Special Feature
When Should We Measure Work of Breathing in the ICU?
By Robert M. Kacmarek, PhD, RRT
Before specifically addressing the question of when measurement of the work of breathing is clinically useful in the ICU, a brief review of the concepts surrounding work of breathing is necessary. (See Table 1.) The total inspiratory work of breathing includes both lung and chest wall work.1 These both may be affected by changes in compliance and resistance of the lung/chest wall system as well as the addition of an artificial airway or attachment to a mechanical ventilator.2 The term "imposed work of breathing" is usually used to identify the work increase caused by artificial airways and ventilator systems.
To directly measure either lung or chest wall work, an esophageal balloon must be placed to reflect pleural pressure changes. In the spontaneously breathing patient, changes in esophageal pressure represent the pressure change required to expand the lung as the chest wall pulls the lung by virtue of the relationship between the visceral and parietal pleura.3 In this setting, the negative pleural pressure changes reflect lung work. Chest wall work can only be determined during passive inflation of the lung/thorax system.3 Since there is no muscular contraction, the pressure increase in the pleural space is a result of the force required to expand the thorax. In order for this measurement to be made, patients have to be either sedated to apnea or hyperventilated beyond the apneic threshold to ensure passive chest wall movement. Combining the work calculations from both lung and chest wall provides the total work of breathing.
Work of breathing is a very poor estimate of patient effort.3 It must be remembered that, in order for work to be performed, movement must occur. That is, if one pushes against a wall all day but cannot move it, one may be exhausted by the effort at the end of the day but still not have performed any work. The same is true with patients: A large intrapleural pressure change would reflect marked patient effort but would only translate into a high workload if a large tidal volume were moved. If the tidal volume is small, work performed is low. A better reflection of actual patient effort is the esophageal pressure change itself, or pressure-time index, that is the product of esophageal pressure change multiplied by the time during which the change occurs.4 This calculation is independent of volume and much more reflective of patient effort.
Table 1
Important Aspects of the Work of Breathing
Lung work
Affected by Lung compliance Airways resistance Imposed work from airways Ventilators
Measured during spontaneous breaths
Chest wall work
Affected by chest wall compliance Measured during controlled ventilation
A gross estimate of esophageal pressure change can be obtained from the central venous waveform.5-7 Changes in central venous pressure provide a crude estimate of changes in pleural pressure. Proper esophageal balloon placement (e.g., esophageal vs tracheal placement; proper midthoracic placement) is difficult to achieve, especially when the balloon is attached to a standard esophageal catheter and the patient is unable to swallow because of an endotracheal tube. Use of a modified nasogastric tube with an esophageal balloon makes placement much easier. Proper position must be assured either by having the patient inspire against an occluded airway to identify matching of esophageal and airway pressure changes, or the observation of cardiac contractions on the esophageal pressure tracing.8,9
Clinical considerations
Now back to the central question: When should we measure the work of breathing in the ICU? Table 2 lists the potential indications for the placement of an esophageal balloon for the purpose of measuring breathing work. However, there are few if any data to support the routine measurement of the work of breathing in any setting. No randomized, prospective controlled trials identifying improved clinical outcome have been associated with the measurement of the work of breathing.
Measurement of the work of breathing is essential for research in modes of ventilation and can be very helpful in teaching the effects of changes in ventilatory support methodology, but it is rarely indicated for actual patient management. During controlled ventilation, measurement of esophageal pressure allows assessment of the actual transpulmonary pressure gradient and the extent that hemodynamic measurements may be affected by the development of intrathoracic pressure. Recent emphasis on measurement of the upper deflection point on the airway static pressure volume curve generally replaces the need for the placement of an esophageal balloon for the purpose of transpulmonary pressure estimation,10 while the analysis of the effect of airway pressure on central venous pressure can be used to estimate the effects of airway pressure on hemodynamic measurements.5-7
Table 2
Indications for Esophageal Balloon Placement
During controlled ventilation Transpulmonary pressure measurement
During assisted ventilation Assessment prior to or during weaning Measurement of auto-PEEP
At the current level of understanding and application of controlled ventilation, there appears to be no clinical setting where the placement of an esophageal balloon is recommended as part of patient care. The use of esophageal balloons for the measurement of work of breathing during assisted ventilation is more controversial. A number of studies have been published relating changes in work of breathing (lung only) to ability to wean from ventilatory support.1,11 Unfortunately, none of these studies has demonstrated a useful prediction of weaning success from work of breathing measurements. Many patients with low work loads fail weaning, while others with high work loads do wean successfully. Again, at the current level of available data, routine measurement of work of breathing during weaning cannot be recommended.
Generally, the only time we place an esophageal balloon for assessment of work of breathing is in those settings where patients have chronically failed attempts to wean. In this setting, the esophageal balloon is used to assist in establishing the need for chronic ventilatory support. Frequently in this application high levels of auto-PEEP are identified.12 This diagnostic finding may be the most useful benefit from the placement of an esophageal balloon in the patient failing to wean from ventilatory support.
Frequently, we find auto-PEEP levels in the 10-15 cm H2O range that clearly identify the reason for weaning failure, assisting in the selection of applied PEEP and/or CPAP settings to minimize the effect of auto-PEEP on patient effort, and identifying the effectiveness of approaches to modify auto-PEEP levels (e.g., bronchial hygiene, aerosol therapy). However, because of frequent activation of the abdominal muscles during exhalation when auto-PEEP is present, the auto-PEEP level can be over-estimated.12
Hess and I have previously proposed13 the answering of three questions regarding the evaluation of monitoring technology:
Is it technically possible?
Is it clinically useful?
Is it cost effective?
Although monitoring work of breathing is technically possible, its routine clinical usefulness has not been established. Will it decrease the number of days of mechanical ventilation? If it decreases the number of ventilator days, then it may be cost effective. Until this has been demonstrated, routine use of work of breathing measurements must be questioned.
Based on current available data there are no indications for the routine use of an esophageal balloon for assessment of work of breathing. The only setting in which balloon placement appears useful is in establishing that weaning failure is due to high work loads and auto-PEEP and in evaluating the effectiveness of therapy to modify the level of auto-PEEP.
References
1. Banner M, et al. Crit Care Med 1994;22:515-523.
2. Blanch PB, Banner MJ. Respir Care 1994;39:897-905.
3. Mador MJ, et al. Work of breathing and respiratory muscle function. In: Kacmarek RM, et al. Monitoring in Respiratory Care. Chicago, IL: Mosby-Yearbook Medical Publications; 1993:267-308.
4. Kacmarek RM, Hess D. Respir Care 1994;39:881-882.
5. Walling PT, Savage TM. Br J Anaesth 1976;48: 475-479.
6. Hylkema KS, et al. Crit Care Med 1983;11:271-275.
7. Flemale A, et al. Eur Respir J 1988;1:51-57.
8. Baydur A, et al. Am Rev Respir Dis 1982;126:788-791.
9. Trop D, et al. J Appl Physiol 1970;29:283-287.
10. Amato MBP, et al. Am J Respir Crit Care Med 1995; 152:1835-1846.
11. Fiastro JF, et al. Chest 1988;94:232-238.
12. Smith TC, Marini JJ. J Appl Physiol 1988;65: 1488-1499.
13. Hess D, et al. Perspectives on monitoring in respiratory care. In: Kacmarek RM, Hess D, Stoller JK. Monitoring in Respiratory Care. Chicago, IL: Mosby-Yearbook Medical Publications; 1993:1-11.
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