Clear fluids, not breakfast, before surgery

CID AWRATION of gastric content continues to be a A major cause of morbidity and mortality in anesthesia. Although most agree that the risk can be reduced by increasing the pH and decreasing the volume of gastric juice, what is a safe volume or pH? Recently (l), questions have been raised as to the validity of the “at risk” concept for acid aspiration, i.e.: pH <2.5 and gastric volume >25 ml (0.4 ml/kg) as a cause of chemical pneumonitis (2). In primates (3), seiwe chemical pneumonitis was observed only with an aspirated volume of >0.8 ml/ kg of instilled fluid with a pH=l.O; severe pneumonitis was not seen when a smaller volume of fluid with a pH=l.O was aspirated. If these results were extrapolated to humans, the critical volume for aspiration with serious sequels would be increased to 50 ml(0.8 ml/kg) of fluid with a pH=l.O. The amount of aspirate with a pH=2.5 which will produce chemical pneumonitis is unknown at present. At the present time we still lack a full understanding of the physiology and pathophysiology of gastric emptying (4). Gastric emptying is different for liquids and solids. Emptying of liquids from the stomach is believed to be primarily a function of the proximal stomach exerting pressure on the fluid volume through sustained contractions. The role of the pylorus in regulating the emptying of liquids is of little importance (5,6,7). In contrast, emptying of solids is regulated by the distal stomach and the gastroduodenal junction (4). Recent studies have suggested that the presence of fat in the upper intestine delays gastric emptying, and that this effect is regulated in part by increasing the resistance to flow offered by the pylorus (4). This may be the reason for the longer delay in emptying of solids as compared to liquids from the stomach. Many factors including the composition of gastric contents, pH, temperature and osmotic pressure all influence gastric emptying (8). Unfortunately, several studies have found similar factors to have contradictory effects on gastric emptying. The differences are often difficult to explain, but may be related to different volunteer / patient populations, measuring techniques, pre-existing drug treatment and the composition and caloric content of the test meals. Nearly all studies on gastric emptying have been performed on healthy volunteers. Very few studies have considered patients with pathological conditions as may be seen in anesthetic practice. Gastric emptying of liquids is 95% completed within one hour. Of interest to the anesthesiologist is that gastric emptying occurs more rapidly in the morning than in the evening (9). This could imply that a longer fast be required for patients whose surgery is scheduled later in the day. Clinical studies to answer this question have not been performed. In healthy subjects over 70 years old, gastric emptying is also somewhat slower (10). Gastric emptying of solids or liquids in obese patients, although usually delayed, have not shown the expected delay in all studies (11, 12, 13). There is, however, agreement that emptying of solids will be delayed in obese subjects who are at least 63% in excess of ideal weight (12). Some investigators have found gastric emptying rates for men and women to be similar (14), while others have found women to have slower emptying rates (15). This difference may be related to the menstrual cycle in women. Interestingly, many studies of pregnant women have not shown a delayed gastric emptying of liquids (16, 17). These results suggest that the guidelines for fasting of fluids in elective pregnant patients need not be different than for nonpregnant patients. Pain and emotional stress delay gastric emptying, but no studies have attempted to quantitate the degree of stress to the amount of residual gastric juice. In this issue, Dr. Ssreide and his coworkers present their timely and interesting study of gastric