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 Table of Contents  
Year : 2019  |  Volume : 17  |  Issue : 3  |  Page : 295-301

Preoperative ultrasound assessment of gastric content in obese patients

1 Department of Anaesthesia and Intensive Care, Faculty of Medicine, Al-Azhar University, Cairo, Egypt
2 Department of Radiodiagnosis, Faculty of Medicine, Al-Azhar University, Cairo, Egypt

Date of Submission27-Jun-2019
Date of Decision22-Aug-2019
Date of Acceptance15-Sep-2019
Date of Web Publication26-Nov-2019

Correspondence Address:
Usama I Abotaleb
214 Faisal Street, Alharam, Giza, 12555
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/AZMJ.AZMJ_90_19

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Background Gastric ultrasound is a simple bedside test that can be used for assessment of patients at risk of aspiration.
Aim To compare gastric volume and content in obese with nonobese patients after 8-h fasting using gastric ultrasonography.
Patients and methods This study was conducted on 100 patients, comprising 50 obese and 50 nonobese, of either sex, having American Society of Anesthesiologists status I–III, with age of at least 21 years, and with no other causes for delayed gastric emptying. Gastric ultrasound was performed by our co-author radiologist. Quantitative and qualitative assessment of antrum, time taken to complete the examination, and number of patients at risk were compared between the two groups. Moreover, we compared residual volume in supine position with right lateral decubitus in the same group.
Results Obese patients had significantly higher gastric volume and total number of patients at higher risk of aspiration; however, both groups were comparable regarding volume/kg. In qualitative assessment, obese patients included lower number of patients with empty stomach and higher number with particulate or solid contents. In both groups, right lateral decubitus volume estimation was higher than supine position volume estimation.
Conclusion A considerable percentage of fasted preoperative obese patients have risky gastric content, and ultrasound scanning is recommended.

Keywords: gastric content, gastric ultrasound, obese, preoperative, residual gastric volume

How to cite this article:
Ramadan OA, Sabra MM, Abotaleb UI, Elfeshaw MS. Preoperative ultrasound assessment of gastric content in obese patients. Al-Azhar Assiut Med J 2019;17:295-301

How to cite this URL:
Ramadan OA, Sabra MM, Abotaleb UI, Elfeshaw MS. Preoperative ultrasound assessment of gastric content in obese patients. Al-Azhar Assiut Med J [serial online] 2019 [cited 2020 Jul 10];17:295-301. Available from: http://www.azmj.eg.net/text.asp?2019/17/3/295/271687

  Introduction Top

Pulmonary aspiration of gastric contents is a major perioperative insult [1] that can lead to serious complications carrying the risk of high morbidity and mortality [2]. Complications may be respiratory or nonrespiratory; respiratory complications include severe pneumonia [2] and acute respiratory distress syndrome, whereas nonrespiratory complications include brain damage and multiple organ dysfunction [3].

The guidelines for healthy adults consider a minimum safe fasting duration of 8 h for meat or fatty meal, 6 h for a light meal, and 2 h for clear fluids [1].

These guidelines help limit the risk; however, they are not applicable or reliable in emergencies, or for patients with certain greater risk of medical conditions [2], for example, obese, pregnant, diabetes mellitus, neuromuscular disease, chronic renal or hepatic disease, and gastroesophageal reflux disease [4]. Some patients also may not follow fasting guidelines owing to difficult communication and mental or psychological diseases [1],[3].

Despite following these guidelines, perioperative pulmonary aspiration occasionally occurs in low-risk fasted healthy patients, and its incidence is ∼1 : 4000 [1],[3]. Furthermore, there is no specific fasting time guidelines for high-risk patients [1].

Although a full stomach is a major risk factor, preoperative assessment of the nature and volume of gastric content is usually based on patient’s history alone [1], and there is a growing interest in an objective bedside tool to assess and limit pulmonary aspiration risk [2],[3]. Gastric scintigraphy is not suitable for everyday use. Simple bedside procedures such as gastric content aspiration are unreliable and uncomfortable if used for awake patients [1].

Gastric ultrasound examination as a simple bedside test is finding a place as an emerging tool for aspiration risk assessment. It is used to answer a well-defined clinical question in a short time to guide patient management and eventually improve patient outcome, which is typically, is the patient’s stomach empty or full? It can detect the nature of the gastric content, that is, empty, clear fluid, or solid, and when fluid is present, its volume can be calculated [1],[2]. It can be used with accurate results in obese patients [5].

