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Journal of Parenteral and Enteral Nutrition
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Original Communications

Gastric Residual Volume (GRV) and Gastric Contents Measurement by Refractometry

Wei-Kuo Chang, MD, PhD*, Stephen A. McClave, MD{ddagger}, Chung-Bao Hsieh, MD{dagger} and You-Chen Chao, MD*

From the * Division of Gastroenterology, Department of Internal Medicine, Tri-Service General Hospital, Taipei, Taiwan, Republic of China; {dagger} Division of General Surgery, National Defense Medical Center, Taipei, Taiwan, Republic of China; and the {ddagger} Department of Medicine, University of Louisville School of Medicine, Louisville, Kentucky

Correspondence: You-Chen Chao, MD, Division of Gastroenterology, Tri-Service General Hospital, No. 325, Cheng-Kung Rd., Sec. 2, Neihu 114, Taipei, Taiwan. Electronic mail may be sent to weikuohome{at}hotmail.com.

Background: Traditional use of gastric residual volumes (GRVs), obtained by aspiration from a nasogastric tube, is inaccurate and cannot differentiate components of the gastric contents (gastric secretion vs delivered formula). The use of refractometry and 3 mathematical equations has been proposed as a method to calculate the formula concentration, GRV, and formula volume. In this paper, we have validated these mathematical equations so that they can be implemented in clinical practice. Methods: Each of 16 patients receiving a nasogastric tube had 50 mL of water followed by 100 mL of dietary formula (Osmolite HN, Abbott Laboratories, Columbus, OH) infused into the stomach. After mixing, gastric content was aspirated for the first Brix value (BV) measurement by refractometry. Then, 50 mL of water was infused into the stomach and a second BV was measured. The procedure of infusion of dietary formula (100 mL) and then water (50 mL) was repeated and followed by subsequent BV measurement. The same procedure was performed in an in vitro experiment. Formula concentration, GRV, and formula volume were calculated from the derived mathematical equations. Results: The formula concentrations, GRVs, and formula volumes calculated by using refractometry and the mathematical equations were close to the true values obtained from both in vivo and in vitro validation experiments. Conclusions: Using this method, measurement of the BV of gastric contents is simple, reproducible, and inexpensive. Refractometry and the derived mathematical equations may be used to measure formula concentration, GRV, and formula volume, and also to serve as a tool for monitoring the gastric contents of patients receiving nasogastric feeding.

Early enteral nutrition has been demonstrated to improve wound healing, preserve intestinal mucosal integrity, and decrease the length of hospital stay. Many critically ill patients cannot tolerate nasogastric tube feeding and thus develop manifestations of feeding intolerance, including vomiting, abdominal distension, and aspiration.1–4 The measurement of gastric residual volume (GRV) obtained by nasogastric tube aspiration is used widely to monitor gastric emptying in clinical practice.5–9 However, the traditional use of GRV is inaccurate and cannot differentiate components of the gastric contents (gastric secretion vs delivered formula).10,11 Therefore, developing a method to determine the total (unknown) volume in the stomach and to differentiate the exact volumes of gastric secretion and delivered formula would significantly improve the evaluation of treatment in patients receiving nasogastric feeding.

Current methods for the measurement of gastric emptying of patients receiving nasogastric tube feeding are unsuitable for routine clinical use.9–13 The use of scintigraphy or magnetic resonance imaging to measure the gastric emptying is limited by the expense and availability of the equipment.13,14 Monitoring gastric emptying using the marker dilution technique requires the intake of special dye and a spectrophotometer to determine the concentration of phenol red.15

