Enhancing Interpretation of Gastric Residual Volume by Refractometry
* Division of Gastroenterology, Department of
Internal Medicine, Tri-Service General Hospital, National Defense Medical
Center, Taipei, Taiwan, Republic of China; and Correspondence: Stephen A. McClave, MD, Professor of Medicine, Division of Gastroenterology/Hepatology, University of Louisville School of Medicine, 550 S. Jackson Street, Louisville, KY 40202. Electronic mail may be sent to samcclave{at}louisville.edu. The traditional practice of gastric residual volumes (GRVs) is flawed in its design and conception, is poorly standardized in its technique, is an inaccurate measure of gastric emptying, and serves as an insensitive marker for regurgitation and aspiration. The refractive index of a solution (like an enteral formula) is a physical property of that solution, which is remarkably constant and reproducible under varying conditions of concentration, pH, and temperature. Refractometry may be performed quickly and easily at the bedside, requires only a small representative sample of aspirated solution, and provides valuable measurements that can be used to calculate both the true total volume of contents and the specific volume of formula remaining in the stomach. Refractometry complements the use of GRVs as a monitor for patients receiving enteral feeding and should improve the accuracy with which patients at risk for aspiration may be identified. No aspect of the practice of gastric residual volumes (GRVs) has been standardized in the literature. Several studies have shown that the values obtained for GRVs may be dramatically altered by variation in technique.1–3 GRVs may be artifactually reduced with use of small syringes, with feeding tubes that have certain design characteristics (collapsible silicone material, narrow-caliber <12 French, fewer infusion ports in the distal end), or situations where the tip of the feeding tube is adhered to the gastric mucosa or positioned away from the pool of gastric juice.1–3 Two additional issues that limit the applicability of GRVs include the fact that the entire true volume of contents remaining in the stomach may not be collected upon aspiration and that the practice focuses on supposed volume of formula remaining without taking into account the 4000 to 5000 mL of salivary and gastric juices delivered daily to the stomach.4 As a result of these factors, little data exist to support a correlation between GRVs and gastric emptying, volume of gastric contents, incidence of aspiration and regurgitation, or changes in the infusion of enteral tube feeding.5 Any device, modification, or change in practice that can enhance the reliability of GRVs might prove to be a valuable tool in both the monitoring and the evaluation of treatment for patients receiving nasogastric feeding.
When light passes obliquely from one medium into another of different optical density, the beam is bent or refracted. The degree to which the light is refracted is called the refractive index. As the concentration of a solution increases, its refractive index changes proportionately (Fig. 1). The refractive index represents a physical property of the medium or fluid related to the number, charge, and mass of the vibrating particles in that fluid through which the light is transmitted (Fig. 1).6–8 The refractive index of a pure substance, also known as the Brix value (BV), is a measurement of the total soluble solids in solution (correlating closely with the molar fractions of the components) that remains constant under standard conditions of temperature and pressure.9–11 The reliability of this physical characteristic can be used to confirm the identity of substances, to analyze mixtures, and to estimate the concentration of nutrients.9,10 Refractometry is used extensively in the agricultural industry. BV measurements have been used in a number of clinical settings to determine the concentration of pure or mixed substances such as drugs, food, fruit juices, and parenteral nutrition solutions.6–11
A variety of commercial refractometers are available on the market and may be ordered via the internet. The instrument, purchased for as little as $100 to $150, is typically <20 cm in length, is handheld, and resembles a spyglass or small telescope (Fig. 2). At the distal end is a slanted window with a hinged flip-up cover. At the proximal end of the device is an adjustable eyepiece that allows for variable focusing of the reading scale.
To operate the instrument and take a reading, a large bead of the solution to be tested is placed on the slanted viewing window, and the hinged cover is gently flipped down or closed (flattening out the bead of solution) (Fig. 3A). The operator then holds the refractometer at eye level and peers through the eyepiece (Fig. 3B). The vertical reading scale is immediately apparent upon viewing and can be brought into sharp focus using the adjustable eyepiece (Fig. 4). The scale has a range from 0–32 and is scored in 0.2 increments. The BV measurement represents the obvious line of demarcation between the white and blue fields (Fig. 4). Usually, the line of demarcation is sharp and clear, but increasing fat content in the solution being tested may render the line slightly less distinct.
