Predictors of weaning outcome

1. Predictors of weaning outcome Muhammad Asim Rana

2. INTRODUCTION • Weaning is the progressive decrease of the amount of support that a patient receives from the mechanical ventilator. However, it is more commonly used to describe the entire process of decreasing the amount of support that a patient receives from the mechanical ventilator, assessing the patient's clinical response, and discontinuing mechanical ventilation.

3. Discontinuation of mechanical ventilation is a two-step process. • 1) First, patients who may be ready to wean are identified using various predictors of weaning outcome. • 2) Weaning is then initiated in those patients.

4. IMPORTANCE OF PREDICTORS  • It is desirable to have accurate, objective predictors of weaning outcome that can be applied early in a patient's clinical course because clinicians tend to underestimate readiness to wean. In several randomized, controlled trials that compared weaning techniques, most patients were able to tolerate discontinuation of mechanical ventilation on the same day that their ability to wean was first assessed.

5. When assessed early in a patient's clinical course, predictors of weaning outcome can help prevent unnecessary prolongation of mechanical ventilation by identifying the earliest time that a patient is able to resume and sustain spontaneous ventilation

6. Conversely, by identifying patients who are likely to fail weaning, predictors of weaning outcome can prevent a premature weaning attempt that could result in cardiovascular, respiratory, or psychological distress. Finally, the predictors may provide insight into the reasons for ongoing ventilator dependence.

7. PREDICTORS • Numerous measures have been proposed as predictors of weaning outcome. • These predictors are assessed during spontaneous breathing and used to decide whether a trial of weaning is warranted.

8. Rapid shallow breathing index (RSBI)  • The ratio of respiratory frequency (f, also called the respiratory rate) to tidal volume (VT) is called the rapid shallow breathing index (RSBI). In other words, RSBI = f/VT. Measurements of f and VT can be obtained using a hand-held spirometer attached to the endotracheal tube, while the patient breathes room air spontaneously for one minute.

9. Using the RSBI as a predictor of weaning outcome is based on the observation that f increases and VT decreases immediately following discontinuation of ventilator support in patients who fail weaning. • The likelihood of weaning failure increases as the RSBI increases.

10. Physical examination  • one of the most helpful methods of judging the likelihood of successful weaning is to conduct a careful physical examination when the patient is breathing spontaneously. • Evidence of increased effort includes nasal flaring, accessory muscle recruitment, recession of the suprasternal and intercostal spaces, or paradoxic motion of the rib cage and abdomen (ie, abdomen moves inward during inspiration).

11. The chest should be auscultated to detect new wheezing or crackles. Dyspnea and changes of mental status, blood pressure, heart rate, cardiac rhythm, or respiratory rate should be identified. An elevated respiratory rate is a sensitive sign of respiratory distress if it is carefully counted over a one minute period; however, bedside estimation of tidal volume is inaccurate. Finally, the patient should be evaluated for cyanosis, although this is not an accurate sign.

12. Arterial oxygenation  • Measurements of gas exchange are frequently considered when deciding whether to initiate weaning. • Do not consider discontinuation of ventilator support if a patient has significant hypoxemia (eg, PaO2 <55 mmHg when the FiO2 is >0.40), although this approach has not been validated in clinical studies.

13. Several indices derived from an arterial blood gas (ABG) have been proposed as predictors of weaning success: • An arterial oxygen tension (PaO2) ≥ 60 mmHg with a fraction of inspired oxygen (FiO2) ≤ 0.35 • An alveolar-arterial (A-a) oxygen gradient of <350 mmHg

14. Another index that was proposed as a predictor of weaning outcome is the arterial/alveolar oxygen tension ratio (PaO2/PAO2). • A PaO2/FiO2 ratio >200 mmHg • The PaO2/PAO2 is more stable when the FiO2 changes than the A-a oxygen gradient.

15. Minute ventilation  • A minute ventilation below 10 L/min was considered a predictor of weaning success. It has since proven to be a poor predictor of weaning outcome, with a high rate of false positive and false negative results. As an example, one prospective cohort study found that a minute ventilation less than 10 L/min had positive and negative predictive values of 50 and 40 percent, respectively, suggesting that flipping a coin could more accurately predict weaning outcome.

16. Maximal inspiratory pressure  • Maximal inspiratory pressure (PImax) is a global assessment of the strength of all the respiratory muscles. It was considered a predictor of weaning outcome after a study reported that a PImax of -30 cmH2O or less predicted successful weaning and a PImax value higher than -20 cmH2O predicted weaning failure.

17. However, subsequent studies have demonstrated poor sensitivity and specificity, probably because PImax assesses the strength of the respiratory muscles without considering the demands being placed upon them.

18. Compliance • Respiratory system compliance is estimated during condition of zero gas flow by: • Compliance = VT / (plateau pressure - PEEP)

19. In a prospective cohort study, a respiratory system compliance of 33 mL/cmH2O (normal 60 to 100 mL/cmH2O) had a positive and negative predictive value of only 60 and 53 percent, respectively.

20. Occlusion pressure  • The airway pressure that is measured 0.1 sec after the initiation of an inspiratory effort against an occluded airway is called the airway occlusion pressure (P0.1). • It is a measure of respiratory drive whose usefulness as a predictor of weaning outcome is uncertain due to conflicting data.

21. In normal subjects, P0.1 values are less than 2 cmH2O. Several studies have demonstrated that patients who have a P0.1 greater than 4 to 6 cmH2O usually fail weaning, whereas patients with a lower P0.1 usually wean successfully. • In contrast, other studies have found P0.1 to be an inaccurate predictor of weaning outcome.

