Chest
Volume 108, Issue 5, November 1995, Pages 1338-1344
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Clinical Investigations in Critical Care
Breathing Frequency and Pattern Are Poor Predictors of Work of Breathing in Patients Receiving Pressure Support Ventilation

https://doi.org/10.1378/chest.108.5.1338Get rights and content

Objective

To evaluate the relationships between directly measured work of breathing (WOB) and variables of the breathing pattern commonly used at the bedside to infer WOB for intubated, spontaneously breathing patients treated with pressure support ventilation (PSV).

Design

In vivo measurements of the WOB were obtained on a consecutive series of adults. Breathing frequency (f), tidal volume (vt), the index of rapid, shallow breathing (f/V T), the duration of respiratory muscle contraction expressed as the ratio of inspiratory time over total respiratory cycle time (ti/ttot), and a breathing pattern score (applied to approximately 50% of the patients) which ranks f, Vt, sternocleidomastoid muscle activity, substernal retraction, and abdominal paradox on a scale were variables of the breathing pattern were also measured. The greater the breathing pattern score, the lower the WOB and vice versa.

Setting

Surgical ICUs in two university teaching hospitals.

Patients

Sixty-seven adults (42 men and 25 women, aged 20 to 78 years) who had acute respiratory failure from various etiologies were studied. All patients were breathing spontaneously receiving continuous positive airway pressure and PSV.

Interventions

Intraesophageal pressure (indirect measurement of intrapleural pressure) was measured with an esophageal balloon integrated into a nasogastric tube. Vt was obtained by positioning a flow sensor between the “Y” piece of breathing circuit and the endotracheal tube. Data from these measurements were directed to a bedside respiratory monitor (Bicore; Allied Healthcare Products; Riverside, Calif) that calculates WOB using the Campbell diagram. Patients received PSV at levels deemed reasonable to unload the respiratory muscles. All measurements were obtained after 15 to 20 min at each level of PSV, averaged over 1 min, and then variables of the breathing pat tem were regressed with directly measured values for WOB.

Results

All breathing pattern variables poorly predicted WOB as evidenced by the low values for the coefficients of determination (r2). Breathing frequency correlated positively with WOB (r=0.47, pc0.001) and predicted or explained only 22% (r2=.22) of the variance in WOB. Vt correlated negatively and f/vt and Ti/ttot each correlated positively with WOB. However, these variables predicted only 20 to 27% of the variance in WOB. The breathing pattern score correlated negatively with WOB and predicted only 43% of the variance in WOB. A prediction model taking all variables into consideration using multiple regression analysis predicted only 50% of the variance in WOB; thus, it too was a poor to moderate predictor of WOB.

Conclusion

Our data reveal that WOB should be measured directly because variables of the breathing pattern commonly used at the bedside appear to be inaccurate and misleading inferences of the WOB. The clinical implication of these findings involves the traditional and empirical practice of titrating PSV based on the breathing pattern. We do not imply that the patient's breathing pattern should be ignored, nor undermine its importance, for it provides useful diagnostic information. It appears, however, that relying primarily on the breathing pattern alone does not provide enough information to accurately assess the respiratory muscle workload. Using the breathing pattern as the primary guideline for selecting a level of PSV may result in inappropriate respiratory muscle workloads. A more comprehensive strategy is to employ WOB measurements and the breathing pattern in a complementary manner when titrating PSV in critically ill patients.

Section snippets

Materials and Methods

Sixty-seven adults (42 men and 25 women) admitted to the surgical ICU who were diagnosed as having acute respiratory failure from various etiologies were studied after obtaining informed consent from the patient's family. The study was approved by the Institutional Review Boards at Shands Hospital at the University of Florida Medical Center and at the Jackson Memorial Medical Center at the University of Miami. Patients in the study population were diagnosed as having moderate to severe forms of

Results

Measured values for WOB ranged from 0 to 2.2 J/L. The level of PSV applied ranged from 5 to 50 cm H2O. All breathing pattern variables poorly predicted WOB as evidenced by the low values for the coefficients of determination (r2). For each variable, r2 predicts or explains the amount of variance in WOB. A variable with an r2 value between 0.64 and 0.81 is considered high and thus, a fairly good predictor.19 The f correlated positively with WOB and predicted only 22% of the variance in WOB (

Discussion

The main finding of this study is that, for adults with abnormal pulmonary mechanics and loaded respiratory muscles who are in respiratory failure and being treated with PSV, the f and the breathing pattern are poor inferences of the WOB. Our study reveals that WOB should be measured directly because f, for example, appears to be an inaccurate and misleading variable from which to infer the respiratory muscle workload. The clinical implication of these findings brings into question the

References (40)

  • KacmarekRM

    The role of pressure support ventilation in reducing work of breathing

    Respir Care

    (1988)
  • BrochardL et al.

    Inspiratory pressure support compensates for the additional work of breathing caused by the endotracheal tube

    Anesthesiology

    (1991)
  • PetrosAJ et al.

    The Bicore pulmonary monitor

    Anaesthesia

    (1993)
  • TokiokaH et al.

    The effectiveness of pressure support ventilation for mechanical ventilatory support in children

    Anesthesiology

    (1993)
  • BannerMJ et al.

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

    Crit Care Med

    (1994)
  • BaydurA et al.

    A simple method for assessing the validity of the esophageal balloon technique

    Am Rev Respir Dis

    (1982)
  • CampbellEJM

    The respiratory muscles and the mechanics of breathing

    (1958)
  • AgostoniE et al.

    Energetics

  • BrochardL et al.

    Inspiratory pressure support compensates for the additional work of breathing caused by the endotracheal tube

    Anesthesiology

    (1991)
  • BrochardL et al.

    Inspiratory pressure support prevents diaphragmatic fatigue during weaning from mechanical ventilation

    Am Rev Respir Dis

    (1989)
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    This paper was presented in part at the annual meeting of the American Society of Anesthesiologists, October 12, 1993, Washington, DC.

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