The reading is immediate, with the duration of the procedure limited only to the time required to place the bead and hold the instrument up to the eye. Only drops of solution are required for an accurate measurement, after which the distal end may be splashed clean with water and dried quickly with a dry tissue (Fig. 5). The instrument is then ready for another measurement on the same or a different patient.
No sterilization at any time is required. As no part of the instrument touches the patient and no portion of the drops of solution actually tested are returned to the patient, there is no risk for transmission of infectious disease (obviously all of the aspirated GRV not involved in the testing may be returned to the individual patient). Periodically the instrument should be recalibrated. After placing a bead of distilled water on the slanted window, a finger screw on top of the instrument is turned to reset the line of demarcation to zero.
The BV measurement of distilled water is zero and thus is used as the set point for calibration of the instrument. In contrast, BV measurements of 0.9% normal saline, saliva, and gastric juice have been shown to be slightly higher at 1.0 ± 0.1, 1.3 ± 0.4, and 1.9 ± 0.6, respectively.12 Five percent dextrose is higher still at 5.2 ± 0.2.12 When 2 or more solute components are added, the resulting BV measurement of the total solution closely approximates the sum of the BV measures of the individual components. The BV measurement of a typical enteral formula, Osmolite 1.0 (formerly Osmolite HN, Ross Division of Abbott Laboratories, Columbus, OH), for example, is 23.2 ± 0.3, which is nearly the sum of the measures of its 3 components: carbohydrate (17 g/dL) at 12.4 ± 0.6, fat (4.1 g/dL) at 6.0 ± 0.1, and protein (5.3 g/dL) at 6.3 ± 0.1.11 Performing refractometry on several commercially available dietary formulas of similar component constituency [Osmolite 1.0 (Ross Division, Abbott Laboratories), Vital HN (Ross Division, Abbott Laboratories), Vivonex (Novartis Nutrition, Minneapolis, MN), and Isosource (Novartis Nutrition)] produces BV measurements that are of close proximity (Table 1).11 The BV readings for each of these 4 formulas have been shown to remain remarkably constant under varying conditions of pH (pH = 1, pH = 4, pH = 7, and pH = 8) and temperature (4°C, 25°C, and 37°C).11 Only at extremes of pH in an acidic environment are the BV measures altered (lower than expected), due most likely to the denaturization of the protein component.11
Interestingly, dilution of dietary formulas has a very reliable and consistent effect on BV measurements, a relationship that can be used in clinical practice. When the concentration strength of a dietary formula is reduced by a percentage amount, the corresponding BV measurement is reduced by nearly the same fractional degree. As part of an earlier study, full-strength Osmolite 1.0 was diluted from 100% to 75%, 50%, 25%, 12.5%, and 0% using distilled water.11 The corresponding BVs of each serial dilution were shown to be 23.2 ± 0.3, 17.8 ± 0.2, 12.0 ± 0.1, 3.0 ± 0.1, and 0.0 ± 0, respectively (R2 = 0.99, slope = 0.24).11 The BV decreased significantly in a linear manner as dilutional water was added (Fig. 6).11
This tight linear relationship can be used to calculate the concentration of the dietary formula in a given setting. The graph of this mathematical relationship can be described by the equation y = ax + b; where y = BV, x = % concentration of formula, a = slope = 0.24, and b = y intercept = 0 (Fig. 6).11,12 By rearranging the equation and solving for x, the concentration of dietary formula can thus be determined by the final equation: % concentration of formula = BV/0.24.11,12 The same linear relationship between consecutive serial dilutions of a formula by distilled water and the corresponding decreases in BV measurements described for Osmolite 1.0 was also shown for 3 other formulas (Vital HN, Vivonex, and Isosource).11 In addition, serial dilution of all 4 of these formulas by gastric juice produced BV measurements that were nearly identical to those values obtained by serial dilution with sterile water.11 These data suggest that not only can the concentration of dietary formula be readily estimated by the use of refractometry, but that these measurements can be expected to be reliable through varying ranges of concentration, pH, or temperature, and under different conditions of storage, preparation, and administration of virtually any of the commercial dietary formulas.11 The value of these basic physiologic principles and physical characteristics of a solution