1. Lord, Linda M. MSN, NP, CNSN

Article Content

Aspiration pneumonia, caused by inhalation of material into the airway, can be one of the most lethal complications of enteral tube feedings. This series focuses on minimizing aspiration, first by reviewing the problem and who's at risk. A future article will cover the signs and symptoms of feeding intolerance and evidence-based techniques for reducing aspiration risk in patients on tube feedings.


About aspiration

Microaspiration occurs frequently in many healthy adults. 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's called 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 aspiration risk.


Antegrade aspiration materials originate from the oropharynx, and include oropharyngeal secretions such as nasal mucus and saliva. Antegrade secretions contain high levels of bacteria that can colonize the trachea, leading to pneumonia in critically ill, mechanically ventilated patients.2


Retrograde aspirated materials originate from lower in the gastrointestinal (GI) tract, and can contain high levels of bacteria if the patient has reduced gastric acid, which normally inhibits bacterial growth. 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


Compared with parenteral nutrition, enteral nutrition has lower rates of hospital-associated infection and maintains gut mass and function. The challenge is determining when to hold enteral nutrition to prevent aspiration, and when to safely continue delivery to provide the nutrition necessary for improved patient outcomes.5 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.


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. Identifying which techniques have actually been shown to lower aspiration risk is the key to delivering tube feedings safely and not holding them unnecessarily.


Incidence and statistics

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


Studies 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 and $40,000.8,9 HAP mortality varies, and is 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 attributed to HAP; others showed no difference.8-10 Many critically ill patients with HAP die of their underlying disease. Higher mortality in patients with HAP was seen in those with medical rather than surgical conditions, those with bacteremias, and those receiving ineffective antibiotic treatment.11,12



GRVs have consistently been used to determine aspiration risk. A syringe is used to aspirate a gastric feeding tube; the volume of feeding formula and fluids obtained is documented every 4 to 12 hours. GRV is considered the volume of residual tube feeding in the patient's stomach; if the volume is allowed to accumulate to a certain level, the patient's aspiration risk will increase. Guidelines advocate GRV limits ranging from 200 to 500 mL.13-16 Remember that GRV not only includes the tube-feeding formula but also normal upper GI secretions. Saliva is secreted at about 0.5 mL/minute (or 700 to 800 mL/day) when appetite is unstimulated, and 7 to 8 mL/minute (or 1 to 2 L/day for three meals) when the patient is eating. Gastric secretions have been measured at 1.5 to 2 L daily. For the tube-fed patient who is N.P.O., this equates to about 100 mL/hour of upper GI secretions that are present in the stomach, even before accounting for the volume of tube-feeding formula delivered.


Detecting aspiration

A common method used for aspiration detection in tube-fed patients is obtaining tracheal aspirates via suctioning and determining whether they show 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. However, glucose-positive tracheal aspirates aren't specific and were more likely to be associated with elevated blood glucose levels.17


Another popular method of detecting aspiration was to add blue food coloring to the tube-feeding formula and then 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, and some infants turned blue, became acidotic, and died with this method.18 (The dye was transported to the mitochondria of cells and interfered with the ATP cycle, causing acidosis and death. The FDA subsequently prohibited 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 used. In one technique, yellow microscopic beads are 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 used these two techniques to determine which bedside measures can be employed to lower aspiration risk.


Research investigations and GRVs

A study recorded GRVs in 20 healthy volunteers receiving nasogastric (NG) tube feedings and eight stable patients receiving gastrostomy tube (G tube) feedings.21 Researchers compared an elevated GRV (identified as 150 mL or greater) to physical exam and X-ray. 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.


The study found no correlation between elevated GRV and physical exam or X-ray, but did find a correlation between the physical findings and X-ray. They also found no difference in GRV between patients placed in the supine versus right lateral decubitus position (which was thought to improve flow of gastric contents through the pylorus). GRVs were lower in patients with G tubes compared to nasally positioned tubes. The researchers 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. They recommended that tube feedings proceed while the patient was closely monitored with continued GRV checks, physical exam, and X-ray.


A follow-up investigation in 2005 used yellow microscopic beads in the tube-feeding formula for patients with NG and G tubes.6 This investigation showed no correlation between GRV and aspiration, and found less aspiration in the G tube group compared with the NG tube group. Because an NG tube intubates the upper and lower esophageal sphincters, researchers speculated that it may allow the upward migration of the tube-feeding formula.22


A study of patients receiving tube feedings by either the gastric or small bowel route found that risk factors that correlated with aspiration were low backrest elevation, vomiting, gastric feedings (as opposed to small bowel feedings), a low Glasgow Coma Score, and gastroesophageal reflux disease (GERD).7 Patients who had pepsin-positive secretions were at greater risk for pneumonia. Paralytic agents and a high sedation level also increased the risk of pneumonia.


In another study, the researchers investigated the association between aspiration (pepsin-positive secretions) and GRV.23 Patients who had more than 40% tracheal aspirates positive for pepsin had significantly more incidence of GRV of at least 200 mL on two occasions or 250 mL on one occasion. The researchers demonstrated that although aspiration can occur without high GRV, it occurs more frequently when GRVs are high. They 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 unless medically contraindicated. 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 and if the GRV was 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 oversaw 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




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