Point of care gastric ultrasound (POCGUS) for the assessment of preoperative gastric residual volume in fasting patients scheduled for elective surgery - An observational study

Pulmonary aspiration of gastric contents is a rare but, a serious complication of anaesthesia. Significant pulmonary complications happens in almost half of patients who aspirate and almost 50% of deaths in anaesthesia are directly due to aspiration of gastric contents, which occurs more commonly in patients with risk factors, at the time of induction of anaesthesia or during airway instrumentation. The presence of gastric contents increases the risk of aspiration and as a result practice guidelines for preoperative fasting have been designed to provide adequate time for gastric emptying in patients undergoing surgery.

The practice guidelines of American Society of Anesthesiologists (ASA) for healthy adults consider a minimum fasting duration of 2 hrs for clear fluids, 6 hrs for a light meal and 8 hrs for a fatty meal, fried foods or meat to be safe. However , uncertainty still exists regarding the exact gastric residual volume(GRV) that places the patients at increased risk of aspiration despite following standard fasting guidelines.

Many techniques have been described to assess the gastric contents like gastric content aspiration,  paracetamol absorption, electrical impedance tomography, radio labelled diet, magnetic resonance imaging and the gold standard being gastric scintigraphy. Each modality has its pros and cons. Gastric ultrasound will be performed using a low frequency curvilinear array probe. With the advent of newer portable ultrasound (US) machines, it is possible to accurately assess the gastric contents non- invasively. Antrum It is the region of the stomach which is most amenable to ultrasonographic examination and also the portion which is most consistently identified (98- 100% cases) on ultrasonography (USG). Antral cross sectional area (CSA) calculated from gastric ultrasound highly correlates with scintigraphy and therefore cross sectional area of gastric antrum accurately predicts gastric fluid volume with the same precision as scinitgraphy. It thus helps the anaesthetist to anticipate the aspiration risk at bedside and guide anaesthetic and airway management more appropriately. Scanning was first done in the supine position followed by right lateral decubitus position. According to the appearance of the contents , qualitative assessment was done followed quantitative analysis. For the latter , antral cross sectional area (ACSA) was measured at the level of the aorta using ultrasound machine. All images were obtained between peristaltic contractions of the antrum. ACSA was measured in right lateral decubitus position using traditional two diameter method (TDM) . This involved measuring two perpendicular diameters of the antrum, from serosa to serosa, longitudinal or craniocaudal (CC), and the anteroposterior (AP) using the formula developed by Bolondi et al  in which

ACSA = (CC × AP) × π) / 4, with π value = 3.14. Later, using ACSA , gastric volume was calculated using the formula,

Gastric volume (mL) = 27 + 14.6 ACSA (cm2) - 1.28 age (years) which is given by Perlas et al.

Hence, the  present study was undertaken to ascertain the gastric residual volume (GRV) by point of care gastric ultrasound  in fasting patients scheduled to undergo elective surgery and correlated the same with the hours of fasting and any underlying co-morbidities.