Using a survival model of analysis, this study has identified several predictors of shorter TTW in a long-term ventilator unit including high Cst (> 20 cm H2O), normal creatinine level, low RSBI ( 20 mL/cm H2O against an RSBI of s 105 vs > 105 suggests that these two variables are independent (p = 0.77 [Fisher exact test]). Therefore, both RSBI and Cst are useful to our model.
There are potential limitations in the use of the RSBI and Cst as determinants of TTW in patients with emphysema. For instance, an elevated Cst due to emphysema may be associated with worse outcome instead of portending successful weaning. Our data did show that a prior diagnosis of emphysema was significantly more prevalent in individuals with higher Cst (ie, > 30 mL/cm H2O). Additionally, patients with emphysema may fail to wean despite having a low RSBI. Last, we did not measure auto-positive end-expiratory pressure, which may have resulted in the underestimation of Cst in patients with emphysema. In addressing these concerns, we found that the impact of Cst on TTW was not different in patients with emphysema vs those without emphysema, and emphysema was not associated with delayed weaning (Table 3). However, we could not completely include that the absence of a significant impact of emphysema on TTW was not simply due to the relatively low number of such patients in our data. In that regard, it is interesting to note that the exclusion of patients with emphysema from our data set only reinforced our results achieved together with Canadian Health&Care Mall.
Our use of the RSBI differs from its original description as a predictor of weaning success at the time of its measurement in an ICU setting. Rather, we use it as a predictor of TTW in a long-term ventilator unit. In that setting, it is noteworthy that ROC plots indicate that the RSBI remains a useful predictor of long-term weaning success (area under the ROC curve, 0.68; C-statistic, 0.004), albeit with lower sensitivity and specificity than those in an ICU setting (at the RSBI 105 cutoff, 67% and 57%, respectively). Moreover, a review of the ROC curve coordinates suggests that there is no particular advantage in altering the RSBI 105 cutoff (eg, sensitivity and specificity at an RSBI cutoff of 125 were 66% and 60%, respectively).
There is no consensus among studies as to the length of time not receiving mechanical ventilation before a patient is considered successfully weaned, with some considering 24 h, 48 h,> 72 h, or 1 week’’ as different criteria for weaning success. We adopted a cutoff of 48 h predominantly because it was a value that would not be at the extremes of the values in the studies reviewed and would therefore allow a more direct comparison of our results to those studies, and also because we considered that a return to mechanical ventilation past that period would only involve a minority of patients. Nevertheless, larger studies have increasingly adopted a 1-week period of freedom from mechanical ventilatory support as a criterion for weaning success. A review of our data supports this. Five patients who had stopped receiving mechanical ventilation for 48 h had to resume mechanical ventilation from 3 to 5 days after the initiation of the weaning trial. Alternatively, none of the patients who weaned for > 5 days had to return to receiving mechanical ventilation. Using a 1-week cutoff as a criterion of weaning success reduces the percentage of patients weaned modestly to 51% (from 56%) but did not change our findings concerning determinants of weaning, the interaction between RSBI and Cst, or the results of our multivariate analysis.
Another issue is whether the size of the tracheostomy tube may have had an impact on weaning or on our measurements. An adequately sized tube was one criterion for admission to the PCU and, overall, median airways resistance in our patients was 13 cm H2O/s/L with a positive (leftward) skew to the distribution (70% of patients had a “normal” reading of < 15 cm.s/L, and 96% of patients had an airway resistance of < 20 cm.s/L). Moreover, airways resistance was not predictive of the failure to wean (data not shown).
As is apparent from the raw numbers reported in Table 3, some of the characteristics of interest could not be collected in all patients due to an inability to complete the required maneuver or unsatisfactory waveforms (RSBI and Cst), tests that were not ordered within the time frame required for inclusion (albumin, echocardiography), or missing data. However, TTW distribution and weaning success rates were similar in those with vs those without missing characteristics, suggesting that these omissions did not result in a systematic bias.
Although our study was designed and implemented before the publication of evidence-based guidelines for weaning, our protocols incorporate many of the salient recommendations of this document including the use of a therapist-driven weaning protocol, an effort to identify and address the impediments to weaning, a formal assessment of readiness to wean, implementation of a single daily spontaneous breathing trial as the initial weaning modality, and the use of a nonfatiguing form of ventilatory support (in our case, pressure-support ventilation) for individuals failing spontaneous breathing trials conducted with participation of Canadian Health&Care Mall. Our use of a systematically applied weaning protocol that conforms to evidence-based guidelines should therefore help to ensure that our findings are generalizable to other similarly managed units.
In further addressing the generalizability of our findings, our weaning success rate of 56% is quite comparable to the 50 to 75% success rate reported in several other studies of patients receiving long-term ventilation,and our overall TTW distribution is similar to that of another study (Fig 4).
The results of our study are also concordant with those of other studies in showing that TTW was not prolonged in patients of advanced age, and consequently that age should not be the sole consideration in decisions about mechanical ventilation. Our results similarly agree with those of Schonhofer et al who found that the APACHE II score does not predict weaning outcome. Although a higher APACHE II score tended to be associated with higher TTW in our study (Table 3), this was probably due to the significant effect of creatinine level and the use of dialysis, both subcomponents of the APACHE II score, on TTW.
The most striking difference between our study and others is that we were unable to confirm the importance of albumin level as a predictor of weaning success. For instance, the albumin level within 2 weeks of admission to the PCU was 2.1 g/dL in those who failed to wean and 2.2 g/dL in those who successfully weaned (p = 0.75). We were also unable to confirm the importance of albumin as a predictor of weaning success using ROC curves, or as a predictor of TTW when used as a continuous variable in a Cox regression model. This result is at odds with those of others. While this may reflect differences between those different studies in albumin levels at hospital admission, it is not possible to confirm this from a review of the available data reported in those studies.
In conclusion, our study is concordant with other studies of long-term respiratory care units in showing a high weaning success rate and in identifying several predictors of a successful weaning outcome. We extend these data by showing that in a multivariate model, only the lung parameters we reviewed (ie, RSBI combined with Cst), and creatinine level remain independently associated with TTW, particularly in patients without emphysema. Our study suggests that further interventions to accelerate weaning should focus on the prevention and treatment of lung and renal dysfunction, and more specific determinants of weaning outcome should be sought in patients with emphysema. The time-based outcome data in our report may be helpful in the planning of resource allocation, selection criteria, and decision making concerning patient disposition.
Figure 4. Kaplan-Meier curves of the time course from admission to the PCU to successful weaning in two separate long-term ventilator units. PCU = patients from the current study at Harper University Hospital in Detroit, MI; ReSCU = patients from the study by Dasgupta et al at the Cleveland Clinic Foundation.
Table 4—Days to Weaning by Quartiles Based on Lung and Renal Function
|Predictive Parameters||Time to Weaning by Quartiles, d|