YANG 1991: Predicting Weaning Outcome (1991)

“Rapid shallow breathing, as reflected by the f/VT ratio, was the most accurate predictor of failure, and its absence the most accurate predictor of success, in weaning patients from mechanical ventilation.”

— The Yang and Tobin Study Group

1. Publication Details

  • Trial Title: A Prospective Study of Indexes Predicting the Outcome of Trials of Weaning from Mechanical Ventilation.
  • Citation: Yang KL, Tobin MJ. A prospective study of indexes predicting the outcome of trials of weaning from mechanical ventilation. N Engl J Med. 1991;324(21):1445-1450. doi:10.1056/NEJM199105233242101.
  • Published: May 23, 1991, in The New England Journal of Medicine.
  • Author: Karl L. Yang, M.D., and Martin J. Tobin, M.D.
  • Funding: National Heart, Lung, and Blood Institute (NHLBI) and others.

2. Keywords

Mechanical Ventilation, Weaning, Extubation, Rapid Shallow Breathing Index (RSBI), Respiratory Failure, Predictive Index.

3. The Clinical Question

In adult patients being considered for weaning from mechanical ventilation (Population), which predictive index (Intervention) is most accurate in predicting the success or failure of a weaning trial (Outcome) compared to traditional indexes (Comparison)?

4. Background and Rationale

  • Existing Knowledge: Weaning patients from mechanical ventilation is a critical process in the ICU. Traditional predictors of weaning success, such as minute ventilation (VE) and maximal inspiratory pressure (PImax), were known to be frequently inaccurate, leading to premature or delayed extubation attempts.
  • Knowledge Gap: There was a need for a more accurate, simple, bedside index to predict which patients would succeed or fail a trial of spontaneous breathing, thereby improving the safety and efficiency of the weaning process.
  • Proposed Hypothesis: The authors developed and hypothesized that two new indexes—the ratio of respiratory frequency to tidal volume (f/VT), and a composite index called CROP (Compliance, Rate, Oxygenation, Pressure)—would be more accurate predictors of weaning outcome than traditional measures.

5. Study Design and Methods

  • Design: A prospective, two-phase, observational cohort study to develop and then validate predictive indexes. This was not a randomized trial.
  • Setting: A single medical ICU in the United States.
  • Trial Period: Not specified in the publication.
  • Population:
    • Inclusion Criteria: Adult patients who had been mechanically ventilated and were deemed ready for a weaning trial by their primary physician.
    • Exclusion Criteria: Patients with tracheostomies or those receiving ventilation via a face mask.
  • Intervention: Measurement of multiple respiratory parameters during the first minute of a spontaneous breathing trial (SBT) via a T-piece, including the novel f/VT ratio and CROP index.
  • Control: Comparison was made against traditional indexes (VE, PImax) measured at the same time.
  • Management Common to Both Groups: All patients underwent a standardized SBT. The outcome (weaning success or failure) was determined by clinical criteria, independent of the index measurements.
  • Power and Sample Size: The study included a derivation cohort of 36 patients and a prospective validation cohort of 64 patients.
  • Outcomes:
    • Primary Outcome: The accuracy of each index in predicting weaning success (defined as the ability to sustain spontaneous breathing for 24 hours after extubation) or weaning failure.
    • Secondary Outcomes: Sensitivity, specificity, and predictive values of each index.

6. Key Results

  • Enrollment and Baseline: 100 patients were studied in total.
  • Trial Status: The study was completed as planned.
  • Primary Outcome: In the validation cohort, the f/VT ratio (Rapid Shallow Breathing Index or RSBI) was the most accurate predictor of weaning outcome. A threshold of 105 breaths/min/L best discriminated between success and failure.
  • Secondary Outcomes: The f/VT ratio had a sensitivity of 0.97 and a specificity of 0.64 for predicting weaning success. Its predictive accuracy, as measured by the area under the ROC curve (0.89), was significantly better than that of the CROP index (0.78), PImax (0.61), and minute ventilation (0.40).
  • Adverse Events: Not applicable.

