Auscultation. In 2005, the National Patient Safety Agency of the United Kingdom issued a patient safety alert in which they reported that, over a two-year period, 11 deaths and one case of serious harm resulted from malpositioned small-bore nasogastric feeding tubes.31 The alert clearly stated that “auscultation of air insufflated through the feeding tube (‘whoosh test’)” must not be used to assess placement of nasogastric feeding tubes and emphasized that use of this method had, in several cases, wrongly suggested that a tube was correctly placed, resulting in significant harm when feedings were initiated. Even so, this method continues to be widely promulgated and used.
In fact, several current nursing textbooks and manuals continue to recommend the use of auscultation, either by itself,32 in combination with aspiration of gastric contents,33, 34 or as a preliminary check prior to X-ray to avoid “ordering an X-ray when the tube is clearly incorrectly placed”35—the latter being a practice that can falsely suggest improper placement, resulting in unnecessary removal of the tube. In our survey of policies and procedures, auscultation proved to be the most widely practiced means of assessing enteral feeding tube placement, with 17 of 28 New England hospitals using this method.
pH testing. In 1989, investigators testing the effectiveness of measuring the pH of aspirated secretions to assess placement of a feeding tube found that 80.8% of aspirates from enteral feeding tubes placed in the stomach had pH paper readings of 1 to 4, while 87.4% of aspirates from tubes placed in the intestines had pH paper readings of 6 to 9.21 Although X-rays revealed seven cases of pulmonary placement, only one aspirate of the seven was tested; it had a pH paper reading of 8. Numerous studies support the use of aspiration and pH testing of aspirates to confirm proper placement of enteral feeding tubes.17, 22, 23, 36
At first glance, aspirating a small amount of tube secretions and testing the pH of the aspirate seems to be an easy and highly reliable method for confirming enteral feeding tube placement, and it is recommended for this purpose in a number of current nursing texts.29, 37-42 There are, however, significant limitations to this method. Medications, such as H2-receptor antagonists or proton pump inhibitors, which are frequently prescribed to prevent stress ulcers and gastric reflux, can raise gastric pH as high as 6.24 And in one study of 52 patients fed by nasogastric (50) or nasoenteric (2) tubes, 42% of patients were receiving one or the other of these medications.17
Likewise, commercially prepared feeding formulas, which typically have a pH close to 6.6, have an alkalizing effect on stomach secretions.25 In a 1986 study of 366 gastric aspirates from 20 patients receiving nasoenteral feeding with Osmolite or Isocal and no other gastric acid buffering, 96% of aspirates from the 10 patients receiving gastric feeding and 43% from the 10 patients receiving intestinal feeding had a gastric pH greater than 3.5.26 In a 2002 study of 80 patients receiving continuous tube feeding, the mean pH of gastric aspirate as measured by pH test strips was 5.7.25
Because the pH level of gastric aspirate is often elevated by either medication or enteral feeding formulas, pH testing isn't always a reliable indicator of gastric placement and may lead the examiner to falsely suspect that the feeding tube is in the lung or small intestines. To prevent alkalizing enteral feeding formulas or medication from confounding pH testing, wait at least one hour after administering feedings or medications before assessing pH, and flush the tube with 30 mL of air immediately before aspiration to clear it of other substances.43
Another limitation of pH testing may explain why so few hospitals in our convenience survey included it as part of their policy: at many hospitals, pH testing is considered a point of care procedure, which, like blood glucose testing, requires annual evaluation of staff skill and competency. It is, therefore, more expensive and resource intensive to allow this practice at the bedside.
