Jejum pré-operatório de 8 horas ou de 2 horas: o que revela a evidência?

Abstract

Insulin resistance is a transitory phenomenon of the metabolic response to trauma. In uncomplicated operations it lasts for 2-4 weeks postoperatively, and is directly related to the magnitude of the injury. The fasting status caused by conventional fasting protocols aggravates this resistance and may induce hyperglycemia. Conventional preoperative fasting time may aggravate this resistance and increment the elevation of glycemia especially because it is frequently longer than the expected 6-8h and may reach 10-16 hs. Additionally, overnight fasting may cause variable degrees of dehydration depending on the extension of the fasting period. Recently, various societies of anesthesia and nutrition have changed their guidelines to propose a reduction of preoperative fasting to 2h with clear fluids containing carbohydrates. These new protocols (ACERTO, ERAS) are based on the safety of this routine as consistently demonstrated by various randomized trials and a meta-analysis.

Fasting; Preoperative care; Insulin resistance; Gastric emptying; Glucose

REVIEW ARTICLE

Preoperative fasting of 8 hours or 2 hours: what does evidence reveal?

José Eduardo de Aguilar-Nascimento, TCBC-MT^I; Francine Perrone^II; Leicia Íris de Assunção Prado^II

^IFull Professor, Department of Clinical Surgery, School of Medical Sciences, Universidade Federal de Mato Grosso, MT, Brazil

^IIGraduate student in Health Sciences, School of Medical Sciences, Universidade Federal de Mato Grosso, MT, Brazil

Correspondence address

ABSTRACT

Insulin resistance is a temporary phenomenon of metabolic response to trauma. In uncomplicated operations, it lasts 2-4 weeks postoperatively, and is directly related to the magnitude of the injury. The fasting status caused by conventional fasting protocols aggravates this resistance and may induce hyperglycemia. Conventional preoperative fasting time can aggravate insulin resistance and increment the elevation in serum glucose levels, especially because it is frequently longer than the expected 6-8 hours and may reach 10-16 hours. Additionally, overnight fasting can cause varying degrees of dehydration depending on the length of the fasting period. Recently, various anesthesia and nutrition societies have changed their guidelines to propose a reduction in preoperative fasting to 2 hours with carbohydrate-containing clear fluids. These new protocols (ACERTO, ERAS) rely on the safety of this regimen as consistently demonstrated by several randomized trials and a meta-analyses.

Key words: Fasting. Preoperative care. Insulin resistance. Gastric emptying. Glucose.

INTRODUCTION

The postoperative recovery of patients who underwent gastrointestinal tract operations is still a great challenge for the surgeon. Nonetheless, a large part of the routines concerning perioperative care in abdominal surgery have hardly changed over time. This has been motivated by old conceptions, medical paradigms that linger in surgery and eventually generate insecurity in the professionals involved in the treatment of those patients³.

The benefits of a six- to eight-hour fast, for instance, as a way to prevent the risk of gastric aspiration during anesthetic induction (Mendelson's syndrome)⁴, have been questioned by many authors^5-6. In fact, that practice was instituted on the basis of reports on cases of bronchopulmonary aspiration in situations when anesthetic induction was performed in urgent operations. The concept was extended to elective operations on the basis of another study from the 1950's, which defined 25 mL as the maximum gastric contents to ensure there would be no risk of bronchoaspiration during anesthetic induction⁷.

Studies in the evidence-based medicine era have sought to investigate whether scientific support exists for many of those practices. The design and results of those studies eventually led to a conceptual review confronting empirically established principles with the strength of evidence. Therefore, the objective of the present review is to address the roads already traveled and our present standing, and to evaluate the impact of shortened preoperative fasting in the management of surgical patients.

The rationale for shortened fasting times

Preoperative night fasting was instituted when anesthetic techniques were still rudimentary, in order to prevent pulmonary complications associated with vomiting and gastric contents aspiration. The reason for that routine was to ensure gastric emptying and prevent bronchoaspiration on anesthetic induction (Mendelson's syndrome)⁷.

A review of last-century textbooks shows that the dogma of preoperative fasting of six to eight hours was established mainly from the association made by Mendelson, in 1946, between feeding and pulmonary aspiration of gastric contents during labor under general anesthesia². Two syndromes were described by that author: the first consists of solid food aspiration leading to airway obstruction and death, or massive atelectasis; the second, named after him, derives from the aspiration of liquid gastric contents when laryngeal reflexes have been depressed by general anesthesia. Patients with that syndrome develop cyanosis, tachycardia and tachypnea. Mendelson demonstrated, in rabbits, that the syndrome depended on acidic pH of the aspirated material to occur². The risk for the so-called Mendelson's syndrome prompted the creation of routines with prolonged fasting periods in the preoperative of elective operations. Empirically, periods longer than 8-12 hours were adopted for safety – a policy that, as will be seen, has gradually changed over the years.

In the early 19th century, patients were allowed to have a little cup of tea a few hours before surgery. After the publication of Mendelson's work² at the end of the Second World War, the guidelines for preoperative fasting changed and the rule of fasting from midnight was adopted for patients whose operation was scheduled for the morning period; and a light breakfast (tea and crackers) was allowed for patients undergoing surgery in the afternoon. That conduct was implemented for convenience. Thus, patients began to fast for long periods (10 to 16 hours or longer)^8,9. However, many current guidelines (American Society of Anesthesiologists–ASA; Norwegian National Consensus Guideline–NNCG; Association of Anaesthetists of Great Britain and Ireland–AAGBI) recommend clear fluids two hours before surgery. The ASA, for instance, recommends more liberal guidelines with regard to fasting, allowing clear fluids (water, tea, coffee and pulp-free juices) up to two hours before surgery¹⁰.