7. Medical Statistics

  • Analysis Principle: Analysis of diagnostic test accuracy.
  • Statistical Tests Used: Sensitivity, specificity, predictive values, and receiver-operating-characteristic (ROC) curves were used to assess the accuracy of each index.
  • Primary Outcome Analysis: The predictive performance of each index was compared.
  • Key Statistic(s) Reported: For the f/VT ratio (RSBI), a threshold of <105 breaths/min/L was identified as the optimal cutoff.
  • Interpretation of Key Statistic(s):
    • Sensitivity:
      • Calculation: 97% of patients who ultimately succeeded in weaning had an RSBI <105.
      • Clinical Meaning: The test is very good at identifying patients who will likely succeed (very few false negatives).
    • Specificity:
      • Calculation: 64% of patients who ultimately failed weaning had an RSBI >105.
      • Clinical Meaning: The test is moderately good at identifying patients who will fail (a significant number of false positives).
  • Clinical Impact Measures: Not applicable in the traditional NNT sense. The “value” is in the improved accuracy of a clinical decision-making tool.
  • Subgroup Analyses: Not performed.

8. Strengths of the Study

  • Study Design and Conduct: The prospective design with a separate validation cohort is a methodologically strong approach for developing and testing a diagnostic or predictive tool.
  • Simplicity and Practicality: The f/VT ratio is a simple index that can be easily and quickly measured at the bedside without complex equipment.
  • Objective Outcome: The outcome of successful extubation for 24 hours was objective and clinically meaningful.

9. Limitations and Weaknesses

  • Internal Validity (Bias): No major limitations to internal validity.
  • External Validity (Generalizability): The study was conducted in a single center and primarily in a medical ICU population, which may limit generalizability. The study was conducted before the widespread use of pressure support for spontaneous breathing trials.
  • Other: The specificity of the index was only moderate, meaning a high RSBI does not guarantee failure, but rather indicates a high risk of failure.

10. Conclusion of the Authors

“Rapid shallow breathing, as reflected by the f/VT ratio, was the most accurate predictor of failure, and its absence the most accurate predictor of success, in weaning patients from mechanical ventilation.”

11. To Summarize

  • Impact on Current Practice: This was a seminal, practice-changing study that is one of the most cited papers in the history of critical care. It introduced the Rapid Shallow Breathing Index (RSBI), which immediately became the global standard for assessing readiness for weaning from mechanical ventilation. It replaced older, less accurate measures and is now an integral part of virtually all ICU weaning protocols.
  • Specific Recommendations:
    • Patient Selection: For adult patients on mechanical ventilation who are being considered for a spontaneous breathing trial.
    • Actionable Intervention: Measure the respiratory frequency (f) and tidal volume (VT) during the first minute of an SBT and calculate the f/VT ratio (RSBI). A value <105 breaths/min/L suggests a high likelihood of weaning success.
    • Expected Benefit: A more accurate and reliable prediction of weaning outcome, leading to safer and more efficient liberation from mechanical ventilation.
  • What This Trial Does NOT Mean: This trial does not mean that an RSBI <105 guarantees weaning success. It is a predictive tool that must be used in conjunction with overall clinical judgment.
  • Implementation Caveats: The RSBI should be measured during unassisted breathing (e.g., on a T-piece or with minimal pressure support/CPAP) to be valid.

12. Context and Related Studies

  • Building on Previous Evidence: This study was designed to systematically evaluate and improve upon the inaccurate weaning predictors that were in use at the time.
  • Influence on Subsequent Research: The RSBI has been validated in hundreds of subsequent studies across diverse patient populations and settings. Research since this trial has focused on refining the optimal cutoff value, understanding its performance during different types of SBTs (e.g., pressure support), and combining it with other predictive measures.

13. Unresolved Questions & Future Directions

  • Unresolved Questions: What is the optimal RSBI cutoff in different patient populations (e.g., COPD, post-operative)? How does the use of pressure support during an SBT affect the interpretation of the RSBI?
  • Future Directions: Research continues to explore more advanced weaning predictors, including diaphragmatic ultrasound and other physiological measures, but the RSBI remains the simple, bedside standard against which all others are compared.

14. External Links

15. Framework for Critical Appraisal

  • Clinical Question: The question was of fundamental importance to daily ICU practice, addressing a major source of clinical uncertainty.
  • Methods: The prospective observational design with a validation cohort was the appropriate and rigorous method for testing a predictive index.
  • Results: The trial had a clear and convincing result, demonstrating the superior accuracy of the novel f/VT ratio over all existing standard predictors.
  • Conclusions and Applicability: The authors’ conclusion is strongly supported by the data. The results are highly applicable and have had a profound and lasting impact on the practice of mechanical ventilation and weaning in ICUs worldwide.

16. Disclaimer and Contact

This summary is provided by the Academic Committee of ESBICM (ACE) to facilitate the understanding of this study; readers are advised to refer to the original trial document for a deeper understanding. If you find any information incorrect, or missing, or it needs an update or have a request for a specific critical care trial summary, kindly write to us at academics[at]esbicm.org.

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