Effect of Feeding-Tube Properties on Residual Volume Measurements in Tube-Fed Patients 
From the St. Louis University School of Nursing, St. Louis, Missouri; Correspondence: Norma A. Metheny, RN, PhD, FAAN, St. Louis University School of Nursing, 3525 Caroline Mall, St. Louis, MO 63104. Electronic mail may be sent to methenna{at}slu.edu.
Background: The effect of feeding tube size and port configuration
on the ability to measure gastric residual volume (GRV) is poorly understood.
In addition, there is confusion about the need to measure GRVs during feedings
into the small bowel. This study sought to (1) compare the volume of gastric
contents obtained from small-diameter feeding tubes and large-diameter sump
tubes concurrently positioned in the stomach and (2) describe the distribution
of GRVs during small-bowel feedings. Methods: For the first
objective, GRV measurements were made from 10-Fr tubes (n = 645) and
14-Fr or 18-Fr sump tubes (n = 645) concurrently present in 62
critically ill patients. Sixty-milliliter syringes were used to measure GRVs
from the 10-Fr tubes; the fluid was returned to the stomach and measurements
were repeated from the large-diameter sump tubes. To address the second
research objective, 890 GRV measurements were made from 14-Fr or 18-Fr gastric
sump tubes (not connected to suction) in 75 critically ill patients who were
receiving small-bowel feedings. Results: When GRVs were >50 mL, a
linear regression equation indicated that volumes obtained from the
large-diameter sump tubes were about 1.5 times greater than those obtained
from the small-diameter tubes concurrently present in the stomach, p
< .001. Gastric volumes Tube-feeding protocols usually call for frequent measurements of gastric residual volume (GRV) to assess intolerance to feedings and likelihood of aspiration, presumably by estimating gastric emptying.1–3 Although the paracetamol absorption test is more accurate for this purpose, it is impractical for regular use. Likewise, other methods of measuring gastric emptying (eg, radionuclide gastric emptying studies) are rarely indicated. Thus, it is likely that GRV measurements will remain a major component of tube-feeding protocols,4 despite disagreement as to their efficacy.5–8 Most protocols also include a specific GRV threshold above which feedings are temporarily discontinued; unfortunately, there is little consensus on how large this "cutoff" point should be. Values as low as 50 mL and as high as 500 mL have been cited3,6,9–11; as a result, there is great variability in clinical practice. Because a high GRV is thought to predispose to regurgitation and aspiration of gastric contents, feedings are often interrupted when an arbitrarily determined GRV threshold has been reached. This has provoked a concern about frequent, and perhaps unnecessary, feeding interruptions. Deciding how to strike a balance between these 2 concerns is important and requires a careful review of research findings from GRV studies. In order to make reasonable comparisons, one must first consider the equipment and procedures used to make these measurements.
Effect of Feeding-Tube Properties on GRV Measurements It is important to explore the effect of feeding-tube properties on the ability to accurately measure GRVs before attempting to identify a specific GRV threshold for temporarily withholding feedings. For example, it is conceivable that a GRV of 150 mL obtained from a small-diameter tube (with only 1 to 3 ports) may represent a more significant problem than a GRV of 150 mL obtained from a large-diameter sump tube with multiple ports.
Increased Gastric Secretions During Small-Bowel Feedings
Objectives
Sample and Setting This project represents a component of a study currently in progress to detect aspiration in critically ill, mechanically ventilated, tube-fed patients. For this component of the study, 137 critically ill tube-fed patients were recruited from 5 intensive care units in a Midwestern medical center. Among the inclusion criteria were continuous feedings through a nasally or orally inserted tube and radiographic evidence of tube site. The first research objective included 62 patients receiving continuous gastric feedings via a small-diameter tube while a large-diameter gastric sump tube was concurrently present to administer medications. The second research objective included 75 patients who were receiving small-bowel tube feedings while a large-diameter gastric sump tube (not connected to suction) was concurrently present. Of these 75 patients, most (73%) were continuously fed in the duodenal bulb; the rest were fed in the second portion of the duodenum (20%), third portion of the duodenum (5%), and jejunum (2%). Participants in the project received continuous feedings at rates ranging between 10 and 70 mL per hour (mean = 42 mL/h). Patients with gastrostomy and jejunostomy tubes were excluded. Seventy-nine percent of the 137 patients were entered into the study within the first day of tube feedings (the remainder was enrolled 2–5 days after feedings were started); 98% were receiving a H2 receptor antagonist or a proton pump inhibitor. Characteristics of patients included in the project are described in Table I.
