Criteria abstracted from The Users' Guide to Medical Literature, from the Health Information Research Unit and Clinical Epidemiology and Biostatistics, McMaster University

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Article Reviewed:

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Comparison of predictors of extubation from mechanical ventilation in children

Manczur TI, Greenough A, Pryor D, Rafferty GF.

Pediatr Crit Care Med 2000; 1:28-32.

Reviewed by Shekhar Venkataraman MD, University of Pittsburgh, Children's Hospital of Pittsburgh

Review posted April 12, 2001

I. What is being studied?

Study objective:

The study was designed to determine whether the results of objective respiratory measurements were better predictors of extubation failure that were readily available clinical data.

Study design

This was prospective, observational study of a cohort of patients. A risk of extubation failure was calculated based on respiratory measurements made at the time of extubation.

Selection of patients:

The following patients were included:

1. Mechanically ventilated for at least 24 hours
2. Children were deemed stable and when suction, physiotherapy, and changes in posture had been performed > 2 hours before the measurements were taken.
3. All patients had an arterial catheter for a blood gas measurement before extubation

Patients were excluded if any prior attempts at extubation were unsuccessful of if long term noninvasive ventilation had been required.

Study methods:

Respiratory measurements were made at the time of extubation. The timing of extubation was decided by the physician taking care of the patients based on the child's clinical condition, blood gas analysis, and ventilatory parameters but no criteria were specified.

Measurements:

All measurements were made both during assisted ventilation at the time of extubation and on CPAP. The physicians were blinded to the results of the measurements. On assisted ventilation, the variables measured were dynamic compliance and the effective tidal volume delivered to the patient. On CPAP, the variables examined were arterial carbon dioxide tension, alveolar-arterial oxygen tension ratio, spontaneous tidal volume in mL/kg, minute volume in mL/kg, respiratory rate, the ratio of the inspiratory time to the total respiratory cycle time, and the maximum inspiratory pressure with an occluded airway. They also estimated a Pbreath, which is the estimated pressure of a spontaneous breath to generate the same tidal volume as the mechanical breaths the patient was receiving.

Outcome variable:

Successful extubation (no reintubation within 24 hours). No criteria used to reintubate was specified but it appears to have been left to the physician caring for the patient.

Analysis:

Cut-off values that determined the highest sensitivity and specificity were determined and receiver-operating characteristic curves were generated from a range of cut-off values.

II. Are the results of the study valid?

Note: These questions follow from Randolph AG et al. Understanding articles describing clinical prediction tools. Crit Care Med 1998;26:1603-1612. [abstract]

1. Was a representative group of patients completely followed up?

It is difficult to determine whether a representative sample of patients was followed up. The authors did not present any data on the characteristics of patients eligible but not included. 32% of the patients included were liver transplant patients and 28% were patients with a neurologic diagnosis. But, all patients included in the study were followed up adequately after extubation.

2. Were all potential predictors included?

No, but the variables selected has been examined in several adult studies and some recent studies in children. While sensitivities and specificities were calculated, these were not translated into probabilities of extubation failure.

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

Yes. They examined the sensitivity and the specificity for different cut-off values for each of the variable.

4. Were outcome variables clearly and objectively defined?

Yes, but the decision to reintubate seems to have been made at the discretion of the physician caring for the patient.

III. What are the results?

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

The extubation failure rate was 12.8%. This compares to previously published extubation failure rates in children. The other results are summarized in the table below.

Values of extubation indices with maximum sensitivity and specificity in predicting extubation failure, and the area under the resultant ROC curve

Variable	Value	Sensitivity (%)	Specificity (%)	Area under ROC curve
a/A ratio	<0.4	50	64	0.59
Tidal volume (mL/kg)	<6	71	85	0.80
Minute volume (mL/kg)	<180	57	85	0.77
Respiratory rate	<24	57	85	0.60
Dynamic compliance (mL/cms H2O/kg)	<0.5	43	80	0.54
CROP index	>0.78	43	89	0.68
Pbreath	<7.65	57	90	0.70
RSBI (bpm/mL/kg)	>8.1	43	88	0.55

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

There is no model development in this study. There is also no validation of these cut-off values in a different sample of patients.

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

Confidence intervals for the point estimates are not provided. Since the sample size is small, the confidence limits are likely to be quite large.

IV. Will the results help me in caring for my patients?

1. Does the tool maintain its prediction power in a new sample of patients?

This was not examined in this study.

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

The patients in this study are similar to some of the patients in my practice. The timing of extubation reflects the clinical practice in that institution.

3. Does the tool improve your clinical decisions?

This study helps in understanding the factors that could predict extubation failure. The cut-off values are not very useful without validation. These variables should be validated across several other centers with different practices before they can be accepted universally.

Additional comments regarding the study validity:

1. It is clear from this study and previously published studies (1, 2) that the adult indices - the rapid, shallow breathing index (respiratory rate/spontaneous tidal volume) and the CROP index - have poor predictive capacity. Age-related differences in the respiratory rate is the major reason for these indices to be poor predictors in children. On the other hand, Baumeister's study (see PedsCCM review) found CROP to be useful in children but CROP was measured in a nonstandard manner making the reliability of CROP in that study highly suspect.
2. Based on previous studies in adults and children, extubation success seems to depend on the adequacy of spontaneous efforts and the ability to sustain this effort. Therefore, it is not surprising that an adequate spontaneous tidal volume (5 ml/kg or more) is a major determinant of extubation success.
3. Unlike previous studies in children, this study did not find dynamic compliance to be very predictive.
4. Minute ventilation, which reflected the sum of the ventilator and spontaneous breaths had the next best area under the curve. This finding is in agreement that the level of ventilator support is an important factor to consider when a patient is extubated. A high level of ventilator support (>30%) can mask the patient's ability to sustain spontaneous breathing and is associated with a high risk of reintubation (1,2).
5. Due to the small sample size, the power of the tests to discriminate is probably low. A larger study is needed to address whether other variables can be discriminating.

References

1. Khan N, Brown A, Venkataraman ST. Predictors of extubation success and failure in mechanically ventilated infants and children. Crit Care Med. 1996;24(9):1568-79. [abstract]
2. Venkataraman ST, Khan N, Brown A. Validation of predictors of extubation success and failure in mechanically ventilated infants and children. Crit Care Med. 2000;28(8):2991-6. [abstract] [PedsCCM EBJC Review]
3. 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):1445-50. [abstract]
4. Baumeister BL, el-Khatib M, Smith PG, Blumer JL. Evaluation of predictors of weaning from mechanical ventilation in pediatric patients. Pediatr Pulmonol. 1997;24(5):344-52. [abstract] [PedsCCM EBJC Review]

 

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