Ideally, clinical measurement of GRV and the composition of gastric contents needs to be convenient, reliable, and widely available and would not expose the subject to ionizing radiation or chemical agents. The refractometer is a handheld device about 20 cm in length that permits bedside measurement of gastric contents.16 The refractive index of a refractometer, also known as the Brix value (BV), is a measurement that is constant for a pure substance under standard conditions of temperature and pressure.17,18 The BV, a measure of total soluble substance in solution, can be correlated closely with the molar fractions of the different components in a mixture.17,18 BVs have been used widely to determine the concentrations of substances such as drugs, fruit juices, enteral formula, and parenteral nutrition (PN) solutions.17–22 The use of refractometer to obtain BVs and the 3 mathematical equations (Table I) has been proposed to measure the formula concentration, GRV, and formula volume in the stomach.16,23,24 However, the theoretical mathematical model has not been previously validated in the in vivo experiments. In this paper, we verified the correctness of the model by performing a validation experiment. We propose the use of refractometry and mathematical models to monitor the gastric contents of patients receiving enteral tube feeding.


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  		Table I Mathematical equations for gastric contents calculation

 


   MATERIALS AND METHODS
Top
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 
Patients
Sixteen patients (11 men and 5 women; mean age 54 ± 25 years) receiving nasogastric tube feeding at the Tri-Service General Hospital, Taiwan, were enrolled in this study. The admission diagnoses of these patients included neurologic conditions (n = 10), head and neck tumors (n = 2), and malignancy of the esophagus, ovary, lung, and cervix (n = 4). At admission, each patient was in stable condition and did not have fever, sepsis, bleeding, surgery, mechanical obstruction of the gastrointestinal tract, or hemodynamic instability. This study was approved by the institutional review board and conducted at the Tri-Service General Hospital, National Defense Medical Center, Taiwan. Subjects were fully informed about the purpose of this study, and informed consent was obtained.

Gastric Contents in Vivo Experiment
No premedication, such as glucagon or hyoscine-N-butylbromide, that could affect the gastric emptying was given in this study. After an overnight fast, each patient had a 14-Fr nasogastric tube inserted. An endoscope (GIF 200, Olympus, Tokyo, Japan) was introduced into the stomach to directly visualize and assist nasogastric tube aspiration of the gastric contents for analysis (Figure 1). To improve visibility during endoscopy, we infused 20 mL simethicone into the stomach via the nasogastric tube. After 10 minutes, all remaining gastric juice was aspirated from the stomach via the nasogastric tube.


Figure 1
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  		FIGURE 1. An endoscope was introduced into the stomach to directly visualize and assist the nasogastric tube and thus facilitate obtaining gastric juice (1A). Study designs for formula content measurement in the in vivo and in vitro experiments (1B).

 
Each patient then received polymeric diet (Osmolite HN, Abbott Laboratories, Columbus, OH) via the nasogastric tube feeding. This formula, with concentration of 300 m Osm/kg, provides 1 kcal/mL and derives 54.3% of its calories from carbohydrate, 16.7% from protein, and 29.0% from fat. To mimic the residual juice remaining in the stomach, a bolus of 50 mL of distilled water was infused into the stomach, followed by 100 mL of dietary formula (Osmolite HN). A 60-mL syringe was used to mix the gastric contents thoroughly by drawing the gastric contents into the syringe and then returning them to the stomach 3 times. After mixing, a 1-mL volume of mixed gastric contents was obtained for a BV measurement (BV1). After this, the first dilution test was performed. Fifty milliliters of distilled water was infused into the stomach, the gastric contents were thoroughly mixed, and then another 1 mL of gastric juice was aspirated for the second BV measurement (BV2). A bolus infusion of dietary formula (100 mL) and water (50 mL) into the stomach was repeated, followed by subsequent BV measurement (BV3 and BV4).

Formula Contents in Vitro Experiment
In the in vitro experiment, 50 mL of distilled water and then 100 mL dietary formula (Osmolite HN) were added to a beaker (Figure 1). The beaker contents were thoroughly mixed, and the BV measurement (BV1) was made using a 1-mL sample. After this, 50 mL of distilled water was added, mixed thoroughly, and then another 1-mL sample was obtained for the second BV measurement (BV2). The addition of dietary formula (100 mL) and distilled water (50 mL) into the beaker was repeated, followed by subsequent BV measurements (BV3 and BV4).

