How Should We Interpret Weaning-Predictive Indices?
For the past three decades investigators have employed numerous physiologic and clinical parameters in an attempt to predict weaning and extubation outcome: 1. Measures of oxygenation and gas exchange (PaO2/FIO2, PaO2/PAO2, oxygenation index, alveolar-art
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15.1 Introduction For the past three decades investigators have employed numerous physiologic and clinical parameters in an attempt to predict weaning and extubation outcome: 1. Measures of oxygenation and gas exchange (P a02/F102' Pa02/P A02' oxygenation index, alveolar-arterial O2 gradient, dead-space fraction, pH) 2. Simple measures of capacity and load (vital capacity, tidal volume; respiratory rate; minute ventilation; maximal voluntary ventilation; negative inspiratory force (NIF) or maximal inspiratory pressure (MIP); static or dynamic compliance; maximal expiratory pressure) 3. Integrative indices (frequency-tidal volume ratio or fIV T ; CROP index; weaning index; inspiratory effort quotient) 4. Complex measurements of capacity and load requiring special equipment (airway occlusion pressure or PO•l ; Po./P1max; gastric intramucosal pH; work of breathing; Pdi/Pdimax; P/P1max) To date, several hundred papers have been published and more than 50 individual parameters have been studied. Only recently have investigators turned their attention to the study of how weaning predictors should be used to improve clinical decision-making. In evaluating the utility of these parameters it is crucial to consider the outcome being examined (e.g., weaning versus extubation), the patient population (e.g., medical, surgical), and the disease process (e.g., acute lung injury, cardiac disease, COPD, neurologic disease) under investigation. The methodology, timing, and reproducibility of the measurements must be carefully analyzed. In addition, numerous major limitations have been identified. For example, nearly all studies are observational, with parameters measured and then correlated with outcome. In the majority of studies physicians performing weaning and extubation were not completely blinded to the results of the predictor. Under these circumstances, the parameter itself may have influenced the decision to wean or extubate (or not) and therefore contaminated the results. Perhaps the most compelling question is whether or not parameters provide additional predictive data that lead to improvements in outcome, such as a decrease in weaning and extubation failures or avoidance of prolonged mechanical ventilation. In uncontrolled fashion, many investigators have used one or
J. Mancebo et al. (eds.), Mechanical Ventilation and Weaning © Springer-Verlag Berlin Heidelberg 2003
15 How Should We Interpret Weaning-Predictive Indices?
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more weaning parameters to decide whether or not to initiate weaning trials (Brochard et al. 1994; Esteban et al. 1997; Esteban et al. 1999; Esteban et al. 1995). Similarly, randomized controlled trials of protocol-directed weaning have included standard parameters as part of a multi-component screen but have not investigated the precise role of these tests (Ely et al. 1999). This has raised the important question: Can spontaneous breathing trials be initiated when standard clinical criteria are satisfactory (e.g., hemodynamic stability, adequate oxygenation), or must further measurements (e.g.
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