Predicting eventual success or failure to wean in patients receiving long-term mechanical ventilation

To determine if the ultimate ability of a long-term ventilated patient to wean can be predicted at the time of his or her admission to a long-term ventilator unit.

Two-phased prospective study.

Long-term ventilator facility, university-affiliated.

Adults ventilated for an average of 3 weeks, who did not have sepsis and who did not have chest tubes or progressive neurologic impairment.

On admission to the long-term ventilator unit, historic factors, ventilator history, and the following laboratory and metabolic tests were obtained: electrolytes, serum calcium, magnesium, and phosphorus, WBC, hemoglobin, albumin, total protein, transferrin, oxygen consumption, carbon dioxide production, respiratory quotient, and dead space/tidal volume. The patients were then placed in a weaning protocol utilizing increasing duration of pressure support ventilation during the day with complete rest at night. Forty-two days after enrollment in the study, representing three times the duration of the weaning protocol, the patients who successfully weaned were compared to those who remained ventilator dependent (n = 20). Patients who died or were transferred to another institution were excluded from this phase of the study, because we were trying to develop parameters that would be predictive of successful weaning. A parameter was considered to be predictive, and retained for the scoring system, if it produced at most 15% false-positives and false-negatives. A score of 0 was then assigned to the threshold value that produced no false-positives; 2 to the threshold value that produced no false-negatives and 1 to the intermediate values. The scoring system was then applied to a new prospective group of patients (n = 72).

Of all the parameters evaluated, only the following satisfied the false-positive and false-negative requirements; static compliance, airway resistance, dead space to tidal volume ratio, PaCO2, and frequency/tidal volume. Applying these, in the scoring system, to the initial group of patients, demonstrated that a score greater than 3 was associated with failure to wean; a score less than 3 was associated with successful weaning, and a score of 3 was not predictive. Using these thresholds, the data were applied to the new prospective group of patients, which again demonstrated that a score of greater than 3 was associated with failure to wean in all cases. A score less than 3 was again associated with successful weaning but there were two false-positives. The sensitivity, specificity, and positive predictive and negative predictive values for the scoring system were 1.0, 0.91, 0.83, and 1.0, respectively. None of the individual parameters included in the scoring system demonstrated equivalent statistical results. All but two of the patients who died prior to finishing the weaning period had weaning scores, which suggested that they would not be successfully weaned.

Parameters that are generally available, when combined into a scoring system, can predict at the time of admission to a long-term ventilator unit, in most cases, whether a patient will eventually wean. The scoring system resulted in no false-negatives and an acceptable number of false-positives. None of the individual parameters were as reliable as the scoring system as a whole.