2000 OPEN FORUM Abstracts
Monitoring Work of Breathing
Richard Branson BA, RRT University of Cincinnati Medical Center, Cincinnati, Ohio
Classically, work is force x distance. In the respiratory system, force is the pressure generated and distance is the volume of gas moved during breathing. The work of breathing is measured using the pressure volume curve. This measurement requires placement of an esophageal balloon and either measurement of the chest wall compliance or use of an estimated (normal) chest wall compliance.
The work of breathing measurement is difficult and can be difficult to reproduce. The normal work of breathing is 0.5 to 0.7 joules/L. However, work can also be expressed in joules/min in order to take into account respiratory rate. Despite the fact that 'normal' values for work of breathing are known, the measurement of the work of breathing has never been shown to predict weaning success.
Surrogates for the work of breathing include simple measurements such as the frequency/tidal volume ratio and respiratory rate. Other surrogates include the pressure time product, tension-time index, and oxygen cost of breathing. The oxygen cost of breathing is determined by subtracting the oxygen consumption during controlled ventilation from the oxygen consumption during spontaneous breathing, The difference is typically though to represent oxygen consumed by the respiratory muscles. This measurement is difficult and prone to numerous confounding factors.
Work of breathing measurements may be made in the laboratory to compare ventilators or in patients to compare techniques of ventilation. The measurement of the work of breathing is an important research tool, but has yet to obtain the simplicity to be used routinely. Additionally, the work of breathing has failed to prove useful in predicting weaning success or determining optimal levels of ventilatory support.
This lecture will discuss the measurement of the work of breathing, normal values, potential uses and pitfalls. Surrogates for the work of breathing will be described and compared to the work of breathing measurement. The role of work of breathing measurements will be reviewed in light of the current literature.