In this step, Dr Kariem El-Boghdadly, Consultant Anaesthetist at Guy’s and St Thomas’ Hospital, and Honorary Senior Lecturer at King’s College London, discusses how point-of-care gastric ultrasound can help with decision making when planning an airway management strategy.
Pulmonary aspiration of gastric contents occurs in approximately 1 in 2000 to 1 in 3000 elective surgeries, and up to 1 in 900 emergency surgical procedures. It is associated with a high incidence of in-hospital mortality and accounts for half of airway-related mortality following anaesthesia. To reduce the potential risks of pulmonary aspiration in the elective setting, fasting guidelines recommend abstinence from solids for more than six hours and clear fluids for more than two hours.
There is significant inter-individual variability in the gastric emptying rate: 4.5% of patients who adhere to fasting guidelines have gastric content that would leave them at risk of pulmonary aspiration. In the emergency setting, gastric emptying is even less predictable, making clinical decision-making about airway management increasingly uncertain.
Point-of-care gastric ultrasound is an emerging diagnostic tool that may provide qualitative and quantitative information on gastric content to help guide clinical decision-making, and determine, for example, the need to perform a Rapid Sequence Induction.
The aim of gastric ultrasound is to visualise the gastric antrum, which is the most dependent area of the stomach to which gastric content will gravitate towards. It lies infero-posteriorly to the left lobe of the liver and sits anterior to the pancreas and the aorta. It consists of five distinct layers (mucosa, muscularis mucosae, submucosa, muscularis propriae, and serosa): the most sonographically visible of which are the latter two outermost layers.
Gastric ultrasound is indicated in patients with uncertain fasting status (e.g. cognitive dysfunction, unclear history) or those with either known or suspected delayed gastric emptying (e.g. diabetes, pain, pregnancy, obesity). It is performed with the patient in the right lateral decubitus position using a low-frequency (1–5MHz) curved array ultrasound transducer placed in the epigastrium. Qualitative assessment of the content of the gastric antrum can reveal an empty antrum, and antrum containing fluid, or an antrum with solids. The landmarks sought, from posterior to anterior, are the vertebral bodies, the long axis of the abdominal aorta or inferior vena cava, the pancreas and the left lobe of the liver with the short axis of the gastric antrum next to the liver.
Figure 1. Ultrasound images of different antral qualitative appearances (L: liver, SMA:superior mesenteric artery)
- A. An empty (Grade 0) gastric antrum with a thick muscularis propriae layer and no intraluminal content. This appearance is consistent with a low risk of pulmonary aspiration.
- B. Gastric antrum containing fluid with some air bubbles.
- C. Gastric antrum after recent ingestion of solids, with a ‘frosted glass’ appearance. The anterior antral wall is visible, but there are no clear structures seen deep to the anterior antral wall.
- D. Gastric antrum containing solids, with heterogeneous echogenicity representing different consistency of solids consumed. Note the thin, hypoechoic muscularis propriae compared to figure A.
Antral clear fluid appears hypoechoic (black) on ultrasound, while thick fluids such as milk or juice containing pulp appear more echoic and homogenous in nature. The antrum will be distended and the muscularis propriae becomes thinner. The antral cross-sectional area can be calculated using the trace calliper function and the volume of fluid quantified with validated mathematical models.
A volume of < 1.5 ml kg–1 is consistent with baseline gastric secretions (Grade 1), while a volume of > 1.5 ml kg–1 is associated with a volume that is greater than baseline secretions (Grade 2). A Grade 1 antrum is associated with a lower risk of pulmonary aspiration, while a Grade 2 antrum or any thick fluids, is associated with a higher risk of pulmonary aspiration.
Gastric ultrasound has been shown to be feasible in patients with a high BMI and similarly is feasible and reproducible in the parturient, despite technical challenges. Finally, this tool can also be used in paediatric patients, although a linear probe is required.
Point-of-care gastric ultrasound is readily learned, carries a high sensitivity and specificity, and has been shown to modify anaesthetic management. Armed with information of the true content of the gastric antrum, clinicians may make more informed decisions on the safety of anaesthetic interventions.
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