Pulmonary aspiration of gastric contents is one of the most feared complications of anaesthesia, as it was the first cause of mortality related to general anaesthesia in France in 1999 . Greater understanding of the pathophysiology of this complication has allowed reduction of its incidence in recent years. Prevention of aspiration depends on preoperative fasting as well as on identification of patients at risk and on the choice of an appropriate and well tolerated anaesthetic protocol whenever predisposing factors are identified.
Preoperative fasting guidelines were proposed and thereafter updated in 2005, leading to liberalization of fasting guidelines, which apply in many situations in anaesthetic practice such as elective surgery, pregnant women not in labour and children [2–4]. However, these recommendations do not apply to emergency surgery, as it is well known that acute pain significantly decreases gastric emptying, leading to increased gastric contents and, hence, increased risk of aspiration . Volume and caloric value of the last ingested meal, chronology of the events – particularly the time between the last meal, time of the accident and induction of anaesthesia – medications administered and the level of pain the patient is experiencing are many factors that do not allow us to know whether the stomach is full or empty at induction of emergency anaesthesia, even many hours after the last meal. Therefore, in this context, it would be of interest to be able to perform a bedside, simple, safe and reliable noninvasive examination of the gastric contents and volume prior to induction of anaesthesia in order to contribute to the assessment of the risk of pulmonary aspiration of the patient.
Real-time ultrasonographic measurement of the cross-section antral area is currently used for the assessment of gastric emptying in diabetic or dyspeptic patients and for clinical investigations in obstetrical anaesthesia [6–12]. In fact, this technique is a valid alternative to scintigraphy for measurement of gastric emptying [6,13–15], with low interobserver and intraobserver variability . Comparison of the ultrasound method with a radiographic contrast study of the stomach proved that ultrasound was reliable in establishing both the presence and absence of food .
Previous studies [7,15] using repeated measurements of antral cross-sectional area for the assessment of gastric emptying in volunteers recorded significant variability among participants as regards the values of antral area, mainly during the postprandial period. This correlates with ultrasonographic measurements of antral volumes performed before and after ingestion of solid or liquid meals in volunteers, showing wide interparticipant variation after ingestion of a meal, whereas interparticipant variability was more narrow during a fasting period . Nevertheless, a recent preliminary study  performed in 36 volunteers reported a close-to-linear relationship between measured antral area and volume of ingested water up to 300 ml.
The aim of this preliminary crossover blinded study was to assess whether a single standardized real-time ultrasonographic measurement of gastric antral cross-sectional area would be able to discriminate between a fasting stomach, as defined by the gastric contents after 12 h fasting, and a full stomach, as observed 2 h after ingestion of liquid or a solid meal in healthy volunteers.
Volunteers provided institutionally approved informed consent to participate in this crossover study. Exclusion criteria from the study were upper gastrointestinal tract diseases, history of diabetes mellitus, chronic renal failure, hypothyroidism, pregnancy, neurological disease, BMI above 30 kg m−2, opioid medication and use of medication affecting gastric motility.
In this crossover study, ultrasonographic measurement of the antral cross-section area was performed by one ultrasonographer who was blinded to the status (fasting or not) of the volunteer, in the morning, in three situations whose order was randomized: after 12 h fasting, 2 h after a solid meal of 300 kcal (two slices of white bread with 10 g butter and 10 g jam) and 2 h after ingestion of 200 ml of nonclear liquids (milk or fruit juice with pulp). Concealment of the order of the sessions was ensured by the use of coded, sealed opaques envelopes. All participants were studied in a semi-upright position with the head of the bed elevated to 45°  and with the ultrasound transducer applied with minimal abdominal compression.
Real-time ultrasonography (GE Healthcare, Piscataway, New Jersey, USA; Logiq e, fitted with a 2–5.5 MHz probe) was used to measure the diameters of the gastric antrum in the sagittal plane passing through the aorta, as previously described [6,7]. Longitudinal (D1) and anteroposterior (D2) diameters of a single section of the gastric antrum were determined, using the abdominal aorta and the left lobe of the liver as internal landmarks to obtain the same standardized scanning level consistently. At each observation, three measurements were done using the mean values of the longitudinal and anteroposterior diameters to calculate the antral area in order to improve the accuracy of the calculation of the outcome variable of this study, that is, antral area, by reducing confusing factors due to the movements of the volunteer and peristaltic activity, as previously established [7,15,17,18]. Measurements were always taken from the outer profile of the wall because the lumen of the antrum is usually very narrow in fasting patients and the inner side of the wall is difficult to outline. As the cross-section of the gastric antrum presents an elliptical shape, its area was calculated in all participants using the following formula:
where D1 and D2 are the longitudinal and anteroposterior diameters, respectively.
The measurements of the gastric antrum were obtained between antral contractions to provide a measure of the relaxed width of the antrum.
Data were analysed using the Statistical Package for Social Science version 12.0 (SPSS Inc., Chicago, Illinois, USA).
Median antral areas were compared for the three sessions using the Friedman test, followed by post-hoc Wilcoxon tests with Bonferroni adjustments for multiple comparisons when appropriate . The level of significance was set at a P value of less than 0.05.
To estimate the discriminating power of ultrasonographic measurement of the antral area for the diagnosis of gastric vacuity, as defined by gastric contents after 12 h fasting in healthy volunteers, receiver operating characteristic (ROC) curves were plotted for fasting vs. nonfasting data, fasting vs. liquid meal data and fasting vs. solid meal data. The areas under the ROC curves were calculated.
Twenty-two volunteers were included. Patient characteristics are presented in Table 1. Sixty-five measures were performed: 22 after 12 h fasting, 22 after a solid meal and 21 after a liquid meal. For one volunteer, 2 h after ingestion of liquids, reliable measurement of the antral area was not possible because identification of the antral section by the ultrasonographer was difficult. Figure 1 shows an example of measurement of the antral area performed after 12 h fasting and 2 h after a solid meal.
