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Original Research

Weight-Based Estimation of Insertion Length of the Nasogastric Tube in Extremely Low Birth-Weight Infants

Kato, Yuta RN; Hirata, Katsuya MD; Oshima, Yukari RN; Wada, Kazuko MD

Editor(s): Dowling, Donna PhD, RN, Section Editors; Schierholz, Elizabeth MSN, NNP, Section Editors

Author Information
doi: 10.1097/ANC.0000000000000692
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Abstract

Nasogastric (NG) tubes are used in the neonatal intensive care unit (NICU) for various indications such as routes for enteral feeding or medications and gastric decompressions. Thus, NG tubes are routinely inserted in extremely low birth-weight (ELBW) infants for long periods, from just after birth till near-corrected term age. NG tube placement error may occur frequently in neonates,1,2 and misplaced NG tubes may cause various adverse effects, including aspiration (owing to feeding via a misplaced tube in the esophagus), malabsorption (owing to feeding via a misplaced tube in the duodenum), and perforation or injury of the esophagus and stomach, especially in premature populations.3–5 Furthermore, incorrectly placed tubes can be associated with vasovagal responses of apnea and bradycardia. For these reasons, correct positioning of NG tubes within the body of the stomach, below the gastroesophageal junction, is important.

Radiograph confirmation is considered the gold standard to verify the NG tube location.6,7 However, cumulative radiation exposure in pediatric patients is a concern. The commonly used estimation methods for the insertion length of NG tubes are based on the direct distances of the nose-ear-xiphoid (NEX) and from the nose to the earlobe to a point halfway between the xiphoid process and umbilicus (NEMU). Regression equations use height according to age groups (age-related, height-based [ARHB]).8–10 Accumulative current evidence shows that ARHB and NEMU are the superior NG tube insertion length predictors compared with NEX in pediatric populations.9,10 One study proposed a weight-based estimation of NG tube insertion length in neonates,11 and its usefulness was prospectively confirmed by the same group.12 However, evidence of the best practice for estimating NG tube insertion length in more premature ELBW infants is limited.

This study aimed to determine a weight- and height-based estimation formula of NG tube insertion length in low birth-weight (LBW) infants, including ELBW infants.

What This Study Adds

  • We developed a radiography-verified weight-based estimation formula for NG tube length in infants whose weights were less than 1000 g.
  • An objective weight-based formula of NG tube length estimation together with the NEMU method may improve the accuracy of the NG tube insertion procedure.

METHODS

LBW infants (birth weight <2500 g) who were admitted to the NICU of Osaka Women's and Children's Hospital between May 2009 and May 2010 and who required radiography for clinical reasons were prospectively included. Infants with congenital diseases (esophageal atresia, congenital diaphragmatic hernia, gastroschisis, etc), hydrops fetalis, cleft lip and palate, and severe intraventricular hemorrhage were excluded.

The NG tube was inserted by bedside nursing staff for clinical indications according to the unit policy, and the tubes were replaced every week. The tube insertion length was initially determined using NEMU methods and occasionally adjusted based on radiographs or a previously inserted length. Aspirate pH testing and gastric auscultation verified the tube position in the stomach. The NG tube position was checked by the medical physician requesting the radiograph. One consultant neonatologist reviewed all radiographs of the participants whose current body weights (BWs) were less than 2500 g and assessed the NG tube position and appropriate ideal insertion length of the NG tube adjusted based on radiographs, and the infant's current weight (within 1 day) and height (within 1 week) were documented. In the present study, infants' current weight was used for calculations whether or not infants surpassed birth weight. The ideal NG tube position was defined as the tip of the tube at the center of the body of the stomach, which passed the gastroesophageal junction, not reaching the greater curvature (Figure 1).

FIGURE 1
FIGURE 1:
Examples of the patient's radiographs. Actual and ideal NG tube lengths calculated on the radiographs are shown. NG indicates nasogastric; △, tip of the NG tube.

This study was approved by the institutional ethics committee, and written consent was obtained from the parents. A regression model was used for the NG tube insertion length with respect to current weight (within 1 day) and height (within 1 week). For predicting NG tube length based on the current infant weight, 2 different weight-based formulas were used, less than 1000 or more than 1000 g, based on our preliminary analysis. Data management and statistical analyses were performed using R statistical software version 3.5.2 (R Foundation for Statistical Computing, Vienna, Austria). All the reported P values are 2-sided; P values < .05 were considered statistically significant.

RESULTS

Overall, 533 radiographs (152 patients) were available for evaluation during the study period. The median gestational age and birth weight of the 152 patients were 32.6 weeks (22.5-39.6 weeks) and 1478 g (422-2486 g), respectively. All the patients included in the study were Japanese. The median postnatal day to taking radiographs was 6 (0-106). The median number of radiographs per patient was 2 (1-20). Among the radiographs obtained, 246 were taken at weights less than 1000 g and 287 at weights more than 1000 g. Of all the radiographs, 157 had corresponding height values measured within 1 week.