become apparent in the patient receiving enteral tube feeding. If a formula is infused through a tube into the stomach, performing refractometry on subsequent GRVs over time may provide indirect evidence for the degree to which formula is being emptied from the stomach. If the BV measurement of a formula at full strength is approximately 24, then separate aspirated volumes with BV measures of 12 and 6 at different times could be expected to have a corresponding concentration of 50% and 25%, respectively.11,12 In a previous study,12 43 patients receiving bolus nasogastric feeding were monitored and then arbitrarily divided into 2 groups according to observed GRVs; patients with lower GRVs (<75 mL over the monitoring period) were placed in group 1, whereas patients with higher GRVs (>75 mL) were placed in group 2.12 Patients were given 250 mL of polymeric formula (Osmolite 1.0) by bolus nasogastric infusion, followed by BV measurement of gastric contents at 0, 30, 60, 120, and 180 minutes. In both groups, the serial BV measurements decreased steadily over the 3-hour time interval after bolus feeding, suggesting that the concentration of formula was decreasing as more of the formula was being emptied and an increasing percentage of the GRV was made up of endogenous salivary and gastric secretions.12 However, for patients in group 2 the decrease was less, such that at 180 minutes, the mean BV measurement of the GRV was significantly higher than that for patients in group 1 (10.1 vs 5.1, respectively, p < .01).12 These data suggested that a higher concentration of formula remained in the stomach in the group already identified by GRVs to have slower gastric emptying. In this manner, refractometry was able to enhance or support clinical impressions made according to traditional GRVs.
Failure to obtain reliable GRVs (ie, that the sample volume obtained by aspiration does not equal the true total volume of contents remaining in the stomach) interferes with the interpretation of serial BV measurements made over time in a given patient. As described in the previous discussion, decreasing BV measures over time (following a bolus infusion of formula) may be interpreted as the situation where formula is being emptied steadily from the stomach.12 If the true volume of contents remaining in the stomach is not known, however, then an equally plausible alternative interpretation of the same data is that formula is not emptying at all, but is steadily being diluted by continued secretion of endogenous gastric and salivary secretions. Application of refractometry through a "water dilution technique" may be used to help calculate a more accurate GRV (Fig. 7).12,13 If the true volume of contents remaining in the stomach (True GRV) is unknown, it can be indirectly calculated by repeated BV measurements before and after dilution with a known quantity of water.
A given amount of formula in a patient's stomach, diluted by endogenous secretions to a certain volume, will have a given concentration (and thus have a specific BV measurement). Adding a known volume of water (50 mL) does not change the original amount of formula or the volume of endogenous secretions contained in the stomach but only dilutes the concentration further, thus reducing the BV measurements by the appropriate corresponding percentage (Fig. 7).12 The degree to which the BV changes (and the concentration of formula decreases further) in response to dilution by a known volume of water is an effect that can be used very simply to derive the original volume of gastric contents (the True GRV).12 The following equation describes this relationship: [True GRV x Predilution BV] = [(True GRV + 50 mL) x Postdilution BV]. The unknown volume of contents remaining in the stomach can thus be calculated by rearranging the equation to solve for True GRV (Fig. 7): [True GRV = (50 mL x Postdilution BV)/(Predilution BV – Postdilution BV)].12,13 In the previously mentioned study involving bolus feeding,12 the 2 groups of patients were arbitrarily differentiated by traditional use of GRV during the pretest monitoring period. Following the bolus infusion of formula, those patients identified in group 2 (higher GRVs) were shown to have significantly higher mean GRV at 180 minutes (aspirated at the bedside) than those patients in group 1 (72 ± 12 mL vs 18 ± 5 mL, respectively, p < .05).12 Determining the True GRV by the water dilution technique reinforced the clinical impression derived by the traditional aspirated GRV. Patients in group 2 were shown to have a significantly higher mean True GRV at 180 minutes postbolus infusion than those patients in group 1 (106 ± 14 mL vs 67 ± 15 mL, respectively, p < .05).12
