased to 0 J/L and ventilator work increased when the muscles were totally unloaded at a PSV level of 31 +/- 8 cm H2O.

CONCLUSION

We propose an objective and goal-oriented clinical approach for using PSV by directly measuring the work of breathing performed by the patient with an easy to operate, bedside respiratory monitor and then applying pressure support ventilation to decrease the work to appropriate levels. Partially or totally shifting the workload from the respiratory muscles to the ventilator is appropriate under specific clinical conditions.