Overview of Gastric Ultrasound
Aspiration of gastric contents, meaning accidentally inhaling substances from the stomach, can occur during surgery and result in serious, sometimes life-threatening, complications. These include not getting enough oxygen, spasms in the lungs, inflammation of the lung tissue, pneumonia, severe respiratory problems, and even death. The risk of developing pneumonia from this is much higher in surgical patients, leading to a lengthier hospital stay and a significantly higher risk of death during their stay.
One of the largest risk factors during surgery is having food or drinks in the stomach before anesthesia is administered. That’s why doctors recommend fasting before planned procedures. This reduces the chances of inhaling stomach contents during surgery. According to the American Society for Anesthesiologists guidelines, healthy patients should avoid consuming clear liquids for at least 2 hours before surgery, breastmilk for 4 hours, non-human breastmilk, infant formula, or light meals for 6 hours, and fried foods, fatty foods and meats for 8 hours.
However, there are no specific guidelines for people who are at a higher risk of delayed gastric emptying (a condition where the stomach takes too long to empty its contents) or aspiration during surgery. This includes people with diabetes, gastroesophageal reflux disease (a condition where stomach acid frequently flows back into the tube connecting your mouth and stomach), severe obesity, pregnancy, or recent opioid use. It’s also challenging for doctors when patients are unable to provide a clear account of their last meal, as seen in patients who struggle with memory issues, language barriers or non-compliance.
A tool which helps healthcare providers in this situation is gastric ultrasound. This is an easy, quick and non-invasive tool to visualize the stomach content at the bedside, a test which helps medical professionals to determine whether a patient’s stomach is full or empty. It also helps to identify the types of substances (solids, thick liquids, clear liquids) present in the stomach, along with the volume of liquids. This helps doctors to better determine the best time for elective procedures and choose the appropriate anesthesia and airflow management method.
Anatomy and Physiology of Gastric Ultrasound
To be good at performing a gastric ultrasound, you need to understand the structure of the stomach and the organs surrounding it. The stomach is shaped like a “J” and divided into four parts. These areas are called the cardia, fundus, body, and pylorus. The cardia is the region right next to the esophagus, the tube that carries food from your mouth to your stomach. The fundus is the upper section of the stomach, positioned above the cardia. The body is the biggest part of the stomach, where your food gets mashed up with digestive juices. The pylorus is the lower part of the stomach and has a funnel-like shape. It consists of the antrum and the pyloric sphincter, a round muscle that acts like a door between your stomach and your small intestine. The antrum is the part of the stomach before the pyloric sphincter, which stores food before it gets released into the small intestine. Doing a gastric ultrasound, the antrum is the most important part to see.
The wall of the stomach has three layers that can usually be seen clearly on an ultrasound. The serosa is the outermost layer and shows up as a thin, vivid line. The muscularis propria is just below the serosa, appears as a thick, less vivid line, and is visible on an ultrasound. The mucosa is the innermost layer of the stomach and shows up as a thin, vivid line. Sometimes, five layers can be seen, with the innermost two considered as artefacts due to the interaction between the mucosal layer and liquid inside the stomach.
The gastric ultrasound is done using a certain angle of the scanning plane. Therefore, it’s crucial to know the position of the organs around the stomach for this kind of view. The liver is easy to recognize due to its size and blood-supply appearance. It’s normally found on the left side of the screen during the ultrasound. The pancreas is deep to the stomach and has a vivid appearance. The bowel is generally difficult to spot on ultrasound due to gas inside it but is generally found on the right side of the screen.
During a gastric ultrasound, you can also see several major blood vessels, such as the aorta, the superior mesenteric artery, and the inferior vena cava (IVC). These all appear black because they are full of blood. The aorta and the IVC might be challenging to tell apart, as they are both large and at similar depths inside the body. Normally, only one of them should be visible at the same time in the specific angle of the ultrasound scan, as they run next to each other. The aorta is thicker, pumps with your heartbeat, and is typically smaller. The IVC has thin walls, might change shape, and its size varies with your breath, getting smaller when you breathe in and larger when you breathe out. You can see the aorta in front of the bones of your spine and left of the line that runs through the middle of your body. The IVC is at a similar depth, but on the right side.
Why do People Need Gastric Ultrasound
Gastric ultrasound is a tool doctors use to look at what’s inside the stomach. This can help them figure out how much food, liquid, or air is in the stomach. It’s especially helpful when a patient can’t tell doctors if they have eaten or drunk anything recently—maybe because they have memory problems, can’t speak the language, or just can’t remember. It’s also useful for patients who have conditions that might slow down digestion, like diabetes, ongoing use of strong pain medicines, pregnancy, or infections in the belly.
The information from a gastric ultrasound can help medical staff decide if it’s safe to go ahead with a planned surgery or if it needs to be rescheduled. For surgeries that require immediate attention, the ultrasound can help doctors choose the best way to put the patient to sleep and keep their airway open during the procedure.
When a Person Should Avoid Gastric Ultrasound
There are essentially no conditions that completely prevent the use of a stomach ultrasound. However, stomach ultrasound is usually not recommended in some situations. These include patients who have abdominal injuries or bandages on the upper part of their belly, and patients who cannot be safely placed on their right side (this is known as the right lateral decubitus position or RLD).
