This article is the first of a two-part series describing steps to minimize aspiration in the tube-fed patient.

Aspiration is defined as the inhalation of material into the airway. Microaspiration occurs frequently. This is aspiration of small amounts, less than 1 mL, that usually have no clinical significance because most of the material is cleared by the mucociliary action of the tracheobronchial tree or coughing. It becomes significant when the volume of aspirated material leads to hypoxia or pneumonia.

Aspiration pneumonitis occurs when aspirated material causes an inflammatory reaction in the lung tissue. If this process is mediated by an infectious agent, it is labeled as aspiration pneumonia. The presence of bacteria in the aspirated material increases the risk of aspiration pneumonia.1,2 Compromised defense mechanisms in critically ill patients also contribute to the aspiration risk.

Aspirated materials can be antegrade, originating from the oropharynx, and include oropharyngeal secretions such as nasal mucus and saliva. Antegrade secretions contain high levels of bacteria that run the risk of colonizing the trachea leading to pneumonia in critically ill, ventilated patients.2

Aspirated materials can also be retrograde, originating from the gastrointestinal (GI) tract. Retrograde aspirates can also contain high levels of bacteria if gastric acid, an inhibitor for bacterial growth, is diminished. A recent metaanalysis showed that when gastric acid secretion is lowered with the use of histamine-receptor blockers and proton pump inhibitors, the incidence of pneumonia rises.3 Buffering gastric acids with continuous gastric tube-feeding infusions can also promote bacterial colonization.1,4

Clinicians have attempted to minimize aspiration by holding tube-feeding delivery for elevated gastric residual volumes (GRVs) or GI symptoms, manipulating tube-feeding administration schedules, changing patients' positions, using prokinetic agents and small bowel feedings, and converting nasally inserted tubes to stomal tubes. It is important to identify which techniques have actually been shown to lower aspiration risk so that tube feedings can be delivered safely and not held unnecessarily.

Complications

Aspiration pneumonia can be one of the most lethal complications of enteral tube feedings. While enteral tube feedings are seen as a superior method for nutrition delivery over total parenteral nutrition by lowering hospital-associated infection rates and maintaining gut mass and function,5 the risk of aspiration requires attention and preventive measures. Various methods have been employed to both detect and prevent tube-feeding aspiration. Aspiration detection has been accomplished by obtaining oropharyngeal and tracheal aspirates and identifying the presence of gastric secretions, tube-feeding formula, or substance added to the tube-feeding formula. Valid aspirate testing measures have been used to judge techniques purported to lower aspiration risk.

A challenge of tube-feeding administration is determining when to hold delivery to prevent aspiration and when to safely continue delivery to provide the adequate nutrition necessary for improved patient outcomes.

Incidence and statistics

Aspiration pneumonia can be identified by a new onset of fever, increased respiratory rate, leukocytosis, leukopenia, purulent sputum, decreasing oxygenation, and a new or progressing infiltrate on chest film.

Recent investigations of critically ill, tube-fed patients report the presence of gastric contents in 22% to 31% of their tracheal secretions, reflecting retrograde aspiration.6,7 No statistics are available on aspiration pneumonias, but hospital-acquired pneumonia (HAP) has been shown to increase a patient's hospital stay by an average of 11 to 13 days, with an excess cost of over $40,000 per patient.8,9 Mortality rates of HAP vary and are affected by the timely initiation and appropriateness of antibiotic treatment and the severity of the underlying disease. One investigation showed a 27.1% increase in mortality rate was attributed to HAP,8 while others show no difference.9,10 Many critically ill patients with HAP die of their underlying disease. Higher mortality rates in patients with HAP were seen in those with medical rather than surgical conditions, those with bacteremias, and those receiving ineffective antibiotic treatment.11,12

GRVs

GRVs have consistently been used to determine aspiration risk. This involves aspirating with a syringe from a gastrically placed feeding tube and noting the volume of feeding formula and fluids obtained, generally every 4 to 12 hours. It is assumed that GRV reflects the volume of residual tube feeding in the stomach and if the volume is allowed to accumulate to a certain level, the aspiration risk will increase. Tube feedings are held if the GRV is too high. GRV significant enough to reflect inadequate gastric emptying and would warrant holding the tube feeding has arbitrarily been reported from 50 to more than 400 mL,13 although more recent investigations and guidelines advocate limits ranging from 200 to 500 mL.14-16 It is important to note that GRV not only includes the tube-feeding formula; normal upper GI secretions also add to the GRV. Saliva is secreted at about 0.5 mL per minute when unstimulated. This calculates to 700 to 800 mL daily. Saliva secretions increase to about 7 to 8 mL per minute when eating, which increases the daily amount to 1 to 2 L. Gastric secretions also have been measured at 1.5 to 2 L daily. For the tube-fed patient who is NPO, this equates to about 100 mL per hour of upper GI secretions that are present in the stomach, even before accounting for the volume of tube-feeding formula delivered.