To verify that the mathematical equation could be used at a higher GRV, we performed an in vitro validation experiment. Dietary formula (200 mL and 400 mL) was diluted with 50 mL water, the BV was measured, and mathematical calculations were performed.

Formula Concentration Calculation
BV measurements were made using a handheld refractometer (Model N.O.W. 507–1; Nippon Optical Works, Tokyo, Japan) using a Brix scale of 0–32, with increments of 0.2. The refractometer was calibrated with distilled water before each measurement. One or 2 drops of the specimen fluid were placed on a designated window for observation. All measurements were made at room temperature using natural light.


Figure 2
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  		FIGURE 2. Linear regression of Brix values and dietary formula concentrations in serial water dilutions. Dietary formula of Osmolite HN dilution resulted in values from 100% to 75%, 50%, 25%, 12.5%, and 0% of the full-strength formula concentration.

 
The basis of this study was a series of in vitro experiments in which a refractometer was used to measure serial dilutions of a polymeric formula.16,24 BV measurements for serial full-strength polymeric formula (Osmolite HN) dilutions of the dietary formula (100%, 75%, 50%, 25%, 12.5%, and 0%) were made with distilled water and had corresponding BVs of 23.2 ± 0.3, 17.8 ± 0.2, 12.0 ± 0.1, 6.0 ± 0.1, 3.0 ± 0.1, and 0 ± 0, respectively (R2 = 0.99, slope = 0.24; Figure 2). According to this tight linear relationship, the unknown formula concentration can be calculated by the mathematical equation y = ax + b, where

  • y = BV,
  • x = formula concentration (full strength formula concentration %),
  • a = slope,
  • b = y intercept = 0.

Therefore, solving for x:

  • y = ax + b,
  • x = (y-b) ÷ a,
  • x = (BV – 0)/slope,

Formula concentration (x) = BV/slope.

Because the slope of the polymeric formula (Osmolite HN) is 0.24, the unknown formula concentration (full strength formula concentration %) can be calculated by the derived mathematical equation Formula concentration = BV/0.24 (Table I, Equation 1).

Gastric Contents Calculation
We have developed a method using refractometry and a "water dilution test" to measure the calculated the gastric contents.16,23,24 Gastric contents were diluted with 50 mL water, and BV measurements were performed before (predilution BV) and after (postdilution BV) water dilution. The mathematical equations in Table I were used to calculate the GRV and the formula volume and the volumes of gastric secretions.16,23,24

Statistical Analysis
Results are presented as mean ± SD. The Pearson correlation test was used for bivariate analysis between each component of the calculated and true gastric contents (formula concentration, GRV, and formula volume).


   RESULTS
Top
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 
BV Measurement
BV measurements before and after the 50 mL distilled water serial dilution are shown in Table II. BVs (BV1 and BV3) decreased after 50 mL water dilution (BV2 and BV4). In the in vitro and in vivo experiments, BV1, BV2, BV3, and BV4 showed minimal variability and are shown in Table II.


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  		Table II Brix values (BV) of formula (in vitro) and gastric contents (in vivo)

 

Formula Concentration Calculation
Formula concentrations were calculated using Equation 1 (Table III). For example, in the in vitro experiment, a formula volume of 100 mL diluted with 50 mL water yielded a BV1 of 16.0, a true concentration of 67%, and a calculated formula concentration by the equation BV/slope = 16.0/0.24 = 67 (full-strength formula concentration %). Results show the calculated formula concentrations for BV1, BV2, BV3, and BV4 in the in vitro validation experiment were 66% ± 1%, 50% ± 1%, 67% ± 1%, and 58% ± 1% of full strength formula concentration. The formula volume of Osmolite HN 100 mL, 100 mL, 200 mL, and 200 mL diluted with water of 50 mL, 100 mL, 100 mL, and 150 mL obtained the true formula concentrations of 67% (100 mL/150 mL), 50% (100 mL/200 mL), 67% (200 mL/300 mL), and 57% (200 mL/350 mL) of full-strength polymeric formula for BV1, BV2, BV3, and BV4, respectively. The calculated formula concentration is very close to the true formula concentrations (p < .001). Similar results were obtained in the in vivo validation experiment (p < .01).


