Anaesthetics, Critical Care and Pain Medicine, New Royal Infirmary of Edinburgh, Little France, Edinburgh, Scotland EH16 4SU, UK
*Corresponding author. E-mail: tim{at}walsh.sol.co.uk
Accepted for publication: January 7, 2004
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Abstract |
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Methods. We carried out a prospective observational cohort study in a 12-bedded general intensive care unit (ICU). We developed a checklist of metabolic, cardiorespiratory and neurological criteria that suggested that patients should start the weaning process. We performed daily assessments throughout ICU stay and recorded whether the criteria were met. Ultimate ventilator independence was used as the reference standard.
Results. We studied 325 sequential admissions to the ICU. Data were available for 98% of patients; 97% of admissions were mechanically ventilated on admission to ICU. Overall, 205 of the 308 ventilated patients (67%) achieved ventilator independence during ICU admission; the other patients died or were transferred ventilated to other ICUs. Eighty-three per cent of the patients who achieved ventilator independence met the set criteria. Fulfilling the criteria was a moderately strong predictor of ultimate ventilator independence: specificity 89%, positive predictive value 94%, positive likelihood ratio (LR) 7.6. When we analysed data by the day from admission on which patients were examined, the test was a strong predictor of subsequent ventilator independence when criteria were met by day 1 (LR 11.1) or day 2 (LR 6.9), but weaker when met by more than/equal to 4 days (LR <3). Patients who met criteria after more than/equal to 4 days often had prolonged weaning and a high incidence of re-intubation. Patients who achieved ventilator independence without fulfilling the criteria (n=35) had a short duration of mechanical ventilation (median 2 days, interquartile range 13 days). The most frequent reason for failing criteria before ventilator independence was a PaO2/FIO2 ratio less than 24 kPa (49% of cases).
Conclusions. A simple checklist can assist nurse assessment of suitability for weaning and could be used as a trigger to commence a weaning protocol. The day on which criteria are met is a useful way of stratifying patients for likely patterns of weaning.
Br J Anaesth 2004; 92: 7939
Keywords: monitoring, diagnostic test; monitoring, intensive care; ventilation, mechanical; ventilation, weaning
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Introduction |
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Methods |
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Weaning criteria
We used simple bedside criteria regarding metabolic, cardiovascular, respiratory, and neurological status to assess patients (Table 1). The aim of the criteria was to provide a checklist that, if all fields were met, suggested that acute physiological derangement had improved sufficiently to consider starting the weaning process. We termed these weaning criteria and considered that the criteria were met when all criteria were present simultaneously. The criteria were developed locally by a process of literature review and by consensus. First, a group of two consultants and two experienced nurses reviewed published studies, in particular systematic reviews of the literature.3 5 Secondly, a checklist was derived pragmatically and discussed generally among ICU staff. Our original criteria had a higher level of haemoglobin concentration (9 g dl1) and a lower level of PEEP (5 cm H2O) than most staff considered necessary for weaning. We therefore modified the original criteria for these items. The final criteria included a PEEP level 10 cm H2O, reflecting current trends to maintain high levels of PEEP7 in patients recovering from acute lung injury, and a haemoglobin concentration
7 g dl1, reflecting evidence suggesting that restrictive transfusion practice does not prolong ventilation requirements in the critically ill.8
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Existing weaning practice
At the time of the study, weaning decisions were largely physician led. No formal screening protocol for suitability for weaning was used in the unit. Instead, medical staff assessed patients as part of routine daily review. Weaning strategies were individualized to patients, and extubation was only carried out after consultation with medical staff.
Analysis
We calculated the sensitivity and specificity, positive and negative predictive values, and positive and negative likelihood ratios (LRs) for the weaning criteria to predict successful ventilator independence during ICU stay. We also calculated the positive LRs depending on the day from admission that weaning criteria were assessed, and plotted the time taken from first meeting the weaning criteria to achieving ventilator independence.
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Results |
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Data concerning weaning criteria were evaluated for the 308 admissions that were mechanically ventilated on admission to the ICU. A description of ventilation outcomes with reference to weaning criteria for these admissions is shown in Figure 1.
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Patients who met weaning criteria, but failed them on at least one subsequent ICU day
Of the 178 patients who met the weaning criteria there were 17 (9.6%) who did not require subsequent re-intubation, but failed the criteria on at least 1 day after meeting the criteria for the first time. Twelve of these 17 patients did not first meet the criteria for more than 4 days after admission (median (IQR, range) day first met criteria: 8 (3.9; 034) days). Only one of these patients (who first met criteria after 34 days) subsequently died in ICU.
Patients who achieved ventilator independence, but never met weaning criteria
The 35 patients who were ventilated at ICU admission, achieved ventilator independence, but who never fulfilled the weaning criteria had a median (1st, 3rd quartile; range) number of days from admission to achieving ventilator independence of 2 (1, 3; 118) days. The median ICU length of stay of these patients was 4 (2, 6; 119) days. The reasons for not meeting the weaning criteria on the day ventilator independence was achieved are summarized in Table 4.
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Discussion |
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Jaeschke and colleagues have considered how to assess the value of a weaning test in critically ill patients.9 They suggested that study data should be examined in a systematic manner using a number of questions.
Was there an independent, blind comparison with a reference standard?
