Objective: Small volume aspirations are difficult to detect; yet, they may lead to pneumonia, a complication that occurs in 20%-25% of intubated patients. In human studies, investigators have measured oxygen saturation (SpO2) during swallowing in dysphagic patients (a population in which aspiration is common), and found mean SpO2 decreases that varied from 1%-21%. The volume of aspiration was not determined in these studies. Therefore, it is currently not known what volume of aspiration is required to produce a decrease in pulse oximetry. The purpose of this study was to determine if decreases in pulse oximetry can detect aspiration in a mechanically ventilated population undergoing a known volume of forced small volume aspiration.
Design: This was a prospective experimental animal study.
Population, Sample, Setting, Years: This study, compared 44 intubated, mechanically ventilated New Zealand White rabbits with 11 control rabbits. The investigation was conducted in a research laboratory at Saint Louis University from June 2000 through December 2001.
Variables: The dependent variable was pulse oximetry oxygen saturation. The three independent variables were: 1. The fiber content of the formula: (a) fiber-containing formula, (b) fiber-free formula, or (c) 0.9% NaCl. 2. Three periods of forced small volume aspiration at two-hour intervals over a six-hour experiment. 3. Time of observation: first at baseline, then again every 15 minutes for a total of eight observations within each period of forced small volume aspiration. The following research hypotheses were examined: 1. Experimental rabbits that received a formula/gastric fluid mixture through their tracheas would experience decreases in oxygen saturation > 3% from baseline. 2. Control rabbits that received 0.9% NaCl through their tracheas would have no decreases in oxygen saturation > 1% from baseline. 3. Experimental rabbits that received formulas containing fiber (JevityŽ or GlucernaŽ) would have greater decreases in oxygen saturation from baseline than rabbits receiving fiber-free formulas (OsmoliteŽ or PediaSureŽ). 4. Decreases in oxygen saturation from baseline would be progressively greater with each succeeding period of forced small volume aspiration in experimental rabbits.
Methods: Test subjects received an intratracheal instillation of a one-to-one mixture of enteral formula and human gastric fluid whereas controls received 0.9% sodium chloride solution. Each intubated, mechanically ventilated, anesthetized animal received 0.4 ml/kg over the first 30 minutes of the first, third, and fifth hours of a six-hour experiment. Oxygen saturation was recorded at baseline and then again at 15-minute intervals throughout the experiment. Detection of aspiration was operationally defined as a decrease in SpO2 >3.0% from baseline.
Findings: Detection occurred in 152 of the 1,056 experimental measurements. Positive findings occurred in 23 of the 44 experimental animals and zero controls [X2(1)=9.95, p<.01]. Results for the three aspiration events were: first event (20 detections; mean decrease 8.15%), second event (39 detections; mean decrease 8.36%), and third event (93 detections; mean decrease 9.97%). The effects of aspiration on SpO2 were significantly greater over time [F(7,37)=2.58, p<.05]. Following the first two aspiration events, the decreases in SpO2 tended to be transient and to occur at levels between 90% and 96%. Following the third aspiration event, the drops in SpO2 increased in intensity and tended to be less transient. There were no statistically significant differences found between fiber-containing formulas and fiber-free formulas.
Conclusions: The fact that pulse oximetry was able to detect repeated boluses of 0.4 ml/kg in only half of the experimental rabbits suggests that this is the smallest volume of aspiration detectable by a fall in SpO2. Oxygen desaturations were progressively greater with succeeding aspiration events. The fiber contained in JevityŽ and GlucernaŽ is too small to block the small airways or elicit a chemical or immune response sufficient to decrease oxygenation within the six-hour time frame of the experiment.
Implications: Assuming these results are relevant to human studies, this study design would be consistent with three 28 ml aspiration events occurring over a 30-minute period of time in patients weighing 70 kg. Accordingly, there would be about a 50% chance that a small volume (0.4 ml/kg) gastric aspiration in a patient receiving enteral formula may demonstrate transient minor decreases in SpO2 similar to those found in this study. Events that occur faster (i.e. in less than 30 minutes), or in patients whose oxygenation status is already compromised may demonstrate larger decreases in SpO2. In order for nurses to assess if a decrease in pulse oximetry is occurring because of small volume aspiration, they must have a better understanding of the use and limitations of oximetry. If minor transient decreases in SpO2 are occurring that cannot be explained, they should be documented and the patient assessed for signs of aspiration.
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