Obese individuals are essential to study, because they are a growing percentage of the population. One in every three adults in the USA is obese. Gastric emptying time study in this population is needed, and this is the scope of our study. It is interesting to note that the current literature shows conflicting results. Although some studies reported a prolonged gastric emptying time [6], others reported a shorter or similar gastric emptying time in obese and nonobese patients [5],[7].

This study was conducted to assess gastric volume and content in fasted obese patients using gastric ultrasonography.

Our aim was to compare obese with nonobese patients after 8-h fasting before elective surgery. The primary outcome was to see if there is an increase in residual gastric volume in obese patients. Secondary outcomes were to compare the gastric contents, time taken to complete the examination, and compare the results taken while the patient was in supine position with that taken while the patient was in right lateral decubitus (RLD) in the same group.

Sample size was calculated using G*Power program (Kiel University, Kiel, Germany), depending on our primary outcome (residual gastric volume). In a previous study, the residual gastric volume in patients who were not at risk was 56.99±36.48 ml. We planned our study to detect 33% increase in obese patients’ residual gastric volume, and a power of 80% (1−β), at 5% significance level (α). The minimum sample size to study was 48 patients in each group. We investigated 50 patients in each group.

  Patients and methods Top

After ethics committee approval, this observational cohort study was conducted from 1 August 2018 to 30 November 2018 on 100 patients in Al-Azhar university Hospitals. Inclusion criteria were patients aged at least 21 years, of either sex, with American Society of Anesthesiologists physical status I–III, undergoing elective surgery, had the ability to understand and follow the instructions, with no other causes for delayed gastric emptying, and provided approval for contribution in the study. Exclusion criteria were pregnant women, diabetic patients, patients with neuromuscular disease, patients with gastroesophageal reflux, patients with chronic renal or hepatic diseases, patients with mental and psychological diseases, patients with recent upper gastrointestinal bleed, or patients with upper gastrointestinal surgery. Patients who cannot lie flat or on right lateral side comfortably owing to any reason, for example, bone fracture, were also excluded.

Our 100 patients comprised 50 obese and 50 nonobese individuals. Patients were assigned into one of two equal groups, depending on BMI. Obese group included patients who had BMI of more than 30, and nonobese group included patients who had BMI of up to 30.

The night before surgery, patients were reviewed for inclusion criteria, informed about the study, instructed about fasting protocol, and signed the consent. All patients were instructed to fast for 8 h after integrated light dinner and 500 ml clear fluid; the actual number of preoperative fasting hours was recorded.

Gastric ultrasound assessment was performed in the presurgical waiting area before administration of any premedication. Assessment was performed by our co-author radiologist.

All ultrasound examinations were done with SonoSite M-Turbo (Fujifilm SonoSite, Inc., Bothell, Washingtton, USA) portable ultrasound machine. Low-frequency (2–5 MHz) curvilinear probe was used in abdominal scan mode settings to identify gastric antrum.

Ultrasound assessment was firstly done in the supine and then in RLD position [4]. The epigastrium was imaged in a sagittal plane moving the transducer from the left to right subcostal margins ([Figure 1]). Gentle probe tilting, sliding, and rotation were used to detect the antrum and to optimize the image; excessive probe rotation in the oblique views may lead to overestimation of antral size [2],[3]. Two ways were used in our study; the antrum was detected first, and then the liver was identified at a cephalad position to it, aortic pulsation posteriorly, and colon in the posterior and lateral quadrant, or liver and aorta/superior mesenteric artery were identified first and act as important landmarks ([Figure 2]), and then the antrum was identified just below the left lobe of liver and anterior to the aorta and inferior vena cava [4].
Figure 1 Moving the transducer in a sagittal plane.

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Figure 2 Solid contents in the antrum.

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An empty stomach ([Figure 3]) appears flattened and collapsed (bull’s eye appearance) [8]. Liquid is seen as distended antrum with hypoechoic content ([Figure 4]), liquid and air as a starry sky appearance, and solids as distended antrum with hyperechoic or heterogeneous frosted glass with blurring of the posterior wall ([Figure 2]) [2],[5].
Figure 3 An empty stomach.

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Figure 4 Liquid in the antrum.

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A standardized plane at the level of the aorta was used [3]. An image of the antrum was then taken between peristaltic contractions while the antrum was at rest. An ellipse was drawn to include the visualized gastric antrum circumferentially, including the serosa [4]. Cross-sectional area (CSA) was measured using the free-hand tracing tool.

Three readings were taken, and then the average CSA was recorded. CSA of the antrum was applied in the formula described by Perlas et al. [9] to calculate the estimated gastric volume. The validity of this formula in obese patients has been proved [4]:

Volume (ml)=27.0+14.6×RLD CSA−1.28×age [9].