Design
Equipment
Procedure For each patient, the same position (supine, right side-lying, or left side-lying) and degree of the bed's backrest elevation (0°–90°) was maintained for both measurements; this was done to control for any effect these 2 variables might have on pooling of gastric contents. No attempt was made to alter the position or backrest elevation selected by the patients' caregivers. When gastric feedings were in use, residual volumes were first measured from the small-diameter feeding tube; the entire volume was returned to the stomach so that a measurement could be then be obtained from the large-diameter sump tube concurrently present in the stomach. After completion of the GRV measurement from the small-diameter feeding tube, 30 mL of water or normal saline was flushed through the tube to prevent clogging. The GRV from the large-diameter tube was measured next; the amount was recorded and adjusted for the previously instilled flush solution via the small-diameter tube. Last, 30 mL of water or normal saline was flushed through the large-diameter tube to prevent clogging; the gastric sump tube was clamped and the feeding tube was reconnected to the enteral solution setup. When small-bowel feedings were in use, the residual volume was first measured from the small-bowel tube; next, the volume of gastric aspirate obtained from the sump tube (concurrently present in the stomach) was measured. The following procedure was used to measure residual volumes from all types of tubes. The enteral formula flow was paused and 30 mL of air was immediately instilled into the tube with a 60-mL syringe before attempting to obtain an aspirate from the tube. This maneuver was helpful in facilitating aspiration of fluid into the syringe. When aspirate was obtained, it was placed in a clean 400-mL calibrated disposable beaker before attempting to aspirate additional fluid. This process was repeated until no more fluid could be obtained. The total volume of aspirate was measured and recorded. At the end of the measurement procedure, the nurse providing direct care decided whether or not a large GRV would be returned to the patient. According to the protocol developed at the institution where the study was conducted, up to 200 mL of the aspirate was usually returned to the patient and the remainder was discarded. An exception to this rule was that 1 mL of fluid was saved from each tube for pH testing.
Statistical Analyses
To address the first objective, multiple residual volume measurements were made from 62 patients who had 2 tubes simultaneously present in the stomach (a small-diameter feeding tube and a large-diameter sump tube). More specifically, 645 measurements were made from each type of tube. Radiographic confirmation of gastric placement was obtained in all patients; however, no attempt was made to control for the specific location of the feeding tubes' ports within the stomach because it is probable that feeding tubes migrate back and forth between the antrum and fundus within the course of a day of feeding.20
Because residual volumes from the 14-Fr and 18-Fr sump tubes did not differ
significantly (p = .264), they were collapsed into 1 category. The
mean volume of aspirate obtained from the large-diameter sump tubes was
approximately 2 times higher than that obtained from small-diameter (10-Fr)
tubes (45.8 ± 2.9 mL vs 20.1 ± 1.7, respectively,
p < .001; see Figure
1). As shown in Figure
2, GRV levels variably considered to be excessive (ranging between
50 mL and 200 mL) were reached significantly more often when aspirates were
obtained from large-diameter sump tubes. For example, a GRV
The mean pH of aspirates from the gastric feeding tubes was higher than that from the sump tubes (6.3 ± 0.6 vs 5.6 ± .09, respectively, p < .001); also, aspirates from the feeding tubes were formula-colored more often than were aspirates from the sump tubes. Patients fed for 1 day or less at time of entry into the study had a higher mean gastric sump-tube volume than did patients fed for more than 1 day (49.7 ± 3.6 vs 27.9 ± 3.9 mL, respectively, p < .001); this likely reflected the effect of gastric adaptation to tube feedings over time. To address the second objective, multiple residual volume measurements were made from 75 patients who had both a 10-Fr small-bowel feeding tube and a large-diameter (14-Fr or 18-Fr) gastric sump tube. More specifically, 890 measurements were attempted from each tube. As shown in Figure 4, individual gastric sump-tube residual volumes were occasionally in the range of what has been considered "excessive." The mean residual volume obtained from the small-bowel tubes was 4.6 ± 0.3 mL (range, 0–105 mL). The highest small-bowel residual volume (105 mL) was obtained from a tube positioned in the duodenal bulb of a patient diagnosed with ileus. Ninety percent of the small-bowel aspirates were <12 mL. The mean pH of the small-bowel feeding-tube aspirates was higher than that of the gastric sump-tube aspirates (6.7 ± 0.8 vs 5.1 ± 2.1, respectively, p < .001); most of the small-bowel aspirates were formula-colored as opposed to the sump-tube aspirates, which were usually green.
