Affiliations: Miami, FL
Dr. Krieger is Professor of Medicine, University of Miami at MSMC, and Director, Medical Intensive Care Units, Mount Sinai Medical Center, Miami Beach; Dr. Campos is Chief Fellow, Division of Pulmonary and Critical Care Medicine, University of Miami.
Correspondence to: Bruce P. Krieger, MD, FCCP, Pulmonary Division, Mount Sinai Medical Center, 4300 Alton Rd, Miami Beach, FL 33140; e-mail: email@example.com
The “perfect” weaning parameter remains elusive, although the clinically accepted standard over the past decade has been the rapid shallow breathing index (RSBI) of Yang and Tobin.1Other investigators have questioned the sensitivity, specificity,2–3 and accuracy of the RSBI as determined by receiver operating characteristic curves.4 However, these studies did not duplicate the methodology of Yang and Tobin,1 who measured the RSBI using a hand-held, calibrated spirometer connected to the patient by a T-piece set up for 60 s. Rather, measurements were made with patients receiving continuous positive airway pressure (CPAP) and/or with pressure support being applied. Even when the methodology of Yang and Tobin was duplicated, we5 showed that the threshold value for RSBI differed depending on the population being studied.
These seemingly discrepant findings do not negate the value of the RSBI. Rather, they emphasize the need to perform and interpret a diagnostic test (such as the RSBI) in a manner that uses the same methods as were applied in the reference research study. Ideally, the patient population to which a diagnostic test is applied should be representative of those who are being tested. These principles were summarized by Jaeschke et al,6 who addressed how to use diagnostic test articles (specifically the RSBI) in the ICU.
In an earlier issue of CHEST (February 2002), El-Khatib and associates7 reported on the differences in RSBI when measured under three different conditions: CPAP at 5 cm H2O and fraction of inspired oxygen (Fio2) of 0.40; CPAP at 5 cm H2O and Fio2 of 0.21; and a T-piece on room air. To their credit, they confined their study to a defined patient population, postoperative coronary artery bypass revascularization patients. They found a significant difference in the RSBI measured via a T-piece compared to when the same patients breathed through a CPAP mode, regardless of the Fio2. This supports the advice of Jaeschke et al,6 that clinicians should not assume that a diagnostic test is accurate when measured under different conditions and in different patient populations.
The purpose of the study of El-Khatib and associates7was not to derive a new threshold value, but rather to highlight the need to establish threshold values based on the conditions (and patient populations) under which the RSBI is being tested. Unfortunately, their methods contained a minor flaw, although we do not feel that this distracts from their conclusion. Whereas they measured RSBI after 15-min CPAP trials, they compared these values to a 1-min T-piece RSBI measurement (as Yang and Tobin proposed). However, 15 min on CPAP may have altered the patient’s breathing pattern. For example, the tidal volume and respiratory rate at extubation are higher than 15- to 30-min postextubation despite no change in the patient’s clinical status.8 Therefore, the comparisons among the experimental groups of El-Khatib and associates were not rigidly similar. Still, their observations appear valid and emphasize that clinicians should not naively apply a diagnostic test (such as the RSBI) to their patients unless they “read the methodology” and duplicate the test in their patient population.
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