Median, interquartile range and minimum–maximum values of the antral area in the three sessions are listed in Table 2. The antral area was significantly smaller in fasting volunteers in comparison with measurements performed 2 h after ingestion of a liquid or a solid meal, but no significant difference was found between liquid meal and solid meal sessions.
The ROC curves of the performance of the ultrasonographic diagnosis of gastric vacuity are presented in Fig. 2, and the areas under the ROC curves are set out in Table 3. At the cutoff value of the antral area of 320 mm2, performances of the ultrasonographic measurement of the antral area for the diagnosis of gastric vacuity are summarized in Table 4.
In this preliminary crossover study, we reported that a single standardized ultrasonographic measurement of gastric antral cross-sectional area was able to discriminate a fasting stomach, as defined by gastric contents after 12 h fasting, from a full stomach in 22 healthy volunteers, with an excellent performance for the diagnosis of a fasting stomach at the threshold value of an antral area of 320 mm2.
The use of ultrasonography for the examination of stomach contents was previously performed in anaesthesia in women in labour and in the postpartum period [20,21]. In these studies, stomach contents were directly visualized by initially positioning the scanner under the left costal margin and scanning up to the xiphisternum. However, this technique cannot be performed when it is difficult to visualize the stomach contents as a result of excessive air, when the stomach is empty or when the gastric fundus is located behind the costal margin . Thus, Carp et al.  reported that they were unable to identify the stomach contents in 40% of patients scanned, out of 39 patients. In contrast, we were able to measure antral area in 65 patients out of 66. This is not surprising, as the gastric antrum was reported to be visible in almost all patients before and after ingestion of solid or liquid meals, irrespective of gas contents in the fundus [6,17]. Moreover, measurement of the antral area is simpler, even if probably less accurate, than the assessment of antropyloric volume . Hence, repeated measurements of antral area are widely used for assessing gastric emptying time in diabetic and dyspeptic patients and for clinical research in obstetrical anaesthesia [6–12].
To date, one recent preliminary study  evaluated the interest in ultrasonographic measurement of the antral area for the assessment of gastric contents and volume. The authors reported that antral area provided the most reliable quantitative information for gastric volume, in comparison with the cross-sectional measurements of the body and fundus , and they found a close-to-linear relationship between gastric volume after ingestion of up to 300 ml water and antral area. These authors proposed a model for predicting gastric volume values, given a measurement of antral area and given a set of values for demographic variables . Although these preliminary results need to be confirmed in further studies, they are in agreement with those of our study, as regards the high rate of complete cross-sectional view of the antrum, which was of 100% in the study by Perlas et al.  and 21 out of 22 measurements in our study, as well as regards the cutoff value of antral area calculated in our study for discriminating an empty from a full stomach.
With regard to the antral area in fasting volunteers, our results are consistent with those previously published. Benini et al.  reported a mean ± SD basal value of antral section of 319 ± 92 mm2 in 19 healthy participants, and Darwiche et al.  reported a median value of the antral area prior to meal ingestion of 214 mm2 (range 126–263 mm2) in eight volunteers. In contrast, as previously published, antral area showed a wide interparticipant variability in nonfasting volunteers. However, a significant difference was found between fasting and nonfasting (liquids or solid meal) sessions, with regard to the median antral area. This allowed calculation of the performance of its ultrasound measurement for the diagnosis of a fasting stomach, as defined by the gastric contents after 12 h fasting in healthy volunteers.
The performance of the ultrasonographic measurement of cross-sectional antral area for the diagnosis of gastric vacuity was excellent in our study. However, these results have to be put into perspective, as our diagnostic procedure was not compared with a gold standard test. In fact, the current tools used for the assessment of gastric emptying, such as gastric impedance monitoring, paracetamol absorption test and breath test that we used in a previous study , are not easily available at the bedside and do not allow accurate estimation of gastric contents and volume , particularly as regards the gastric volume after prolonged fasting. Scintigraphy is very expensive and cannot be repeated in the same patient because of the risk of cumulative dosage . Hence, these methods are not appropriate for the estimation of the status of gastric contents with respect to our study design. Therefore, we assessed whether measurement of antral area was able to discriminate gastric contents measured after 12 h fasting from gastric contents measured 2 h after ingestion of a solid or nonclear liquid meal in healthy volunteers. According to the current recommendations on preoperative fasting, healthy patients are considered to have an ‘empty stomach’ after 12 h fasting, even if the stomach is rarely totally emptied after many hours of fasting , whereas they are considered to have a ‘full stomach’ 2 h after ingestion of solids or nonclear liquids, that is, with an increased risk of pulmonary aspiration of gastric contents [2,3]. The results of this preliminary study are of interest, as they enable us to suggest that a simple ultrasonographic method could help the anaesthetist in the assessment of gastric contents status and, hence, of the risk of pulmonary aspiration during general anaesthesia, even if the pathophysiology of this complication is multifactorial and aspiration is not always the consequence of a full stomach .
However, the threshold value of antral area of 320 mm2 reported in this preliminary study can currently not be considered for assessing gastric contents status in clinical practice. Particularly, this cutoff value probably varies according to the age and the height of patients . This value has to be validated in further studies including a significantly larger number of patients.
This preliminary study suggests that a single ultrasonographic measurement of antral area could be of interest for the estimation of preoperative gastric contents status. It could help physicians in the assessment of preoperative aspiration risk, in conjunction with clinical data and the history of the patient. However, this method has to be validated in further trials prior to being proposed in clinical practice.
Support was provided solely by instutional sources.
There are no conflicts of interest.
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