Although it was possible to aspirate fluid from small-diameter tubes in approximately 76% of the 645 attempts from the stomach and 70% of the 890 attempts from the small bowel, the volumes were often quite low and likely did not represent the amount of fluid actually present (especially in the stomach). Because 2 tubes were simultaneously present in the stomach, one would anticipate that the residual volumes obtained the tubes would be similar. However, as described above, the mean volume of aspirate obtained from large-diameter sump tubes was more than 2 times higher than that obtained from small-diameter (10-Fr) tubes. In addition, individual high GRVs were observed more frequently when the larger tubes were used. The findings indicate that small-diameter feeding tubes underestimate the volume of gastric contents present during enteral feedings. As indicated earlier, the regression line drawn to relate GRVs from the different tube sizes across the wide range of values indicated that GRVs obtained from the large-diameter sump tubes were approximately 1.2–1.7 times greater than those obtained from the 10-Fr tubes. For example, as depicted in Figure 3, a reading of 150 mL from a 10-Fr tube could be estimated as equivalent to 225 mL from a 14-Fr or 18-Fr sump tube, whereas a reading of 200 mL from a 10-Fr tube could be estimated as equivalent to 320 mL from a 14- or 18-Fr sump tube. GRVs decreased over time, likely because of the effect of adaptation to feedings; however, this finding did not affect the results because our aim was to compare simultaneous readings from the 2 types of gastric tubes. A limitation of the study is that only 1 type of small-bore feeding tube was included; it is possible that tubes with a different port configuration would have allowed the withdrawal of more fluid.
Almost 12% of the gastric aspirates from patients receiving small-bowel
feedings reached levels Very low small-bowel residual volumes (usually <10 mL) during feedings have been reported by other investigators.21,22 Because of these typically low values, small-bowel residual-volume measurements rarely performed during small-bowel feedings.20 are It has been suggested that residual volumes cannot be obtained when transpyloric tubes are correctly positioned.23 Perhaps we were able to aspirate fluid in approximately 70% of the attempts from small-bowel feeding tubes because most of them were positioned in the proximal duodenum (as opposed to the distal duodenum or jejunum); it is also probable that the technique used by the data collectors contributed to the success rate. Some authors have suggested that serial small-bowel residual volume measurements can be used as an indicator of tube location.24–26 For example, a sudden volume increase may be a signal of upward displacement of the tube's ports into the stomach.
According to the above findings, it is concluded that small-diameter tubes may substantially underestimate GRVs; therefore, clinicians should take into account the type of feeding tube used when evaluating the significance of a specific GRV. The results described above may be helpful in estimating actual GRVs when small-diameter tubes are being used. It is also concluded that a small percentage of patients receiving postpyloric feedings will have substantial GRVs. Thus, GRV monitoring may be needed during small-bowel feedings when other risk factors for aspiration are present. The National Institute of Nursing Research, R01 NR05007, supported this study. Received for publication September 20, 2004. Accepted for publication January 27, 2005.
Journal of Parenteral and Enteral Nutrition, Vol. 29, No. 3,
192-197 (2005) This article has been cited by other articles:
|
|
|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
 |
|
|
 |
|
100 mL were found in 11.6% of the 890
measurements made in patients receiving small-bowel feedings; volumes 
Top
MATERIALS AND METHODS
RESULTS
was < .05 (2-tailed). Linear
regression was used to evaluate the relationship between the GRVs measured
from the 2 types of tubes (10-Fr feeding tubes and 14- or 18-Fr sump tubes).
The magnitude of the relationship between the 2 measurement types was assessed
using Pearson correlation coefficient. Statistical software used was SPSS
version 12.0 (Chicago, IL). All statistical evaluations had a sufficiently
large sample size to provide at least 80% power. 
