Objective: To determine the accuracy of three methods (visibility of dye, glucose readings, and pepsin readings made on tracheal secretions suctioned from rabbits) in detecting aspiration associated with tube feedings.
Design: A 2x3x3 experimental design using two concentrations of dye (0.8 ml and 1.5 ml per liter of enteral formula), three glucose concentrations (low, moderate, and high) in enteral formulas, and three observation times (2, 4 and 6 hours).
Population, Sample, Setting, Years: The population consisted of 182 New Zealand White rabbits weighing approximately 3 kilograms; 161 were experimental animals and 21 were controls. Gastric juice used in the experiments was provided by 161 acutely ill hospitalized adults. The study took place in an animal laboratory in a medical center from November 1999 through January 2002.
Outcome Variables: Outcome variables consisted of dye visibility, glucose concentrations, and pepsin concentrations in tracheal secretions obtained at 2, 4 and 6 hours.
Methods: On the day prior to each experiment, human gastric juice was collected from an acutely ill hospitalized adult who had been fasting for at least 4 hours and had received no medications by tube or mouth within the preceding hour. The gastric juice was refrigerated until needed on the following day. On the morning of each experiment, the gastric juice was mixed half and half with one of eight enteral formulas (3 low, 2 moderate, and 3 high glucose content) stained with one of two dye concentrations. The animals were anesthetized, intubated, and mechanically ventilated. Eighty experimental animals received gastric juice mixed with formula containing 0.8 ml of dye per liter; the remaining 81 received gastric juice mixed with formula containing 1.6 ml of dye per liter. The low, moderate, and high glucose formulas were approximately evenly distributed among the 161 experimental animals. Three separate boluses of the mixture were instilled intratracheally via a small catheter introduced through the endotracheal tube into the mainstem bronchus. At the beginning of the experiment, 0.4 ml/kg of the mixture was infused over a 30-minute period. The infusion was then stopped and 90 minutes were allowed to elapse before endotracheal suctioning was performed. The suctioned material was visually inspected for the presence of dye. In addition, the secretions were tested for glucose content by a glucose oxidase reagent strip and for pepsin by an immunoassay. At Hours 2 and 4, this entire process was repeated. Thus, by the end of the 6-hour experiment, each experimental animal had received intratracheally a total volume of fluid equivalent to 1.2 ml/kg. The control animals received only 0.9% sodium chloride instillations into their tracheas, in the same volume and pattern as the experimental animals. Three dye, 3 glucose and 3 pepsin readings were made for each animal during the experiment. As recommended by proponents of the glucose method, a tracheal glucose concentration of 20 mg/dL or greater was said to indicate aspiration of enteral formula. Dye was defined as either present or absent, as determined by its visibility in the suctioned secretions. A tracheal pepsin concentration of 1 microgram per milliliter or greater was said to be positive for the aspiration of gastric juice.
Findings: Overall sensitivity of the dye method was 66.5%; it was 95.7% for the glucose method and 94.7% for the pepsin method. Overall specificity for the dye method was 100%; it was 53.1% for the glucose method and 100% for the pepsin method. Dye was visualized in only about two-thirds of the experimental animals' tracheal secretions; concentration of dye in the instilled mixture had no significant effect on dye visibility. Concentration of glucose in the enteral formulas had no significant effect on the glucose concentration in the tracheal secretions of the experimental animals. For unknown reasons, almost half of the control animals' tracheal secretions contained glucose in concentrations equal to or greater than 20 mg/dL.
Conclusions: The dye method lacks adequate sensitivity and the glucose method lacks adequate specificity. In contrast, the pepsin method is both highly sensitive and specific.
Implications: Neither the dye or glucose methods are satisfactory for detecting aspiration. The laboratory pepsin test is ready for large scale testing in humans to determine its efficacy in predicting aspiration.
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