A total of 246 (<1000 g) and 287 pairs (>1000 g) of radiographs and weights and a total of 157 pairs of radiographs and heights were analyzed. Using linear regression analysis of the ideal insertion length compared with weight or height, formulas that would predict the appropriate NG tube insertion length were derived (Figures 2 and 3).

FIGURE 2
FIGURE 2:
NG tube length and weight. Comparison of the ideal NG tube length and weight with regression lines. NG indicates nasogastric.
FIGURE 3
FIGURE 3:
NG tube length and height. Comparison of the ideal NG tube length and height with a regression line. NG indicates nasogastric.
  • <1.0 kg: 5.3 (±0.28) × BW (kg) + 9.89 (±0.20), R2 = 0.768 (0.712-0.815; P < .001)
  • 1.0 kg < BW < 2.5 kg: 3.0 (±0.12) × BW (kg) + 12.3 (±0.20), R2 = 0.828 (0.788-0.861; P < .001)
  • Height (cm) × 0.33 (±0.02) + 3.51 (±0.56), R2 = 0.869 (0.825-0.903; P < .001)

To simplify for ease of clinical application, the values were rounded. The adjusted formulas for estimating the NG tube length (cm) of LBW infants were as follows:

Weight-based formula:

  • 5 × weight (kg) + 10 (BW < 1.0 kg)
  • 3 × weight (kg) + 12.5 (1.0 < BW < 2.5 kg)

Height-based formula:

  • Height (cm)/3 + 3.5

Using the adjusted formulas, 82.3% (439/533) in the weight-based formulas and 66.9% (103/157) in the height-based formulas were within 1.0 cm of the ideal position in the stomach. Table 1 shows examples of the calculations of NG tube insertion length for each weight.

TABLE 1. - Examples of the Calculations of Nasogastric Tube Insertion Length for Each Weight
Weight, kg Insertion Length, cm
0.4 12
0.6 13
0.8 14
1.0 15
1.5 17
2.0 18.5
2.5 20

DISCUSSION

Various methods are used to estimate NG tube insertion length in LBW infants.13 Among them, the NEMU method is commonly used for the initial estimation of the NG tube length.9,10 However, this procedure may cause concern of reproducibility among healthcare providers who insert tubes. To overcome this, we evaluated the usefulness of the more objective weight- or height-based estimation, which could reinforce the accuracy of the insertion of the NG tube in LBW infants.

This study evaluated 533 radiographs and determined a weight- and height-based estimation formula of the NG tube length in LBW infants, including ELBW infants. In our analysis, the weight-based formula was superior to the height-based formula in estimating the correct NG tube length in the LBW population. Furthermore, because height is not routinely measured and weight is a parameter regularly measured in the NICU, using a weight-based formula in the NICU is convenient and preferable.

Freeman et al11 evaluated radiographs of 138 orogastric tubes and 80 NG tubes in 87 infants who weighed 397 to 4131 g and reported a weight-based formula for estimating gastric tube insertion length in newborns. They proposed the formulas of tube insertion length in centimeters as follows: orogastric = (3 × weight [kg] + 12) and NG = (3 × weight [kg] + 13).11 Our results were quite similar for estimating the NG tube insertion length when the infant weight was more than 1000 g. However, in the population whose weights were less than 1000 g, using the same formula of Freeman et al or the formula derived from infants weighing more than 1000 g in the present study, the estimated insertion length would be too long for the actual ideal length verified on chest radiographs.

The present study proposed 2 different weight-based formulas for predicting NG tube length based on the current infant weight, namely less than 1000 or more than 1000 g. This discrepancy may be explained by the difference in proportion during fetal growth. The femur length-to-head circumference and femur length-to-biparietal diameter ratios steeply increase as gestational age increases.14 The larger proportion of the head in more premature infants is a possible explanation of the relatively shorter NG tube length required in infants with weights less than 1000 g.

This study had several limitations. As this was a single-centered study, the reproducibility of the formula could be of concern. In addition, whether the result could be adapted to other races remained uncertain. Additionally, we did not evaluate the usefulness of the NEMU method in the present study because the NG tube length insertion was conducted clinically: initially with the NEMU method, but when the tubes were replaced, the previous length or previous radiographs were used as reference. Furthermore, infants' body proportions were not considered in the study. The strengths of the study were the relatively large number of patients with valid radiographs, especially including a large number of ELBW infants.

The value of the work was the use of a weight-based estimation for NG tube insertion length, as weight is documented daily and is a more reliable measure when compared with length. In addition to previous studies, for the estimation of NG tube length in infants weighing less than 1000 g, a newly developed weight-based regression formula may be used, although the formula needs to be validated in another sample of premature infants. We propose the application of a weight-based formula for estimating NG tube length derived from the present study, in combination with the NEMU method to improve the accuracy of the NG tube insertion procedure.

References

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Keywords:

enteral nutrition; extremely low birth-weight infant; feeding tube; nasogastric; neonatal; neonate

© 2019 by the National Association of Neonatal Nurses.