Traditionally, overnight fasting was recommended before surgery to reduce the volume and acidity of gastric contents and lower the risk of regurgitation/aspiration during surgery.1,2 However, recent guidelines are more relaxed and permit restricted fluid intake up to a few hours before surgery.3 Optimal perioperative fluid management is also an important component of Enhanced Recovery after Surgery (ERAS) pathways.4,5 These protocols recommend shortening the fasting period as well as encourage ingesting a clear carbohydrate drink 2 to 3 hours before surgery to prevent preoperative dehydration and improve patient outcomes.3,4,6–8
Typical preoperative clear carbohydrate drinks contain carbohydrates at concentrations exceeding 10% and are aimed to stimulate insulin sensitivity and reduce postoperative insulin resistance.9–11 Oral rehydration solution (ORS), originally used to treat children with diarrhea, has a low carbohydrate content (1.35%–3.3% glucose).12,13 The salt/sugar balance in ORS formulations favors rapid intestinal absorption,14–17 and ORS formulations have therefore also been used as a preoperative clear fluid. For example, OS-1 (Otsuka Pharmaceutical Factory, Japan) is popularly used in Japan for preoperative and perioperative fluid and electrolyte management, replacing intravenous fluids in low-risk patients.12,13,18,19
Perioperative fluid and electrolyte management is particularly challenging in obese patients because their gastrointestinal function is considered to be different from that in nonobese patients.20 In particular, several studies have shown that gastric emptying is delayed in obese subjects.6,7,21,22 However, the appropriate fasting period and preoperative fluid protocol have not been clearly defined for obese patients. For example, the American Society of Anesthesiologists guidelines do not mention the safety for fasting before anesthesia in obese patients.6 Although the European Society of Anaesthesiology guidelines state that obese patients can safely follow the guidelines for nonobese patients, the evidence level is a 2 and the recommendation grade is a D.7
The aim of the present study is to investigate the gastric emptying of the proprietary ORS formulation (OS-1) in morbidly obese subjects after standard overnight fasting. We compare the change in gastric fluid volume (GFV) measured using magnetic resonance imaging (MRI) over 2 hours after OS-1 ingestion in morbidly obese subjects and nonobese controls. We hypothesized that GFV would be higher in the morbidly obese subjects compared with controls both after fasting and after subsequent ingestion of OS-1.
Setting and Ethics
The protocol for this study was approved by the ethics committee of the Yotsuya Medical Cube, Tokyo (institutional review board [IRB] No. YMCANES2013-1), and it was conducted in accordance with the Declaration of the Helsinki-Ethical Principle for Medical Research Involving Human Subjects and with the Ethical Guidelines for Clinical Research issued by the Ministry of Health, Labour and Welfare in Japan. Informed written consent was obtained from all participants before enrollment. This article adheres to the applicable EQUATOR guidelines.
The morbidly obese group consisted of 10 morbidly obese adult subjects (5 men and 5 women, with a body mass index [BMI] ≥35) chosen from the Yotsuya Medical Cube. The control group consisted of nonobese adult healthy volunteers (5 men and 5 women, with a BMI between 19 and 25). All subjects were suitable for MRI scanning (eg, no metal implants in the body) and consented to participate in the study. Subjects with diabetes mellitus or gastroesophageal reflux that required medical treatment were excluded. This was a preliminary study conducted to gain insight into the GFV change in morbidly obese subjects. Therefore, only 10 subjects per group were recruited and examined.
Subjects were asked to fast for 9 hours and scanned before and after drinking OS-1 to evaluate GFV changes. Subjects were instructed to not have any food or drink after 12:00 am (midnight) on the day of the examination. Height and weight were measured at 8:30 am, and the examination was started at 9:00 am. The first MRI scan was performed before ingestion (preingestion) to measure the residual GFV. Subjects were then asked to drink 500 mL of OS-1 in the sitting position within 3 minutes. After the ingestion, MRI scans were performed at 0 minutes (just after the end of ingestion) and at 30, 60, 90, and 120 minutes after ingestion.
