**3. Gastric function**

The main tests to study the stomach function are gastric barostat, scintigraphy, tests using an intraluminal capsule, breath test, electrogastrography (EGG) and Water Load Test (WLT).

#### **3.1 Barostat**

Gastric barostat studies are the best-established methods of measuring gastric accommodation, and they are considered as the gold standard.

The barostat is a computerized air pump device that measures fundic relaxation in response to a meal, by monitoring the volume of air within an intragastric bag (a polyethylene balloon [15]) that is clamped at a constant pressure level [16]. The system maintains a constant pressure by fixing it and changes the bag's volume reflecting gastric relaxation or contraction (**Figure 3**). In this way, gastric accommodation response to various interventions is recorded.

The problem is that the barostat is an invasive test [17]: although it is a valuable technique, it is not practical in daily clinical assessment for patients nor for research studies. This is the reason why today non-invasive methods are preferred, and they will be later described in this chapter. Moreover, the barostat balloon can't perfectly measure post-prandial accommodation of the entire stomach, and it is possible that it interferes with the intra-gastric management of the meal [18] (**Figure 3**).

### **3.2 Scintigraphy**

Gastric Emptying Scintigraphy (GES) is a non-invasive test that uses 99mTc-Nanocolloidal as the radioactive substance. The patient is usually given a solid and radiolabeled meal, then data are acquired, and the emptying time is analyzed. A liquid meal with water labeled with indium-111 diethylenetriaminopentacetic acid may also be used. The procedure should be repeated after 1 hour, 2 hours and 4 hours: in fact, delayed gastric emptying is detected with greater sensitivity at 4 hours, and images detect gastroparesis with 30% greater frequency. Imaging at 0,1,2 and 4 hours allows identification of both rapid and delayed gastric emptying, which is important because patients are treated in a different way, even though their symptoms are similar. Moreover, images at 60 minutes have a specificity and sensitivity of 90% for rapid gastric emptying, whereas images at 240 minutes show a specificity of 70% and sensitivity of 100% for delayed one [19]. Delayed emptying is considered to be gastric retention of more than 90% at 1 hour, more than 60% at 2 hours, and more than 10% at 4 hours [20].

#### **3.3 Intraluminal capsule**

The capsule test is performed by ingesting a capsule with the meal: this records intraluminal pH, pressure, temperature and post-prandial gastric contractions, and transmits wireless data to a receiver. In healthy people it is detected in the duodenum after about 5 hours. The test has a concordance of 90% with scintigraphic tests and has excellent sensitivity and specificity especially in detecting gastroparesis [21]. This technique offers a non-radioactive, ambulatory alternative to scintigraphy [22].

#### **3.4 Breath test**

A functional test for the study of gastric emptying (GE) is the breath test (GEBT) [23]. It is a non-invasive method, feasible also in pregnancy and children, that does not use any radiation, it is easily repeatable, and it is based on the measurement of the 13CO2/12CO2 ratio in the exhaled breath after the administration of a standard meal, labeled with 13C-Spirulina or 13C-Acetate or 13C-Octanoic Acid. The procedure is repeated at 45, 90, 120, 150, 180, and even 240 minutes after the end of the meal. By measuring the change in this ratio over time from the premeal value, the rate of 13CO2 excretion can be calculated and the individual's rate of gastric emptying determined.

When compared with scintigraphy, it showed similar values: good agreement was found between the two tests, thus validating the breath test as an alternative method for studying gastric emptying. Based on the study of normal values, the 10th and 90th percentiles of t 1/2 calculated with scintigraphy were used to classify patients as follows: subjects with delayed (t1/2 > 86 min), accelerated (t1/2 < 52 min), or normal (t1/2 52–86 min) [24] gastric emptying.