RLD CSA: cross-sectional area of the antrum in the RLD.

In qualitative assessment, antral content was classified into empty, clear fluid, or particulate/solid [4].

Qualitative assessment of gastric contents, quantitative calculation of residual gastric volume, volume/kg, and time taken to complete the examination were recorded and compared between the two groups. Total number and percent of patients at higher risk were calculated and compared. Moreover, we compared the CSA of antrum in supine position with RLD in the same group.

Time taken to complete the examination was calculated from the start of patient preparation till the end of the examination. Patients at higher risk of aspiration were defined as any patient who had particulate or solid contents on qualitative assessment and/or more than 100-ml residual gastric volume on quantitative assessment [4].

Statistical analysis

SPSS version 17 program (SPSS Inc., Chicago, USA) was used to enter data and for statistical analysis. Data were presented as mean±SD, range, number, and percent. For parametric data, comparison between the two groups was performed using unpaired t-tests, and paired t-test was used for comparisons within the same group. For data collected as proportions, Fisher exact or χ2-test was performed. A P value less than 0.05 was considered statistically significant.

  Results Top

Both groups were comparable with respect to age, height, sex, fasting hours, and time to complete the examination ([Table 1]).
Table 1 Patients’ characteristics

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In quantitative assessment, there was a statistically significant higher gastric volume in obese patients; however, there was no significant difference when we compared volume/kg. Five patients had gastric volume more than 1.5 ml/kg, and three of them were nonobese. In qualitative assessment, there were statistically significant differences with respect to lower number of patients with empty stomach and higher number with particulate or solid contents in obese patients. Total number of patients at higher risk for aspiration was higher in obese patients ([Table 2]).
Table 2 Qualitative and quantitative assessment

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In both groups, when comparing gastric volume in RLD and supine position, it was higher in RLD estimation ([Table 3]). However, this difference was 12.61±6.84 ml in obese group and 11.07±9.02 ml in nonobese group, with maximum difference of 32.41 ml in our study.
Table 3 Comparison between gastric volume in supine and right lateral decubitus positions

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  Discussion Top

In our anesthetic practice, patients are forwarded to operation room after fasting period and then proceed with anesthesia, supposing that complete gastric emptying has occurred. It is wise to modify this practice, especially in patients with higher risk for aspiration [4]. Our question is that, is the fasting duration valid in predicting gastric emptying in obese patients.

Ultrasonography has become an essential tool in the advancement of anesthetic practice and is considered as a point of standard care [10]. Ultrasound assessment of gastric contents is a relatively recent addition to this practice, which has its surge only in the past decade. It is fast and reliable noninvasive tool that can be used for risk assessment and clinical decision making before anesthesia. Moreover, mastering this skill is easy and rapid [4].

Obese patients are of particular interest because they are considered to be at higher risk of aspiration, difficult to ventilate, and/or difficult to intubate. Although the increased adiposity of visceral tissues and greater depth of the stomach (may be≥7 cm) can make the examination more difficult, gastric ultrasound is achievable in 95% of severely obese individuals [3]. Kruisselbrink et al. [5] evaluated the validity of gastric ultrasound and the mathematical model we used for calculation of gastric volume in obese subjects. We built our study to compare gastric volume and content in obese with nonobese patients after 8 h of fasting. According to the WHO guidelines [4], any patient with a BMI of more than 30 kg/m2 was considered obese in this study.

In this study, there were more patients at risk for aspiration and higher gastric volume in obese patients; however, both groups were comparable regarding volume/kg. In both groups, RLD estimation of gastric volume was higher than supine position estimation.

Considering that ultrasound is not used by anesthesiologist for gastric content assessment in our institute, all examinations were done by our co-author radiologist, whereas the anesthesia physicians took only the opportunity to train. Time to complete the examination was accepted, and the technique can be done even in most emergency surgical operations.

We used curved array low-frequency probe (2–5 MHz) with abdominal pre-sets because it is the most suitable for gastric ultrasound in adults. It provides adequate penetration to identify the related anatomical landmarks. Only the thick muscularis propria is reliably observed, along with the characteristic location of the antrum, allowing differentiation of the stomach from other thinner parts of the gastrointestinal tract with less prominent smooth muscle layer [3].

However, a linear high-frequency probe (7–12 MHz) can be used in pediatric patients less than 40 kg. Antrum location is more superficial, so a linear high-frequency transducer would provide better resolution and improved antral tissue layer differentiation [3],[11].