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  		Table III Formula concentrations calculated by mathematical equation

 

GRV Calculation
GRVs were calculated using Equation 2 (Table IV). For example, in the in vitro experiment using an Osmolite HN at a formula volume of 100 mL diluted with 50 mL water, we obtained a predilution BV (BV1 = 16.0) and a postdilution BV (BV2 = 12.0; Table II). The unknown GRV (predilution volume) can be calculated using the mathematical equation GRV = (50 mL x postdilution BV)/(predilution BV – postdilution BV) = (50 x 12.0)/(16.0 – 12.0) = 150 mL. In the in vitro validation experiment, the calculated GRVs were 158 ± 10 mL and 335 ± 21 mL for the first (total volume of predilution BV1) and second (total volume of predilution BV3) water dilution tests, which were close to the true GRVs of 150 mL and 300 mL, respectively (p < .001). Similar results were obtained in the in vivo validation experiment (p < .01).


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  		Table IV Gastric residual volumes (GRVs) calculated by mathematical equation

 

Formula Volume Calculation
Formula volumes were calculated using Equation 3 (Table V). For example, in the in vitro validation experiment, a formula volume of 100 mL diluted with 50 mL water yielded a calculated GRV of 150 mL (Table IV), and from a calculated formula concentration (67% of full strength formula concentration; see Table III), the unknown formula volume could be calculated via Equation 3, Formula volume = GRV x formula concentration = 150 mL x 67% = 101 mL. In the in vitro validation experiment, the calculated formula volumes (derived from the BVs) were 105 ± 6 mL and 223 ± 13 mL in the first and second dilution tests, which were close to the true formula volumes of 100 mL and 200 mL, respectively (p < .001). Similar results were obtained in the in vivo validation experiment (p < .001).


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  		Table V Formula volumes calculated by mathematical equation

 

To verify that the equations could be used at a higher GRV, our results showed that the calculated residual volumes were 211.3 ± 3.9 mL and 420.3 ± 11.4 mL, which were close to the true total volumes of 200 mL and 400 mL, respectively (R2 > 0.98). Calculated formula volumes were 214.4 ± 3.9 mL and 425.8 ± 11.5 mL, which also were close to the true formula volumes of 200 mL and 400 mL, respectively (R2 > 0.98; data not shown).


   DISCUSSION
Top
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 
Handheld refractometers are precision optical instruments that measure the degree that light bends as it passes through the interface between 2 substances of different densities. The refractometer operates on the principle that, as the concentration of a solution increases, its refractive index and BV changes proportionately.17,18 The BV of a solution is remarkably reproducible under varying conditions. The BV has been shown to have an excellent correlation with the concentration of commercially available dietary formula (Osmolite HN, Resource, Vital HN, Vivonex, and Isosource) across varying conditions of pH (pH 1, pH 4, pH 7, and pH 8) or temperature (4°C, 25°C, and 37°C).21 The reliability of these physical characteristics of BV can be used to confirm the identity of substances, to analyze mixtures, and to measure the dietary formula concentrations under different conditions of storage, preparation, and administration of dietary formulas for patients receiving enteral feeding.

In the in vivo validation experiment, the overestimate of the formula concentration (derived from BV1) may be due to retained soluble substance (simethicone, endogenous secretions, or food debris) in the fasting stomach. The simethicone contained high concentrations of dissolved nutrients and had a correspondingly high BV of 22. After serial procedures of formula and water dilution, the formula concentration (derived from BV2, BV3, and BV4) became close to the true formula concentration (Table III).