Gastric Emptying Measurements With MRI
MRI scans were performed using a Signa HDxt1.5T Ver23 scanner (GE Healthcare, Japan) at the Yotsuya Medical Cube in Tokyo. Transaxial scans of the upper abdomen using heavy T2-weighted MRI parameters were acquired, and the gastric liquid area on each slice was identified by a distinct positive contrast against the surrounding tissues and was outlined. GFV was determined by multiplying the sum of these areas by the slice thickness (Figure 1).23 Data acquisition and image analysis were performed by a trained radiology technician blinded to the study design.
Data are presented as mean ± SD for continuous variables and as frequency for categorical variables. GFV values were compared between morbidly obese and control groups and also between preingestion and postingestion time points. Data were tested for normality, and the difference between groups was tested using unpaired Student t tests without assuming homoscedasticity and Fisher exact tests, as appropriate. GFV differences at different time points were compared using paired t tests and 1-sided t tests. Correlations between residual GFV and body weight (BW) or BMI were tested across all subjects using Pearson correlation testing with 95% confidence limits. Statistical significance was considered as P < .05. Statistical analyses and graph plotting were performed with Microsoft Excel 2013 and R version 18.104.22.168 Because of the preliminary nature of this research, correction for multiple testing across time points was not performed.
The demographics of the 2 groups are shown in Table 1. Significant differences were found between the groups in body weight (BW) and BMI (P < .001). In the morbidly obese group, there were 3 cases of hypertension and 7 cases of sleep apnea syndrome reported as comorbidities. All subjects completed the study, and there were no apparent side effects or complications in any of the subjects.
Residual GFV at preingestion was significantly higher in the morbidly obese group (73 ± 30.8 mL) than in the control group (31 ± 19.9 mL, P = .001; Figure 2, Table 2). Drinking 500 mL of OS-1 significantly increased GFV in both groups (0 minutes), with GFV reaching significantly higher levels in the morbidly obese group (561 ± 30.8 mL) than in the control group (486 ± 42.8 mL, P < .001; Figure 2, Table 2). GFV declined rapidly in both groups and reached fasting baseline levels by 120 minutes (50 ± 29.5 mL in the morbidly obese group and 30 ± 11.6 mL in the control group); GFV at 120 minutes was significantly higher in obese subjects compared with controls (P = .038). Furthermore, although there was no significant difference in GFV between preingestion and 120 minutes in the control group (P = .891), GFV was significantly lower at 120 minutes than it was at preingestion in the morbidly obese group (P = .015). Furthermore, correlation analyses showed a significant correlation between residual GFV and body weight (r = .66, 95% confidence interval, 0.31–0.85, P = .001), and between residual GFV and BMI (r = .60, 95% confidence interval, 0.21–0.82, P = .005).
Residual GFV in the Morbidly Obese
As hypothesized, morbidly obese subjects had a higher residual GFV compared with normal subjects after 9 hours of fasting. Fasting residual GFV showed a wide variation in the study groups (coefficient of variation, 42% in obese and 64% in normal subjects). We also found a significant linear correlation between residual GFV and BW across our study population. These results confirm previous reports that have also observed a higher GFV in obese subjects and have postulated that the larger stomach size in obese subjects accounts for the higher GFV.20,25–28 Further analysis of our results after normalizing GFV to BW showed that there were no significant differences in GFV/BW values between morbidly obese subjects and controls (morbidly obese, 0.57 ± 0.25; controls, 0.52 ± 0.33; P = .68).
Gastric Emptying in the Morbidly Obese
As hypothesized, morbidly obese subjects had a higher GFV compared with normal subjects immediately after OS-1 ingestion. However, our results also showed that gastric emptying in morbidly obese subjects was not delayed and reached fasting baseline values by 120 minutes, similar to the nonobese controls. Our observations fail to support any speculation that decreased gastric emptying is a concern in obese subjects to a greater extent than that for nonobese subjects. Indeed, some studies have previously reported the gastric emptying rate to be similar in obese and nonobese subjects.29,30 Notably, GFV at 120 minutes was observed to be significantly lower than the residual GFV (preingestion) after 9 hours of fasting in morbidly obese subjects. Although the clinical significance of this reduction is not clear, we speculate that gastric secretions accumulating during the 9-hour fast contributed to the residual GFV, and these secretions were emptied along with the OS-1 and had not fully accumulated at 120 minutes after OS-1 ingestion.