Aspiration detection

A common method used for aspiration detection in tube-fed patients is obtaining tracheal aspirates via suctioning and determining whether there is evidence of tube-feeding formula. An earlier method was to test the aspirate with glucose oxidase reagent strips because glucose is found in tube-feeding formulas. This method was discounted by Kinsey et al. who determined that glucose-positive tracheal aspirates were not specific and were more associated with elevated blood glucose levels.17 Another popular method was the addition of blue food coloring to the tube-feeding formula and visually inspect the tracheal aspirate for the color blue. This method had widespread usage even though it had never been tested for sensitivity or reliability. Many clinicians were concerned that tube feedings were being turned off too frequently with this method. In 2000, Maloney et al. reported fatalities in infants that turned blue and became acidotic with this method.18 It was discovered that the dye was transported to the mitochondria of cells and interfered with the ATP cycle causing acidosis and death. An FDA Public Health Advisory was issued in 2003 prohibiting FD&C blue dye No. 1 from being added to tube-feeding formulas in high-risk patients.19

Recently, improved tracheal aspirate techniques have been utilized. One technique uses yellow microscopic beads added to the tube-feeding formula and then detected by calorimetric fluoroscopy.6 Another technique uses Pepsin immunoassay measurements in tracheal aspirates, as Pepsin is the major enzyme in gastric fluid.20 Over the last decade, several research investigations have utilized these two techniques to determine which bedside measures can be employed to lower aspiration risk.

Research investigations and GRVs

McClave et al. studied 20 healthy volunteers receiving nasogastric (NG) tube feedings and 8 stable patients receiving gastrostomy tube (G tube) feedings and checked GRVs.21 They compared an elevated GRV, identified as 150 mL or greater to physical exam and radiographic film. The physical exam focused on the abdomen for the presence of abdominal bloating or distension, increased tympany, and the absence of bowel sounds. Radiographic films were used to detect distended gastric bubbles or bowel loops and increased air/fluid levels.

This group found no correlation between elevated GRV and physical exam or radiographic film, but there was a correlation between the physical findings and the film. They also found no difference in GRV between the supine and right lateral decubitus position. It had been thought that the right lateral decubitus position might improve flow of gastric contents through the pylorus. They did discover that GRV were less with G tubes compared to nasally positioned tubes. They recommended at that time that a GRV 200 mL or greater with NG tubes or 100 mL or greater with G tubes should raise concern and point to the possibility of inadequate gastric emptying, and risk for regurgitation and aspiration. However, the researchers felt that tube feeding should proceed while the patient is closely monitored with continued GRV checks, physical exam, and radiography.

Dr. McClave's group then published a follow-up investigation in 2005 with the purpose of determining the relationship between GRV, physical findings, and aspiration risk.6 Aspiration was identified by adding yellow microscopic beads to the tube-feeding formula in NG tube and G tube-fed patients. This investigation showed no correlation between GRV and aspiration and there was less aspiration with the patients fed via a G tube compared to an NG tube. An NG tube intubates the upper and lower esophageal sphincters and may allow the upward migration of the tube-feeding formula.22

Metheny et al. recently identified pneumonia risk factors for patients fed tube feedings by either the gastric or small bowel route.7 The researchers determined aspiration by obtaining tracheal aspirates and testing them for the gastric enzyme, Pepsin. As a result of this study, the risk factors that correlated with aspiration were low bed rest elevation, vomiting, gastric feedings (as opposed to small bowel feedings), a low Glasgow Coma Score, and GI reflux disease (GERD). Patients that had the Pepsin-positive secretions were at greater risk for pneumonia. Paralytic agents and a high sedation level also increased the risk of pneumonia.

Metheny et al. continued research in this area and investigated whether there was any association between aspiration (Pepsin-positive secretions) and GRV.23 The researchers identified frequent aspirators as those patients with more than 40% tracheal aspirates positive for Pepsin. Research findings showed that frequent aspirators had significantly more incidence of GRV of at least 200 mL x 2 or 250 mL x 1. The researchers demonstrated that although aspiration can occur without high GRV, it occurs more frequently when GRVs are high. Metheny then used these data to design an Aspiration Risk Reduction Protocol and studied critically ill, mechanically ventilated patients.24 The protocol stated that tube-fed patients were to have the head-of-bed (HOB) elevated 30 degrees or higher. A written provider's order was encouraged along with the hourly recording, on the bedside flow sheets, of the patients' HOB angles by the direct care nurse. GRV checks were to occur every 4 hours. If the GRV was 200 mL or greater, a prokinetic agent was to be given x 1 and if still 200 mL or greater, it was to be repeated. If the GRV continued 200 mL or greater, then the feeding tube was to be converted to a small bowel feeding tube. An advanced practice nurse was used to oversee that the protocol was being followed and also assisted the ICU nurses with bedside insertion of small bowel feeding tubes. Results showed that the HOB elevation 30 degrees or higher increased from 38% to 88% with the protocol in place. The conversion to small bowel feeding tubes increased from 39% to 68%, aspiration (identified by Pepsin positive secretions) decreased from 88% to 39%, and pneumonia decreased from 48% to 19%. The author strongly feels that GRV should remain in nursing protocols.25

Part 2 of this series will cover the signs and symptoms of feeding intolerance, clinical issues related to GRVs, and evidence-based techniques to minimize aspiration risk in tube-fed patients.

REFERENCES