Dimitris Raptopoulos, DVM, PhD, DVA, DECVA, Professor; Ioannis Savvas, DVM, PhD
Clinic of Surgery, Faculty of Veterinary Medicine, Aristotle University of Thessaloniki
In 1883, Baron Joseph Lister, an English surgeon wrote, "While it is desirable that there should be no solid matter in the stomach when chloroform is administered, it will be found very salutary to give a cup of tea or beef-tea two hours previously". Most anaesthetists and surgeons followed this advice for many years. In 1946, Mendelson published a very well known paper on high incidence of pulmonary aspiration during general anaesthesia, in obstetric patients. Shortly after this publication, the practice of "nil per os after midnight" was established (Pandit et al. 2000). Thereafter, prevention of aspiration of gastric contents became one of the cornerstones of safe anaesthetic practice in humans. About 10% of the anaesthetic mortality is related to gastro-oesophageal reflux (GOR) during induction of anaesthesia. Strict preoperative fasting rules to ensure an empty stomach at induction had been a major concern for the anaesthesiologists. However, over the past two decades, several authors have questioned the scientific basis of these rules (Morgan 1984; Spence 1989; Engelhardt and Webster 1999; Ljungqvist and Soreide 2003).
In human medicine, major risk factors for GOR include: a high ASA physical status score, emergency surgery, pregnancy, ingestion of a meal within 3 hours, opioids, obesity, and type of surgery. About 10-20% of GOR cases during induction result in aspiration pneumonitis. The incidence of aspiration pneumonitis varies among the hospitals between 0.7 and 10.2 per 10,000 of general anaesthetics. The gastric contents aspirated into the lungs may cause mechanical obstruction of the airways, chemical inflammation (very high/low pH), and infection (Harrison 1978; Hovi-Viander 1980; Gibbs and Modell 1992; Warner et al. 1993; Turner 1996; Engelhardt and Webster 1999; Ng and Smith 2001). It has been shown that in rhesus monkeys a maximum volume of 0.4 ml kg-1 with a pH of 2.5 may be aspirated without severe respiratory consequences. This corresponds to a total volume of about 25 ml in an adult woman in pregnancy. The authors had proposed that a gastric content volume of at least 25 ml with a pH less than 2.5 indicate a high risk patient (Roberts and Shirley 1974).
In veterinary medicine, there is no reference to aspiration occurring during induction of anaesthesia in dogs and cats, although this should not be assumed as impossible. However, GOR during anaesthesia is the major cause of oesophagitis (reflux oesophagitis) and oesophageal stricture. In a recent study, the overall incidence of GOR during anaesthesia was 16.3 % (39/240), in dogs. In only one of these cases regurgitation occurred, the rest of them being silent ref lux (no gastric contents were observed out of the mouth) (Galatos and Raptopoulos 1995b). In another study (Kushner and Shofer 2003), the anaesthetic complication rate for oesophageal stricture calculated retrospectively was found to be 0.13 % (30/23,295). However, mortality was high (30%) in the stricture cases. It seems that whereas GOR may occur during general anaesthesia, oesophageal stricture is rare, while it is associated with high morality rate. Interestingly, there is no report of reflux oesophagitis after anaesthesia in humans.
CURRENT RECOMMENDATIONS IN HUMAN MEDICINE
In humans, the major methods used to minimize aspiration are control of gastric contents, reduction in GOR, prevention of aspiration, and attenuation of the effects of aspiration. The first two include preoperative fasting, decrease in gastric acidity, facilitation of gastric emptying, and maintenance of a competent lower oesophageal sphincter (LOS). The latter two involve tracheal intubation or the use of other airway devices and application of cricoid pressure (Ng and Smith 2001).
Fasting before surgery is necessary to avoid the risk of regurgitation and vomiting; it is also a legal requirement (Watson and Rinomhota 2002). The fear of aspiration of gastric contents and its life-threatening consequences in patients (aspiration pneumonitis and respiratory failure), has caused many medical practitioners, particularly anaesthesiologists, to rigidly follow conservative (i.e., prolonged) preoperative fasting standards. This is the nil per os (NPO) order for clear fluids/liquids and solids overnight or six to eight hours preceding the induction of anaesthesia. However, this practice neither takes into account the differences in the rate of gastric emptying for solid food (which may exceed six hours) and clear liquids (which is one to two hours), nor the differences in scheduled times of surgery (Watson and Rinomhota 2002).