The antrum was identified in our study; it is particularly agreeable to ultrasound examination owing to its superficial and constant location in the epigastric area with a satisfactory soft tissue acoustic window through the left lobe of the liver. Most importantly, an evaluation of the antrum provides accurate reflection about the content in the entire stomach [3].

Perlas et al. [12] studied swallowed volume and CSA in different parts of the stomach: fundus, antrum, and body. They described the sonographic appearance, determined the CSA of each part, and determined if there is a numerical correlation between volume and CSA in different parts of the stomach. Their findings supported the use of antrum as a marker of intragastric content. Since then, many studies have used antrum for the assessment of intragastric contents [2, 4, 5, 8–10,13–17]. Fundus is more difficult to scan with ultrasound, and gastric body frequently has more air content, which may interfere with the examination [3].

The antrum may appear fuller in the RLD. This apparent increase is probably owing to the effect of gravity [2]. It appears to be a closer fit to linear relationship between volume and CSA with RLD position measurement. We scanned all patients in both the supine and RLD. Although, the antral volume was more while the patient was in RLD, the difference was accepted with maximum difference of 32.41 ml in our study.

Perlas et al. [3] considered semi-sitting position (head elevated 45) is the acceptable second choice if examination in the RLD position is not possible (e.g. trauma), and the supine position is the last.

After determination of the CSA of the antrum based on gastric ultrasound image, numerous mathematical models have been developed to describe the numerical relationship between CSA and gastric volume [3],[12],[13],[14].

The formula we used has been validated against endoscopically guided gastric suctioning and was found to provide a reasonably accurate estimate [9].

There have been variable opinions on the amount of gastric volume that constitutes risk for aspiration [4]. If the volume is less than 100 ml in adults, it is generally safe or at low risk for aspiration. These specific cutoff values for gastric volume were derived from the maximal residual gastric volumes observed in adequately fasted patients without any coexisting risk factors for aspiration [1]. Kruisselbrink et al. [5] considered a volume of 0–150 ml is safe.

Some studies estimated volume/kg; earlier studies considered a volume of more than 0.8 ml/kg as high risk for aspiration [17]. However, later studies stated that a residual gastric volume of more than 1.5 ml/kg puts a patient at high risk for aspiration [3],[4],[15],[18].

In this study, volume/kg was not higher in obese, and in all patients, only five had more than 1.5 ml/kg; two of them were obese. This was in agreement with the results of Kruisselbrink et al. [5]. Gastric volume of 1.5 ml/kg in obese is very high, and it was considered by some authors for average weight patients [3],[14]. However, Sharma et al. [4] reported 1.9 ml/kg average gastric volume, and there was a positive correlation between BMI and gastric volume in their study.

In this study, there are some patients who had delayed gastric emptying in the normal nonobese group after 8 h of fasting. Previous studies have reported full stomach in normal population presented for elective surgery, with an incidence range of 3.5–6.2% [3],[16]. In obese patients, the incidence is higher and our results (24%) are comparable with Sharma et al [4] (28.04%), but their study included patients with multiple risk factors.Numerous recent editorials in major anesthesiology journals have called for greater implementation and teaching of gastric ultrasound in anesthesia practice as a daily practice, to be included in the ultrasound curriculum for anesthesiologists, and to be of great importance in obstetric anesthesia [3],[19],[20],[21].

Sharma et al. [4] recommended routine gastric ultrasound screening of all patients and additional measures taken for patients who are at risk. Up till now, we cannot routinely use ultrasound to assess residual gastric content in all patients, as routine gastric ultrasound in patients with low possibility of a full stomach is controversial [3],[16]. However, we recommend its use in obese patients because aspiration risk is unclear. Ultrasound confirmation of solid content or a high volume of fluid suggests a higher risk for aspiration. This would indicate airway protection from aspiration with endotracheal intubation, not to use supraglottic devices, and possibly a rapid sequence induction of anesthesia [3].

Our findings arouse two worrying questions: (a) why did some patients have risky gastric content in normal population? (Is there are any special risk factors?), and (b) for obese patients, can we recommend more fasting time (although this may decrease the gastric pH<2.5) [2], or will clear fluid consumption 2 h before anesthesia solve the problem? [1]

There are some limitations of our study. It was not randomized because groups were based on BMI, and also it was not blinded. However, this model has high reliability and low variability between observers [3],[22], as all measurements were taken by the radiology physician, and CSA was calculated by the machine.

  Conclusion Top

A considerable percentage of preoperative obese patients have particulate gastric content and/or high gastric volume after 8 h fasting, and gastric ultrasound scanning is recommended.

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Conflicts of interest

There are no conflicts of interest.

  References Top

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  [Table 1], [Table 2], [Table 3]


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