The experimental procedures for patients required an average time that ranged from 15 to 20 minutes. Underestimated GRVs and formula volumes may be due to loss of gastric volume by gastric emptying during the in vivo experimental procedure. In the in vivo validation experiment, there was a higher-than-expected BV (predilution BV1) due to retained simethicone in the stomach, and then an overestimation of the calculated formula concentration and formula volume. Because the water dilution technique is based on the presumption that the total soluble substance should not change before or after water dilution, the stomach-retained soluble substance did not influence the calculation of GRV. For example, GRV could be calculated in the presence of a high concentration of soluble substance, such as retention of a drug (simethicone in the first dilution test) or dietary formula (dietary formula in the second dilution test) in the stomach. This information suggested that retention of drugs or food debris in the stomach would lead to both a higher predilution BV and postdilution BV but would not influence GRV measurement using the mathematical calculation.

GRV is determined by the dynamic balance between input (endogenous saliva plus gastric juice and exogenous formula) and output (gastric emptying) from the stomach. Once the true GRV and formula volume have been calculated, the gastric secretions can be calculated using a mathematical equation (Table I; Equation 4). Application of the gastric contents measurement would be useful for clinical practice, particularly for management of patients receiving enteral tube feeding. For example, in patients who have a high residual volume associated with high food retention, this may be indicative of gastroparesis or delayed gastric emptying. Use of prokinetic agents (ie, metoclopramide or erythromycin) or postpyloric tube feeding may be considered for these patients with delayed gastric emptying. In patients with a high residual volume associated with high gastric secretion, this may suggest that the rate of disappearance of the food was faster than the net rate of disappearance of fluid or increased gastric secretion. Addition of acid-inhibiting agents (ie, H2-blocker or proton pump inhibitor) may inhibit the gastric secretion, with a corresponding decrease in the GRV.

Using the traditional method of aspirated GRVs by a nasogastric tube may lead to artificially decreased values in situations involving small syringes or tube caliber, collapsible silicone material, few ports in the tip of the feeding tube, or when the tip of the feeding tube is either adhered to the gastric mucosa or is not positioned in the pool of gastric juice.25,26 The BV determination samples (1 mL) of gastric contents were thoroughly mixed by using a 60 mL syringe, which was filled and emptied 3 times. Using the method presented in this paper, we can easily obtain the same composition of the gastric contents in the different parts of the stomach in patients receiving nasogastric tube feeding. The value of the techniques described in this study show that accurate measurement of GRV does not rely on positioning the tube tip in the dependent part of the gastric pool.

We have used the BV measurement and water dilution test to monitor patients receiving bolus and continuous enteral feeding. To monitor gastric emptying for patients receiving bolus nasogastric feeding, polymeric diet was administered into the stomach and then the BV of gastric contents measured.16 We found that patients with feeding intolerance had significantly higher BVs, calculated GRVs, and formula volumes than controls. To monitor patients receiving continuous nasogastric feeding, we designed a clinical trial by the water dilution technique and BV measurement.24 We found that patients with feeding intolerance showed a significantly higher formula volume remaining in the stomach. Traditional GRVs obtained by nasogastric tube aspiration in patients receiving continuous enteral feeding failed to differentiate those patients with potential emptying problems from those with sufficient gastric emptying. These results suggest that patients with feeding intolerance can be readily monitored by use of a refractometer and measurement of the BV of gastric contents.

Measurement of the BV of gastric contents is simple, reproducible, and inexpensive. Additionally, little training is required, and the amount of sample solution required per test is only 1 mL. Refractometry and simple mathematical equations may be used to calculate the formula concentration, GRV, and formula volume, as well as to serve as a tool with which to monitor the gastric contents of patients receiving nasogastric tube feeding.

We would like to express our sincere thanks to the Tri-Service General Hospital (TSGH-C94–39) and the National Science Council (NSC-94–2320-B-016–034) for their grants to support to the study.

Received for publication April 14, 2006. Accepted for publication August 15, 2006.

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Journal of Parenteral and Enteral Nutrition, Vol. 31, No. 1, 63-68 (2007)
DOI: 10.1177/014860710703100163
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