Moreover, the concept that fasting produces an "empty stomach" has been shown to be incorrect. Numerous studies demonstrate that fasting neither diminishes gastric volume nor decreases gastric acidity and the risk of pulmonary aspiration is not increased by the preoperative intake of clear liquids. Withholding fluids preoperatively is not only of no benefit to patients but may even be harmful (Dowling, Jr. 1995). A long fasting may lead to thirst, general discomfort, dehydration, and possible hypoglycaemia (Phillips et al. 1994). Preoperative fasting in man may lead to a fluid deficit of about one litre, which may contribute to perioperative discomfort and morbidity (Holte and Kehlet 2002).
Several studies have confirmed that clear liquids are rapidly emptied from the stomach. Gastric residual volume in humans who have had unlimited clear fluids up to 2 hours before induction is not different from, or may even be smaller than, that for humans who have been fasted overnight. Therefore, there is no need for excessive periods of fasting before elective surgical procedures in any patient (Cote 1999). Moreover, potential benefits of reduced thirst, better perioperative experience, improved compliance and reduced hypoglycaemia may be seen (Phillips et al. 1994).
Based on the new data available, several editorials and national anaesthesiology societies recommend a more liberal approach to preoperative fasting guidelines in otherwise healthy patients undergoing elective procedures. The newer recommendations allow the consumption of clear liquids up to two hours before elective surgery, a light breakfast six hours before the procedure, and a heavier meal eight hours beforehand (Strunin 1993; Soreide et al. 1996; Eriksson and Sandin 1996; Warner et al. 1999; Pandit et al. 2000; Ng and Smith 2001; Ljungqvist and Soreide 2003). The application of these new guidelines resulted in no increase of complications in a 3-year study in Norway (Fasting et al. 1998). It should be mentioned that some authors suggest that a light breakfast (tea and toast, for example) may be given 3-4 hours before the procedure (Miller et al. 1983). Finally, non-human milk is similar to solids in gastric emptying time (Warner et al. 1999).
CURRENT RECOMMENDATIONS IN VETERINARY MEDICINE
In dogs, factors that may influence the incidence of GOR include: volume and acidity of gastric contents, age, surgical procedure, drugs used for premedication and/or anaesthesia, and preoperative fasting. Positioning (body tilt on surgical table) does not seem to have any effect. Increased age as well as intra-abdominal surgery was associated with significantly increased incidence of reflux. Moreover, many drugs may affect LOS tone and predispose to GOR (Galatos and Raptopoulos 1995a; Galatos and Raptopoulos 1995b). In mature healthy dogs, it is usually recommended to allow free access to water up to 2 hours before anaesthesia and no food 6 (Bednarski 1996) or 12 hours (Hall et al. 2001) beforehand, although Muir et al. (2000) suggest that food and water should be withheld for approximately 6 hours before surgery. However, there is evidence that increasing the duration of preoperative fasting is associated with an increased incidence of ref lux in dogs. None of 30 dogs fasted 2-4 hours refluxed, whereas 4/30 (13.3 %) dogs fasted 12-18 hours had a reflux episode during anaesthesia (p=0.112) (Galatos and Raptopoulos 1995b). In another study, none of 31 dogs fasted 3 hours refluxed, whereas 6/29 (20.7 %) dogs fasted 10 hours had a reflux episode (p=0.009) (Savvas and Raptopoulos 2000). In the latter study, the dogs had been fed a commercial canned canine diet at the half daily rate. Furthermore, dogs fasted 3 hours had not significantly increased gastric content volume compared to dogs fasted 20 hours, while gastric acidity was reduced (Savvas 2000), which may have a beneficial effect in preventing major consequences in case of GOR during anaesthesia. In contrast, the administration of fat-free cow milk (10 ml kg-1) resulted in a significantly lower gastric content pH.
There is evidence that in otherwise healthy dogs undergoing elective surgery, allowing the consumption of water up to two hours prior to induction of anaesthesia and a light meal 3-4 hours before the procedure may be beneficial. Although the above findings have not been used in a sufficiently large number of clinical cases, it seems that the time has come to abandon the traditional "nil per os after midnight" or nil per os for 6-12 hours prior to anaesthesia, and adopt more liberal guidelines for preoperative fasting in adult healthy dogs undergoing elective procedures.
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