The reference standard in our study was whether the patient was ultimately successfully weaned from mechanical ventilation. We defined this as disconnection from the ventilator, and failure to wean as death without disconnection from the ventilator. We excluded patients who were discharged ventilator-dependent to other acute ICUs as a result of ICU bed shortages, because ultimate weaning outcome in these individuals was not known. We believe that the reference standard used in the study was reasonable, although it was impossible to blind investigators from this outcome. The weaning checklist data were collected independently and clinicians and nursing staff weaning the patients did not see the checklist results. However, complete concealment of the data was not possible or ethical and it is likely that clinicians used some of these data when they made ventilation decisions. Collecting the checklist data independently was the most feasible method of blinding and minimized the chance of bias.
Did the patient sample include an appropriate spectrum of patients to whom the diagnostic test would be applied in clinical practice?
We designed our study to include all patients admitted to the ICU during the study period. We validated the completeness of our dataset against an independent audit database that tracks all patients admitted to the ICU.6 We obtained data on 98% of eligible admissions, and characterized the admissions of the eight patients for whom no data were available. We therefore consider selection bias extremely unlikely. We also characterized the illness severity (APACHE II diagnosis), physical characteristics, ventilation outcome, length of stay, and mortality of the cohort to enable comparisons with other settings. This should enable others to assess the applicability of our findings to other ICU populations.
Did the results of the test being evaluated influence the decision to perform the reference standard?
The properties of the weaning criteria test would be distorted if they had influenced weaning decisions during the study because this would have introduced verification bias.10 During the data collection, weaning criteria were recorded by an individual who was not involved in making weaning decisions, but as discussed above it was impossible to blind clinicians to the components of the checklist as this would have been impractical and unethical. Blinding of test information is a common confounding factor in studies of weaning criteria. Our question related specifically to the association between fulfilling all components of the checklist and ultimate ventilation outcome. Although it is likely that individual physiological variables were used by clinicians during patient evaluation, we think it unlikely that this had a major confounding effect on our reference standard.
What are the sensitivity, specificity, and likelihood ratios for the test?
We have presented the overall sensitivity and specificity of the test for correctly identifying patients who are successfully disconnected from the ventilator and discharged from the ICU. For nurse-led weaning a test is needed that is simple, quick, and gives the nurse at the bedside confidence that the patient will eventually become ventilator independent having achieved his/her current status. The ideal test should have a high specificity, because this allows a high degree of confidence that the outcome of interest, namely ventilator independence, will occur.11 In our population the weaning criteria had, overall, a moderately high specificity (89%), which should allow the ICU nurse to be confident that considering the patient for weaning is appropriate.
Sensitivity, specificity, and predictive values are of limited value for predicting outcomes in weaning studies because they generally apply to single cut-off points. We calculated LR, which allow an assessment of changes in pre-test probability.5 Our overall positive LR of 7.6 and negative LR of 0.18 correlate with clinically important changes in probability suggesting that the weaning checklist may be clinically useful. Plotting the number of patients who were still ventilator dependent in relation to the day from ICU admission on which the criteria were first met (Fig. 2) suggested that this factor was an important determinant of the value of the test. We therefore calculated positive LRs for the weaning criteria depending on the day from admission on which the test was performed. This showed that the test had a large effect on the post-test probability if weaning criteria were met on the first full ICU day, a moderate effect on the second day, and smaller effects on subsequent days. We have used these LRs to simulate the pre- to post-test probability changes at various levels of pre-test probabilities on different days of admission (Table 5). These changes were measured from a Fagan nomogram.12 This simulation clearly shows that if the criteria were met on days 1 or 2 from admission a clinically significant increase in post-test probability occurs irrespective of the estimated pre-test probability of ultimate ventilator independence. When met on day 3 or more the changes in probability are more modest, consistent with positive LRs of 24. These patients took longer to achieve ventilator independence, had a high incidence of re-intubation after first meeting the criteria (31%), and in many cases (46%) failed the weaning criteria on subsequent days. If our weaning criteria were used as part of a daily nursing checklist, broadly two groups of patients would be identified. First, patients who fulfil the criteria during the first 12 days of ventilation are weaned successfully and quickly in the majority of cases. These patients comprised 40% of all admissions during the study period. For these patients the weaning criteria could be used to trigger a reduction in sedation, a spontaneous breathing trial, or protocolized reduction in ventilator support, and subsequent extubation without involvement of physicians. Secondly, the group who first met the criteria in more than/equal to 3 days from ventilation had a more variable weaning duration. For these patients, the criteria could also be used to trigger a reduction in sedation and ventilatory support, but additional evaluation by medical staff, such as a formal assessment of respiratory pattern, cardiovascular responses and comfort, may be advisable. These patients may be less suited to a nurse-led protocol, but the weaning criteria could be used as a trigger to involve more experienced staff or a weaning team.
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We did not formally include a review of sedation in our study, but our data emphasize the importance of daily assessment of sedation status in conjunction with a weaning protocol. Daily cessation or reduction in sedation may have decreased the number of failures as a result of inadequate spontaneous respiratory effort or impaired neurological status. Previous studies have shown that daily sedation cessation reduces ICU length of stay.13 This approach is likely to be particularly effective if combined with regular assessment of weaning criteria to trigger a weaning protocol.
In conclusion, we have shown that a simple bedside weaning checklist can reliably predict patients who achieve ventilator independence, particularly if the day from ventilation on which these criteria were first met is considered. These criteria are a potential method of introducing nurse-led weaning protocols.
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Acknowledgements |
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References |
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