Preoperative fasting involves stopping the oral intake prior to surgical procedures requiring hypnosis or anesthesia [1]. In the postoperative period, eating food by mouth is avoided, until the intestines regain their function in abdominal surgeries [2]. The weakening or abrogation of airway reflexes after deep hypnosis or anesthesia can lead to pulmonary aspiration of stomach contents [3]. The ideal condition to minimize the risk of regurgitation and aspiration is an empty stomach prior to deep hypnosis or anesthesia, which may be impossible to achieve due to continued gastric secretion activity combined with poor physiology due to the various etiologies encountered in the preoperative period.[3]
Conventional points have been cited for gastric aspiration volumes greater than 25 ml with a pH less than 2.5 to be associated with significant pulmonary aspiration leading to respiratory insufficiency [5, 4]. This pH and critical volume were obtained by Roberts and Shirley on a Rhesus monkey and extrapolated to humans [6]. Although gastric pH and volume have traditionally been measured using suction techniques, recent research methods utilizing the pharmacokinetic properties of orally administered water-soluble substances such as paracetamol, magnetic resonance imaging (MRI) and scintigraphy have provided better insights into the physiology of gastric ulcers. Stomach emptying. Newer methods such as gastric ultrasound (GUS) have also been increasingly used to study stomach volumes and emptying.[6]
Principles Of Clinical Guidelines For Preoperative Fasting:
These clinical practice guidelines provide recommendations for discontinuation of solid and liquid food prior to elective diagnostic or surgical procedures performed under deep hypnosis or anesthesia (regional or general) and resumption of oral feeding in the post-procedure period. They aim to reduce the incidence and severity of aspiration pneumonia and other complications (such as hypotension during induction of anesthesia), to prevent delays or cancellations of surgical procedures and also to obtain better patient outcomes. The guidelines are intended for practicing anesthesiologists, surgeons, and other medical and paramedical professionals involved in the care of patients.[7]
It may also be helpful for patients to know what types of foods are considered clear liquids, clear liquids, snacks and heavy meals and guide them in the consumption of nutrients prior to presenting for elective diagnostic or surgical procedures.[7]
Preoperative fasting is defined as the minimum duration of fasting required for oral solids or liquids before administration of anesthesia or sedation necessary to reduce the risk of regurgitation and pulmonary aspiration in patients without risk factors for regurgitation.[7]
An empty stomach is a condition in which the stomach contents reach the basal volume after eating a solid, semi-solid or liquid meal. Basal volume is defined as the volume of stomach contents in an adult after an overnight fast.[8]
Residual stomach volume is the volume of stomach contents at the time of examination or evaluation.[8]
A full stomach is the presence of solid content or a state of increasing liquid stomach contents above basal volumes after eating a solid, semi-solid or liquid meal.[8]
Clear liquid is defined as any liquid that takes less than two hours to empty from the stomach in humans.[8]
Clear liquid is defined as any liquid that takes between 2 and 4 hours to be emptied from the stomach in humans.[8]
A snack is defined as any food item that takes between 4 and 6 hours to be emptied from the human stomach.[7]
A heavy meal is defined as any food that takes more than 6 hours to be emptied from the human stomach [7].
What Is The Acceptable Residual Stomach Volume?
Roberts et al. found that instillation of 25 mL of gastric paracentesis into the right trachea of a rhesus monkey resulted in significant pneumonia. Extrapolating to this result in humans, they stated that inhalation of 25 ml would lead to pneumonia and assumed that a residual stomach volume of more than 25 ml constituted a significant risk of developing pneumonia [6]. Many studies have attempted to determine residual stomach volume, based on values obtained by actual suction (during endoscopy or by gastric tube) and by radiographic techniques. More recent studies have used GUS to determine remaining stomach volumes after fasting. The stratification of fasting duration, definition of residual gastric volume, and methods for its assessment in the current studies were inconsistent, which limited attempts to formulate definitions acceptable to clinical practice.[8, 9]
In a prospective descriptive study using GUS, Perlas and colleagues found that 86 of 200 patients who fasted for 8 hours on solids and 5 hours on liquids had no or minimal residual gastric volume, while seven patients had a high residual gastric volume of 180 ± 83 ml. One of these patients with gastroesophageal reflux disease aspirated gastric contents during extubation [9]. Van de Putte et al conducted a retrospective analysis of 512 patients and classified patients as having an 'empty stomach' (no contents or liquid content < 1.5 mL/kg) or a 'full stomach' (liquid content > 1.5 mL/kg or presence of solid content). ) Based on the GUS, they found that despite adhering to the American Society of Anesthesiologists' fasting guidelines, 32 patients had a full stomach according to these criteria. The average fasting duration for these patients was 10.8 hours and 13.9 hours for liquids and solids, respectively. Eight patients ate heavy meals out of nine patients had solid content while one of them had gastroesophageal reflux disease and Parkinson's disease [10]. Four studies involving pregnant women found residual stomach volumes greater than 25 ml [12 − 11].
Based on current evidence, the classic cut-off of more than 25 ml of residual gastric volume may need to be re-examined as a risk factor for aspiration pneumonia in adults.
Appropriate Fasting Times - When Is The Stomach Empty?
Twenty-one studies were identified [13–15] describing gastric emptying times for different types of foods. Of these, 14 were randomized controlled trials and 7 were observational studies. These studies used different methods to estimate gastric emptying such as the 'gold standard' scintigraphy, MRI, GUS and breath CO2 analysis. Most of the studies were conducted on healthy volunteers and had small sample sizes. Six studies were performed each on gastric emptying of liquids of varying composition, solids of variable content, and the mixture of solids with liquids or meals, respectively, while one study of semi-solids and comparison of solids versus liquid, respectively. These studies showed significant differences in gastric emptying. A study conducted in Indian environment and published in 1999 concluded that adding spices (masala) to liquor reduces gastric emptying time [14].
The conclusions drawn from these studies are as follows:
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Gastric emptying time varies between liquids, solids and semi-solids. For solids, it depends on the cooking method and the size and composition of the ingredients.
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Clear liquids with low calorific value take about two hours to empty.
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Liquids with a high calorific value and cloudy liquids may take more than two hours to empty.
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Solid-liquid mixtures take between 4 and 7 hours to empty.
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Fried food items of similar composition take longer to deflate.
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Scintigraphy cannot be used to estimate residual stomach volume.
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The GUS estimate of the residual stomach volume is calculated through mathematical models based on the antrum cross-sectional area, where the return to baseline values is considered the time of gastric emptying. [15]
The classification of food items through the survey into heavy meals, snacks, and clear liquids was reviewed by an experienced dietitian after reference to available evidence [15].
The classification was reviewed by a dietitian based on the duration of gastric emptying as clear liquids, cloudy liquids, and heavy or light meals. [15]
Fasting On Clear Liquids Two Hours Before The Procedure Vs. Overnight?
The stomach always has a basal volume due to the continuous gastric secretions. The duration of fasting is considered acceptable if, after a meal, the volume of the stomach returns to basal levels. The 20 studies identified by our search regarding clear fluid intake and stomach volumes did not report any cases of aspiration or regurgitation [14]. The studies had a low or very low level of evidence. In eight studies, after an overnight fast, one group received water for medication consumption while the other group received between 100 and 500 ml of water up to two hours before anesthesia [13–15].