PREDICTION TOOL

Criteria abstracted from The Users' Guides to the Medical Literature series in JAMA

Am J Respir Crit Care Med 2009 180: 982-988.[abstract]

Review posted July 19, 2010

1. What is being studied?

   The study objective:

   To evaluate the performance of the respiratory muscle time index (TTmus) to that of the diaphragm tension time index (TTdi). The TTmus is a non-invasive assessment of the load on all respiratory muscles, while the TTdi is an invasive assessment of the load solely on the diaphragm. The TTmus is calculated according to the equation: (mean inspiratory pressure/maximal inspiratory pressure) x (time of muscle contraction/total time of respiratory cycle) (1).

   The TTmus was also compared to other predictors of extubation outcome, such as age, tidal volume, inspiratory time, CROP index (compliance, respiratory rate, oxygenation, and inspiratory pressure), duration of ventilation, PIMAX (maximum inspiratory pressure), FiO2 at extubation and rapid shallow breathing index. Eighty children were recruited and deemed ready for extubation. Their functional residual capacity was then measured following suctioning. The patients were then placed on CPAP, and the variables were measured for 2 minutes. Only 28 patients had a balloon catheter inserted to measure the TTdi. The patients were extubated within four hours of the decision to extubate.

   The study design:

   A prospective cohort study in two pediatric intensive care units in London.

2. Are the results of the study valid?
   
   
   1. Was a representative group of patients completely followed up? Was follow-up sufficiently long and complete?

      Yes. There was a well-represented and broad population of pediatric intensive care unit patients, and all patients that were mechanically ventilated for > 24 hours were eligible for enrollment. All 80 patients were followed for 24 hours, and failing intubation was defined as re-intubation in less than 24 hours.

   2. Were all potential predictors included?

      The authors included many of the potential predictors such as age, weight, height, length of intubation, maximum inspiratory pressure, and maximum FiO2, however they did not mention the presence or absence of an air leak, which can be useful in the prediction of post-extubation failure due to upper airway obstruction. Also, there is no mention of whether any of the extubation failures were due to upper airway obstruction, which was the single most important cause of failure (37%) in a study by Kurachek and colleagues (2). Also the use and results of any extubation readiness tests by the clinical team are not documented.

   3. Did the investigators test the independent contribution of each predictor variable?

      Although the objective of the study was to evaluate the performance of the TTmus to that of the TTdi and other predictors of extubation outcome, there was no multivariate analysis performed to test the independent contribution of each variable, making this largely an association study. Also, the TTdi was measured in only 28 of the 80 patients.

   4. Were outcome variables clearly and objectively defined?

      Yes. The outcome variables were defined as successful versus failed extubation. However the decision to reintubate was made clinically, and therefore subject to clinical bias and opinion. In addition, the reason for reintubation was not clarified; some might have been reintubated for airway obstruction – which we would never expect the TTmus to detect while still intubated.

3. What were the results?
   
   
   1. What is(are) the prediction tool(s)?

      The prediction tool was the measurements of TTmus and TTdi. TTmus is a non-invasive assessment of the load on all respiratory muscles. TTdi is an invasive assessment of the load solely on the diaphragm. The authors found that a TTmus of > 0.18 is 100% predictive of extubation failure. The range of TTmus for successful extubation was 0.02-0.17 and 0.183-0.26 for extubation failure. All patients that had a TTmus greater than 0.18 failed extubation.

      Other predictors that were significantly associated with extubation failure were PIPmax > 27.5 cm H2O, mean PI/PImax < 0.33, Ti/Ttot > 0.45, FRC/kg body weight < 19.15 mL/kg, FiO2max > 0.825 and P0.1 > 4.45 cm H2O. A higher PIP was also found to be a predictor of extubation failure in a study by Newth and colleagues (3).

   2. How well does the model categorize patients into different levels of risk?

      The TTmus model categorizes patients into either an extubation success group or an extubation failure group with 100% sensitivity and 100% specificity, yielding and an area under the ROC curve of 1.0. If the TTmus was greater than 0.18, there was a 100% chance of extubation failure.

   3. How confident are you in the estimates of risk?

      Confidence intervals are not provided in the authors’ analysis, specifically when reporting the area under their ROC curves. With small numbers, one would suspect that the lower 95% confidence interval for the point estimate of an area of 1.0 would be much lower, thereby providing less confidence.

4. Will the results help me in caring for my patients?
   
   
   1. Does the tool maintain its prediction power in a new sample of patients?

      The stated TTmus cutoff value was not validated among additional patients.

   2. Are your patients similar to those patients used in deriving and validating the tool(s)?

      Yes. The patient population is similar to our patient population. Patient diagnoses included liver transplantation, sepsis, and respiratory failure.

   3. Does the tool improve your clinical decisions?

      The utility of the TTmus cutoff of > 0.18 is unknown and needs to be validated among other patients before its use can be recommended. The other predictors that were identified, although significantly associated with extubation failure, may prove useful if subsequently validated.

   4. Are the results useful for reassuring or counseling patients?

      Again, this study is not useful for reassuring patients since it has not been validated. If the next study used a TTmus cutoff value of less than 0.18 in determining extubation readiness and found a similar success rate, then the measurement of TTmus would be helpful.

References:

1. Ramonatxo M, Boulard P, Prefaut C. Validation of a non-invasive tension-time index of inspiratory muscles. J Appl Physiol 1995;78:646-53.
2. Kurachek SC, Newth CJ, Quasney MW, et al. Extubation failure in pediatric intensive care: A multiple-center study of risk factors and outcomes. Crit Care Med 2003;31:2657-64.
3. Newth CJ, Venkataraman S, Willson DF, et al. Newth CJ, Venkataraman S, et al. Weaning and extubation readiness in pediatric patients. Pediatr Crit Care Med Pediatr Crit Care Med. 2009; Jan;10(1):1-11.

Last Updated: July 19, 2010

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