Metabolic effects of fasting

Metabolic response to operative trauma is potentiated by prolonged preoperative fasting. After a few hours of fasting, insulin levels decline and, conversely, an increase in glucagon levels takes place, which determines fast utilization of the small glycogen store (approximately 400 g in an adult individual) that is mostly found in the liver. In less than 24 hours of fasting, hepatic glycogen is totally consumed. Before that, however, gluconeogenesis is activated and muscle protein starts to be used to supply glucose for the tissues that depend exclusively on it for energy (the central nervous system, renal medulla and erythrocytes)^11. Such phenomenon seems to be centrally regulated, and higher ACTH (adrenocorticotropic hormone) secretion by the hypophysis is also stimulated, thus increasing the secretion of cortisol by the adrenal gland. Serum levels of the growth hormone rise in the event of hypoglycemia or a decrease in circulating free fatty acids. Cortisol, along with the decrease in insulin and the increase in adrenergic and thyroid hormones are responsible for the catabolic reactions that supply amino-acids into the bloodstream^12,13.

Another component of the metabolic response to trauma is insulin resistance, which lasts around three weeks after uncomplicated elective abdominal operations^14,15. This manifestation is more intense on the first and second postoperative days and is directly related to the magnitude of the surgery, yet it occurs even in minor operations¹⁵. Preoperative fasting contributes to an increase in insulin resistance, thus potentiating the metabolic stress of operative trauma¹⁶.

Evidence of safety

In a recent systematic review of Cochrane encompassing 22 trials, it became evident that liquid intake in the immediate preoperative period (two to three hours prior to surgery) is safe and not associated with the risk for aspiration, regurgitation and mortality in relation to patients under traditional fasting protocols¹⁷. Preoperative oral liquid feeding was considered to be beneficial to the patient, as it prevents dehydration and thirst. Furthermore, the study stressed that the "nil by mouth from midnight" preoperative routine must be discouraged¹⁷. Nevertheless, traditional preoperative fasting is regarded as indispensable by many surgeons and is still used due to the power of die-hard old concepts and classical paradigms of medicine^6,18.

Aguilar-Nascimento et al conducted a randomized trial with cholecystectomy patients comparing traditional fasting with supplying a carbohydrate-rich (CHO) drink two hours prior to the operation. The results showed that the latter was safe and not associated with anesthetic complications. Furthermore, the patients in the CHO group had fewer gastrointestinal complications and their postoperative hospital stay was one day shorter than that of the control group⁴. Another study by the same investigators showed that, in videolaparoscopic cholecystectomies, the shortened fasting time with CHO two hours prior to the operation reduces insulin resistance and the metabolic response to trauma, thus benefiting the patient¹⁹.

In 2005, The European group ERAS (Enhanced Recovery After Surgery) issued a consensus regarding perioperative care, outlining a number of changes in the traditional forms of care on the basis of randomized, controlled trials and meta-analyses²⁰. The most relevant changes were adapted to the Brazilian setting by the ACERTO project (Aceleração da Recuperação Total Pós-Operatória – Total Postoperative Recovery Acceleration)²¹. Before the project was instituted, patients would spend an average of 16 hours in preoperative fasting; after its implementation, there was a significant decrease in preoperative fasting times, which allowed patients to be operated on with an average time interval of four hours between feeding and anesthetic induction²².

Paradigm shift in a university-affiliated teaching hospital

In the second semester of 2005, a new protocol termed ACERTO (www.projetoacerto.com.br) was implemented at the Hospital Universitário Julio Muller (HUJM) of the Universidade Federal de Mato Grosso. In the array of perioperative care practices included in the protocol was the shift from the traditional fasting period of 6-8 hours to 2 hours. Actually, the time of preoperative fasting for solids was maintained, but patients began to ingest a drink containing maltodextrin (200 mL of water containing 12% maltodextrin; 50 g, 200 kcal, approximately 285 mOsm) two hours prior to surgery. Before the protocol was implemented, an audit of the service showed that patients fasted for 16 hours, ranging from 6 to 24 hours. That time increase was determined by a number of reasons, such as delayed surgery, change of surgery time or period of day and adherence of the patient to more extended fasting. We held a seminar with the participation of anesthesiologists, residents, nurses and dietitians, in addition to surgeons, to discuss the problem and requested an anesthesiologist to present modern guidelines for preoperative fasting. It was clear from the presented guidelines that the two-hour fast shortened by a carbohydrate beverage was not only safe, but also advisable.

Since then, the protocol has been changed and conventional fasting was reserved only for patients with major gastroesophageal reflux disease, intestinal obstruction, morbid obesity and poor gastric emptying of any etiology. The experience of the anesthesia service, with no case of regurgitation during anesthetic induction, has recently been submitted for publication. Anesthesiologists are comfortable with the protocol and, despite a certain degree of resistance by some at the beginning, this new regimen is now routine at the HUJM. Multiprofessional participation, especially of nurses and dietitians, is important in view of the routine changes that this paradigm shift in preoperative fasting entails.

CONCLUSION

The shortening of preoperative fasting with the supply of a carbohydrate-rich (CHO) beverage up to two hours before the operation is being seen as one of the beneficial factors in reducing organic response, insulin resistance, surgical stress and, additionally, improving the patient's well-being. The satisfaction of the patient must also be taken into account, and it is greater when shorter preoperative fasting periods are prescribed. This practice is not only safe, but also essential to faster recovery from operative trauma. Therefore, this reduction in preoperative fasting length should be adopted.

REFERENCES

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Publication Dates

• Publication in this collection
  09 Nov 2009
• Date of issue
  Aug 2009

History

• Accepted
  04 Oct 2008
• Received
  11 Aug 2008