Visualizing gastric aspirates. Aspiration of recognizable gastric content is often part of the process used to confirm placement of an enteral feeding tube. Gastric aspirates are described as being grassy green or colorless, often with sediment. Intestinal aspirates are often yellow or bile stained and either clear or cloudy. Pleural aspirates are described as watery and straw colored, while tracheobronchial fluid is off-white or tan and often mixed with mucus.27 Unfortunately, the overlapping color and physical characteristics ascribed to the aspirates limit this method significantly. In a study of 880 aspirates from nasally placed feeding tubes, observers found it difficult to agree on the aspirates' specific color and clarity characteristics.28 Researchers concluded that the visual characteristics of feeding tube aspirate can be helpful in distinguishing between gastric and intestinal content but is of little value in differentiating between gastrointestinal and respiratory placement.
Gastric contents are also aspirated to assess tolerance to the feeding and risk of aspiration. But in a 2005 randomized trial, risk of aspiration couldn't be determined from measuring residual volume.44 Of the hospital policies we evaluated, it was unclear whether gastric contents were aspirated as a means of checking tube placement or of monitoring gastric emptying.
Measuring feeding tube length. Current practice dictates that at the time of radiographic confirmation of tube site location, the tube should be marked with indelible ink or adhesive tape where it exits the nares.29, 30 On subsequent feedings, if the nurse notes that more of the tube is exposed, the position of the tip should be questioned. But this method should never be the sole means of determining tube placement, because tubes that appear to be securely taped can still migrate.27 Migration is more likely with the commonly used small-bore tubes.
OTHER METHODS OF ASSESSING PLACEMENT
Bilirubin aspirated from a feeding tube is useful in determining placement because bilirubin is normally found in intestinal fluid and, in small amounts, within the stomach if bile is refluxed from the duodenum. Bilirubin is rarely present in tracheobronchial or pleural fluid.22, 45 A colorimetric test for detecting bilirubin is not yet available for use at the bedside.23, 45
Another method that has been investigated for use in mechanically ventilated patients is carbon dioxide measurement, using capnometry at the proximal end of the feeding tube. The color change that indicates the presence of carbon dioxide is considered a sign of pulmonary tube placement.46 In some studies, capnometry has proved remarkably reliable for verifying enteral feeding tube placement in mechanically ventilated patients.15, 47 But in one study of 69 mechanically ventilated patients, capnometry incorrectly identified 16% of tubes as being in the lung, suggesting that the technology needs to be refined.48
Despite decades of research on the subject, primarily by Metheny and colleagues, our literature review uncovered no formal guidelines or practice recommendations from any professional association for inserting and verifying placement of blindly inserted enteral feeding tubes. In 2009, however, the American Association of Critical-Care Nurses (AACN) issued a practice alert on the subject, containing a number of evidence-based proposals.19
During the blind insertion of any feeding tube, the AACN recommends using several methods to assess tube location. These include watching for signs of respiratory distress, visually assessing aspirate, and using pH testing, if available. Before the tube is used to administer feeding or medication, the AACN recommends confirming placement by X-ray.19 Radiographic confirmation is particularly important when small-bore tubes are used because they may produce no symptoms when incorrectly placed and usually require a stylet for insertion, which introduces the risk of tissue perforation if the tube is not correctly positioned.11
Once initial proper placement is confirmed by X-ray, mark the tube's exit site clearly with tape or a permanent marker.19 Since all bedside methods of assessing feeding tube placement have limited efficacy, the AACN suggests that nurses should assess feeding tube placement at four-hour intervals, using the following methods19:
* measure the external portion of the tube
* review routine chest and abdominal X-ray reports
* observe changes in the volume and appearance of feeding tube aspirates
* test the pH of aspirates (if pH strips are available)
* obtain X-rays if tube placement is in doubt
Despite the plethora of evidence that's been available since 1989, nurses continue to use the auscultatory method to assess enteral feeding tube placement, whether out of habit or because of a lack of resources and time. Until a completely reliable, simple, and cost-effective means of assessing placement is available, nurses must be vigilant in using the best evidence to date.
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For 22 additional continuing nursing education articles on research topics, go to www.nursingcenter.com/ce.
Keywords:© 2012 Lippincott Williams & Wilkins, Inc.
enteral feeding; enteral nutrition; feeding tube; feeding tube assessment; tube placement