Components of the work of breathing and implications for monitoring ventilator-dependent patients.

OBJECTIVES: a) To discuss the components of the work of breathing using an established physiologic model (Campbell diagram); b) to describe the requirements of a monitor to measure work; and c) to discuss the implications and relevance for assessing the work of breathing of ventilator-dependent patients. DATA SOURCES: Relevant articles from the medical and physiologic literature are referenced, as well as the authors' experience. STUDY SELECTION: Identified (by authors) laboratory and clinical research establishing the need and physiologic importance for correctly measuring the work of breathing. DATA EXTRACTION: A physiologic model of the various components of the work of breathing is used in conjunction with data from published literature. SYNTHESIS: Diagrams of increasing complexity based on the Campbell diagram depict the physiologic elastic and resistive work of breathing for the lungs and chest wall under normal and abnormal conditions. Decreases in compliance and increases in airways resistance are associated with increases in elastic and resistive work, respectively. A modification of the Campbell diagram to include an additional area depicting the imposed work of the breathing apparatus is suggested; i.e., the additional resistive load imposed on the respiratory muscles by the endotracheal tube, breathing circuit, and the ventilator's demand-flow system during spontaneous breathing. Increases in physiologic and/or imposed work result in respiratory muscle loading, predisposing to increases in oxygen consumption and the development of fatigue and hypercapnia. Measuring work of breathing by integrating the area of the esophageal pressure-volume loop alone underestimates the work of breathing relative to the Campbell diagram and, therefore, should not be used. Because the site of pressure measurement and mode of ventilation influence measurements of the work of breathing as well as compliance, clinicians should be aware of these factors when interpreting measurements. Monitors that are used in clinical practice to assess the work of breathing should be able to measure pressure at the airway opening (between the Y-piece of the breathing circuit and the endotracheal tube), at the carinal end of the endotracheal tube, and in the esophagus (inference of intrapleural pressure); as well as measure flow rate and volume at the airway opening; and calculate the various components of the work of breathing based on the Campbell diagram.
CONCLUSIONS: Accurate measurement of physiologic and imposed work performed by the patient are essential to assess the afterload on the respiratory muscles, diagnose specific work of breathing abnormalities, and monitor the effects of interventions to mitigate respiratory muscle loading. Work of breathing data are useful in formulating objective guidelines for setting the ventilator appropriately to optimize respiratory muscle loads, e.g., selecting an appropriate amount of pressure support ventilation to decrease the work of breathing to a specific level.