Equipment used for Gastric Ultrasound
In order to do an ultrasound of the stomach, almost any ultrasound machine can work. The ideal machine should be able to measure the area of the stomach as seen from a cross-section (a view as though it were sliced). If the machine has settings specifically for abdominal imaging, that would be great to use. It’s also important to adjust the machine to a depth that makes the main blood vessel of the abdomen, the abdominal aorta, visible.
Most patients would need a scanning device (referred to as a “probe”) that emits low-frequency signals (2 to 5 MHz) and has a curved shape. For smaller children, a probe that gives off higher-frequency signals (5 to 13 MHz) might be needed. However, with such a probe, it’s crucial to check that the ultrasound “waves” can go deep enough into the body to give clear images of the abdominal aorta.
Who is needed to perform Gastric Ultrasound?
A trained doctor can do and understand stomach ultrasounds. Sometimes, they might need an extra hand to make sure small kids or patients who can’t move around easily, aren’t able to cooperate, or have been given medicine to help them relax, are in the right position.
Preparing for Gastric Ultrasound
Before starting, the doctor should explain the purpose of the exam to the patient. In simple terms, the epigastrium (upper part of the abdomen) needs to be exposed, while the chest and the pelvic area are kept covered for privacy. To keep the patient’s clothing and sheets clean, they are protected with clean towels to avoid getting any ultrasound gel on them.
The ultrasound machine is positioned in a way that allows the screen to be seen during the examination. Ideally, the doctor will stand on the right side of the patient, with the ultrasound machine on the left. This arrangement makes the procedure easier and more efficient for both the doctor and the patient.
How is Gastric Ultrasound performed
When using ultrasound to examine the stomach and abdominal area, the patient lies flat on their back. The medical expert holds the ultrasound probe in a way that it points towards the patient’s head. The liver is easy to recognize, and it should appear on the left side of the screen. The probe is initially positioned just below the breastbone, and once the liver is spotted, the other abdominal structures are looked for. This includes major blood vessels like the inferior vena cava (a large vein carrying deoxygenated blood from the lower half of the body) or the abdominal aorta (the largest artery in the body), and organs such as the stomach, liver, pancreas.
The expert will pay close attention to the contents of the stomach. They check for solids, thick liquids, or clear fluids. Do remember, just because you’re lying down and nothing unusual is observed, does not mean the stomach is empty. Therefore, this sort of scan can only suggest if the stomach has a lot of content, it can’t confirm if it’s empty.
In the next phase of the exam, the patient will be asked to lay on their right side. Right lateral decubitus (or RLD) is the medical term for this position. Placing patients on their right side allows the contents of the stomach to move to an area of the stomach called the antrum. This moves the contents toward the exit of the stomach and makes it easier to scan and measure the contents of the stomach. What’s in the stomach is assessed again, and any solids or thick liquids are noted.
Solids look different from liquids under an ultrasound. Solid food obstructs the ultrasound waves, making it hard to see structures beyond it. Thick liquids, such as yogurt, appear bright and are straightforward to identify. Clear liquids look dark and are also easy to identify. If the liquid has recently been drunk, there might be some bubbles present, making the liquid look like a starry night. When the stomach is empty, the ultrasound shows a bull’s eye pattern.
If the stomach only has clear liquids, these can be measured and a rough estimate of the volume of these liquids can be calculated. This is important because knowing how much liquid is in the stomach can tell the doctors if the patient is at risk of aspiration (breathing foreign objects such as stomach contents into the lungs) during anesthesia. The normal volume should be less than 1.5 ml of liquid for every kg of body weight. If the volume is larger than this, it could mean the stomach is full.
What Else Should I Know About Gastric Ultrasound?
During surgery, an ultrasound of your stomach is often performed to ensure safety. The importance of this ultrasound depends on the urgency of your surgery. If your surgery isn’t urgent and the ultrasound shows solids, thick liquids or a large amount of clear liquids (more than a small sip of water per kilogram of your body weight) in your stomach, your surgery may need to be rescheduled. This is to avoid the risk of these substances being accidentally inhaled into your lungs during surgery.
However, if your surgery can’t wait or if it’s hard to predict when your stomach will be empty (like in a condition called gastroparesis, where your stomach doesn’t empty as well as it should), the risk of inhaling stomach contents and the best timing for your surgery should be discussed with you and the doctor who is performing your surgery. This is especially important when putting you to sleep for the surgery—the anesthesia plan—must include steps to avoid the risk of inhaling stomach contents.
You should not receive any medication to make you sleepy before your surgery, to keep your airway reflexes intact; these reflexes keep your airway clear and protect you from inhaling anything into your lungs. However, if medication to make you sleepy is required, your airway will be protected using a rapid technique of anesthesia and the placement of a breathing tube into your windpipe (endotracheal intubation).
Depending on the circumstances, a tube may need to be inserted through your nose or mouth and passed down into your stomach (this is known as a nasogastric or oral gastric tube) either before the anesthesia is given or after the breathing tube is placed. This tube helps to drain the contents of your stomach and reduce the risk of inhaling them into your lungs.