Once the more accurate True GRV has been calculated, the specific volume of formula remaining in the stomach can be determined (Fig. 8). The volume of formula remaining in the stomach (Volumeformula) should equal the True GRV multiplied by the % concentration of the formula [Volumeformula = True GRV x % concentration]. BV measurement by refractometry provides the % concentration through the previously mentioned equation: [% concentration = BV/0.24].12,13 Thus the specific volume of formula remaining in the stomach now can be determined easily by the equation Volumeformula = True GRV x (BV/0.24) (Fig. 8).12,13
Being able to determine the volume of formula remaining in the stomach allows for even further "fine tuning" of the interpretation of aspirated GRVs, calculation of the True GRV, and whether problems with gastric retention exist. As expected in the previous study involving bolus infusion of formula followed out for 180 minutes by BV measurements, the patients in group 2 had a significantly higher volume of formula calculated to remain in the stomach than those patients in group 1 (50 ± 8 mL vs 6 ± 2 mL, respectively, p < .05).12 However, by focusing on the volume of formula remaining (Volumeformula) and comparing it to the initial volume of the bolus infusion, specific patients with potential problems of gastric stasis could be identified. Out of the 25 patients in group 1 (lower GRVs), all of whom were interpreted to have no problems with gastric emptying, 1 patient (4%) was calculated to have >20% of the infused volume of formula still in the stomach at 3 hours.12 In contrast, of the 18 patients in group 2 whose significantly higher aspirated GRVs and calculated True GRVs might have generated some concern, 13 patients (72%) were shown to have emptied > 80% of the original bolus volume 3 hours later.12 Such findings, which were not identified by traditional aspirated GRVs, provide assurance to the clinician regarding those patients within the population who appeared to be clearing the formula from the stomach, allowing them to focus closer attention instead on those patients specifically retaining formula. In another separate study,13 this time involving patients on continuous infusion of enteral formula, the water dilution technique and determination of the volume of formula remaining helped differentiate potential problem patients with reduced gastric emptying from a larger patient population. In a similar fashion, patients were arbitrarily placed in 1 of 2 groups according to traditional aspirated GRVs observed during a pretest monitoring period. Being able to calculate the volume of formula remaining in the stomach by these refractometry techniques helped assure that all 22 patients in 1 group (GRVs <75 mL) had Volumeformula that was less than the hourly infusion rate when checked twice 3 hours apart.13 In contrast, 6 of 14 patients in the other group (GRVs >75 mL) twice showed Volumeformula that was greater than the hourly infusion rate, suggesting potential problems in gastric emptying and identifying patients that might need closer scrutiny and monitoring by the clinician. The remaining 8 patients were shown to have Volumeformula less than the hourly rate. None of this information could be ascertained by use of traditional GRVs alone.13
A number of strategies may be developed using refractometry with or without aspirated GRVs. Table 2 shows one strategy where both GRVs and BV measurements are made at regular intervals (ie, every 4 hours), the 2 tests are interpreted simultaneously, and the water dilution technique is reserved only for those patients in whom both markers are elevated.12 Refractometry may be reserved for those patients with GRVs above a designated level or for those who demonstrate clinical signs of intolerance. Refractometry might even replace use of GRVs entirely as a clinical monitor at some point.
At the present time, the exciting potential of this instrument requires validation by different laboratories. Early validation studies by our group (unpublished) using endoscopy are confirming the accuracy with which BV measurements calculate True GRV and volume of formula remaining in the stomach. Hopefully this technology can help confirm or deny a relationship between gastric emptying, volume of gastric contents, intolerance of enteral feeding, and likelihood for aspiration.
Nutrition in Clinical Practice, Vol. 19, No. 5,
455-462 (2004)
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Principles of Refractometry
Operation of the Refractometer
).