Gastric Fluid Volume Measurement
Gastric emptying has been measured and evaluated by several methods, including gastric fluid aspiration via gastric tube,31,32 scintigraphy,33,34 ultrasonography,28,29,35,36 and MRI.23,37 An MRI-based technique was used in this study because it offers the most accurate method to evaluate gastric fluid volume and also overcomes limitations associated with the other measurement techniques.23,37 However, MRI techniques have drawbacks, including being potentially uncomfortable for participants and requiring a trained radiological technician to perform data acquisition and image analysis. In view of these considerations, we limited the study size to 10 volunteers per group in this preliminary study, in accordance with previously published studies.29,38,39 In the absence of a priori power analyses, we performed a post hoc power analysis using a significance level of .05 and an effect size based on Cohen’s d (Table 2). We observed that the study size of n = 10 per group was able to detect significant differences between the morbidly obese and nonobese groups at a power of 0.93 (preingestion GFV) and 0.99 (0 hours of GFV).
The current study was not without limitations. The study was conducted only on morbidly obese Japanese subjects (BMI, 46.3 ± 6.4) and used a specific ORS formulation (OS-1) that is popularly used in Japan.12,13,18,19,39 OS-1 is purported to contain a balanced concentration of 2.5% carbohydrates (1.8% of glucose) and electrolytes, and it reduces insulin resistance despite its low carbohydrate content.40 It has been reported to have faster gastric emptying compared with formulations containing higher carbohydrate content.38–40 The generalizability of the results from this study to other populations or ORS preparations is not known, and caution should be exercised when extrapolating the results to other populations including subjects with higher degrees of obesity, for example, super obese with a BMI >50, or to other ORS preparations or water. Furthermore, all subjects had fasted for 9 hours before OS-1 administration. Residual volumes and gastric emptying characteristics may be different with different fasting periods (eg, 6 hours) and should be taken into consideration during perioperative planning.
This study shows that morbidly obese subjects have a higher residual gastric volume after 9 hours of fasting compared with subjects with a normal BMI. However, gastric emptying after OS-1 ingestion in morbidly obese subjects appears to not differ from subjects with a normal BMI. As in subjects with a normal BMI, GFV reached baseline levels within 2 hours of OS-1 ingestion. Based on the correlation observed between residual gastric volumes and body weight, we speculate that higher gastric volumes in the morbidly obese may only be reflective of their larger body weight. Further studies are required to confirm whether preoperative fasting and fluid management that are recommended for nonobese patients could also be applied to morbidly obese patients.
The study was funded in part by a research grant from Otsuka Pharmaceutical Factory (Tokushima, Japan). Assistance with statistical analysis was provided by Stagen. Editorial support, in the form of medical writing based on authors’ detailed directions, collating author comments, copyediting, fact checking, and referencing, was provided by Cactus Communications.
Name: Toshie Shiraishi, MD, PhD.
Contribution: This author helped design the study and concept, collect and analyze the data, and write the manuscript.
Name: Dai Kurosaki, MD, PhD.
Contribution: This author helped collect and analyze the data.
Name: Mitsuyo Nakamura, MD.
Contribution: This author helped collect the data.
Name: Taiji Yazaki, MD, PhD.
Contribution: This author helped collect the data.
Name: Satomi Kobinata, MD.
Contribution: This author helped collect the data.
Name: Yosuke Seki, MD, PhD.
Contribution: This author helped design the study and concept, and analyze and interpret the data.
Name: Kazunori Kasama, MD.
Contribution: This author helped supervise and interpret the study.
Name: Hideki Taniguchi, MD, PhD.
Contribution: This author helped critically review the manuscript.
This manuscript was handled by: Richard C. Prielipp, MD.
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