22. Work of breathing  • The mechanical work of breathing can be calculated from the intrathoracic pressure that is generated by contraction of the respiratory muscles (or a ventilator) and the VT. • There are insufficient data to recommend that it be measured routinely as a predictor of weaning outcome.

23. In healthy subjects who are breathing at rest, the average work per liter is 0.47 J/L and the average work per minute of ventilation is 4.33 J/min. • Several studies have reported that increased work of breathing (eg, >1.0 J/L or >13 J/min) predicts weaning failure.

24. Integrative indices  • Weaning failure is usually multifactorial; therefore it is not surprising that single measures tend to be unreliable. Indices that integrate several physiologic functions were developed to improve predictive accuracy. There are insufficient data to recommend any index be used routinely to predict weaning outcome.

25. The CROP index • The CROP index integrates thoracic compliance (C), respiratory rate (R), arterial oxygenation (O), and maximal inspiratory pressure (P). Thus, it considers both demands on the respiratory system and the capacity of the respiratory muscles to handle them: •      CROP index = [Cdyn  x  PImax  x  (PaO2  ÷  PAO2)]  ÷  Respiratory Rate

26. in which Cdyn is dynamic compliance, PImax is maximal inspiratory pressure, and PaO2 ÷ PAO2 is a measure of gas exchange. A CROP index >13 mL/breath per min is generally considered to predict weaning success. When the CROP index was prospectively examined, the positive and negative predictive values were 71 and 70 percent, respectively.

27. The pressure-time product Index (PTI) • This is the time integral of respiratory muscle pressure. It is a measure of ventilatory endurance. The minute ventilation needed to bring PaCO2 to 40 mmHg (VE40) is a measure of ventilatory endurance and an estimate of the efficiency of gas exchange.

28. Integrative index of Jabour • incorporates these measures: • Integrative Index = PTI x (VE40  ÷  VTsb) • where VTsb is the tidal volume during spontaneous breathing. • An integrative index <4 units per minute is generally considered to predict weaning success. In a retrospective study, this integrative index had a positive predictive value of 96 percent and a negative predictive value of 95 percent.

29. USING PREDICTORS 

30. It should be emphasized that predictors of weaning outcome are intended to identify patients in whom weaning can begin. They should not be used to justify immediate extubation when successful weaning is forecast.

31. In other words, predictors of weaning outcome should be used in the first step of a two-step approach to discontinuation of mechanical ventilation: • Identify patients who may be ready to wean using predictors of weaning outcome. • Wean those patients whose predictors of weaning outcome forecast success. • This includes performing a weaning trial, assessing the patient's response during the trial, and extubating the patient if the trial is successful.

32. This approach is consistent with the cardinal precept of diagnostic testing - begin with a screening test and follow with a confirmatory test. Evaluation of predictors of weaning outcome can be considered the screening test, while a weaning trial can be considered the confirmatory test.

33. The goal of screening is to not miss anybody with the condition under consideration (in this case, the ability to sustain spontaneous ventilation). Thus, a good screening test has a high sensitivity (ie, a low false negative rate). A high false positive rate is acceptable. The RSBI fulfills these criteria, with a sensitivity of ≥ 90 percent in some studies • . The RSBI also satisfies the desire that a screening test be simple, expeditious, and safe. Measurement of the RSBI requires only a minute or so to perform.

34. SUMMARY AND RECOMMENDATIONS

35. Weaning is the progressive decrease of the amount of support that a patient receives from the mechanical ventilator. • However, it is more commonly used to describe the entire process of decreasing the amount of support that a patient receives from the mechanical ventilator, assessing the patient's clinical response, and discontinuing mechanical ventilation.

36. Discontinuation of mechanical ventilation is a two-step process. • First, patients who may be ready to wean are identified using various predictors of weaning outcome. • Weaning is then initiated in those patients.

37. Proposed predictors of weaning outcome include the rapid shallow breathing index (RSBI), physical examination, arterial oxygenation, minute ventilation, maximal inspiratory pressure, respiratory system compliance, occlusion pressure, work of breathing, and integrative indices

38. We do not consider weaning until patients are hemodynamically stable and have an arterial oxyhemoglobin saturation (SaO2) >90 percent while receiving a fraction of inspired oxygen (FiO2) ≤ 40 percent and positive end-expiratory pressure (PEEP) ≤ 5 cm H2O. • Once these goals are achieved, we suggest that weaning be initiated in most patients when the RSBI is ≤ 100 breaths/min per L (Grade 2C). • The threshold can be increased (eg, 115 to 125 breaths/min per L) if the risk of complications due to continued mechanical ventilation outweighs the risks of reintubation, or decreased (eg, 80 to 90 breaths/min per L) if the risk of reintubation outweighs the risks of continued mechanical ventilation.

39. Clinical variables used to predict weaning success • Oxygenation: PaO2 of 60 mmHg with FiO2 of 0.35 • Alveolar-arterial PO2 gradient of <350 mmHg • PaO2/FiO2 ratio of >200 • Ventilation: • Vital capacity of >10-15 ml/kg body weight • Maximum negative inspiratory pressure <-30 cmH2O • Minute ventilation <10 L/min • Airway occlusion pressure (P0.1) <4-6 cmH2O • Frequency to tidal volume ratio (f/VT) <100 b/min/liter • CROP index >13 ml/breath/min • Integrative index of Jabour et al <4/min • Poor respiratory system compliance • Increased work of breathing

40. Shukran Alhamdullilah