Milk Fasting Times and Aspiration in Infants

Concerns about fasting before anesthesia have created an almost irrational anxiety among anesthesia practitioners with direct consequences to patient care. The fear of pulmonary aspiration drives decision-making around operating room schedules. These decisions, sometimes illogical, are steered by current professional fasting guidelines concerning milk in neonates and infants. There is concern that anesthesia practitioners will be at greater medicolegal risk if they override fasting guidelines, even if there is allowance for practitioner discretion for divergence in clinical practice. The implementation of recent changes in fasting guidelines in children has highlighted both the excessive fear of aspiration and a fear of guideline changes. However, once new guidelines are established, they offer reassurance and comfort for practitioners [1]. A recent review in our journal, Pediatric Anesthesia, of the physiology related to preoperative fasting for breast, formula, and cow's milk [2] gives opportunity for reappraisal of current fasting guidelines in neonates and infants. The incidence of clinically evident aspiration for routine anesthesia is ~0.05% to 0.10%, with higher risk in emergency cases and neonates and infants. However, even when aspiration does occur, serious pulmonary morbidity or mortality is very rare. Discussion around the management of aspiration risk and consequent lung damage often centers around either gastric content volume or neutralization of gastric content acidity. Preoperative fasting reduces gastric content over time. The gastric volume available for aspiration at induction of anesthesia is commonly a key issue in the development of guidelines. The three main contributors to pulmonary damage are volume, acidity, and particulate matter of gastric contents aspirated. Early primate models suggest that volumes above 0.8 mL/kg with a pH of 1 are required to cause morbidity in rhesus monkeys [3]. However, these volumes were neither vomited nor regurgitated; they were directly injected into the monkey lung. Other animal models suggest the minimum gastric volume that resulted in passive regurgitation ranged between 8 and 40 mL/kg [4]. These gastric volumes are far greater than the volume aspirated, and there is huge variability in that volume aspirated, making a target gastric volume as a guideline objective to have considerable uncertainty. The more reasonable assumption, based on gastric emptying times, is that residual volumes 1–1.5 mL/kg (95% centile) after 3 h fasting for breast milk and 4 h for cow's milk are safe [2]. These assumptions do not account for the feed volume and assume a normal distribution rather than the more typical lognormal distribution [5]. There will always be neonates and infants with residual gastric volumes greater than 1–1.5 mL/kg. Yet, the volume contaminating the lungs during an aspiration incident is invariably unknown. Other factors aside from gastric volume contribute to the volume aspirated into the lungs. Inadequate anesthesia or poor control of the airway both contribute to aspiration risk. A prospective one-year survey of nearly 120 000 children at eleven specialist pediatric hospitals in the UK documented 24 cases of aspiration, five required some postoperative ventilation and none had ongoing sequelae. An insecure airway at induction and intubation, during maintenance without an endotracheal tube, or around extubation in the recovery phase as well as light anesthesia were associated with aspiration [6]. We would like to presume that the low incidence of aspiration was attributable to the skills of the pediatric trained anesthesia practitioners. However, concomitant pathology also has an impact on aspiration incidence. There were 12 children who suffered aspiration during the maintenance phase of anesthesia, and five of those cases were associated with increased intra-abdominal pressure [6]. Fasting guidelines should also acknowledge the value of preoperative assessment of comorbidities and airway management planning. And yet most of our concerns continue to center around fasting and gastric volume. Fasting is invariably some variant of the 6-4-2 (bottle milk, breast milk, clear fluid) hour rule. The applicability and usefulness of this rule have been poorly tested. Parents of older children are unreliable historians when it comes to how long their children have fasted [7]. Children who are allowed “sip to send” or clear fluids up to 1 h before theater have a similar incidence of pulmonary aspiration compared to 2-h clear fluid fasting [8], although data for neonates and infants after milk feeds are limited. Ultrasound has been used to measure gastric contents, but concerns remain about the accuracy of measures. Fasting periods in neonates and infants equivalent to individual feeding intervals have been advocated. Such advocacy simply reminds us of how clinical practice can be based on scant scientific evidence [9]. Milk remains a vital source of hydration and energy, and while gastric emptying times approximate adult rates at 9 months of age, similar to that associated with breastfeed weaning, concerns about prolonged fasting remain. The rationale for milk fasts longer than those for clear fluids is grounded in the belief that formula and cow's milk behave like solids with delayed gastric emptying and poorer outcomes if aspiration occurs. The belief that milk forms a coagulum, creating a particle size bigger than 2 mm and therefore delaying gastric emptying and contributing to aspiration pathology, is poorly supported. Although there are differences in gastric emptying based on age, volume, and route of feeding, the overall normal liquid gastric emptying in infants and children younger than 5 years of age is more than 80% at 3 h. Milk, both breast and formula, shows a less pronounced decay, but the variance of both breast and formula milk is similar to that of clear fluids. The definition of what is a “safe” residual gastric content at the end of a fasting time remains in doubt. That volume does not correlate in any way with the volume that might be aspirated. The acidity of that volume is poorly described, the particulate content unknown. Gastric emptying has been described using pharmacologic terms (e.g., first-order elimination for clear fluids and zero order for solids). We use such principles to determine safe drug doses. The dose of succinylcholine is, for example, 2–3 ED90 (effective dose that achieves blockade in 90% of population). This is a dose that is effective in almost everyone. If we were to apply similar principles to gastric emptying and used 3 “ED80” for gastric emptying, then we would seek a fast time of 9 h. A simplified scheme of 2–0 (milk, clear fluids) would have similar validity as the present 4-3-0 (formula milk, breast milk, clear fluids). Current regimens for milk in infants are practical but goaded by fear of pulmonary aspiration rather than scientific persuasiveness. While fasting guidelines are often perceived as “fasting rules,” there is provision for modification. Babies and infants having a milk-based diet could be offered clear fluids beyond their last milk feed. Offering warmed 5% or 10% glucose solutions can improve the hydration and calorie intake of babies awaiting anesthesia and minimize that preoperative distress yoked to hungry neonates and infants. Medicolegal fear driving slavish adherence to guidelines may not be in a patient's best interests because such adherence creates delays, prolonged fasting, and greater distress at induction. Fasting guidelines usually allude to medical discretion, but many anesthesia practitioners take a defensive approach and delay cases over minimal transgressions, sometimes justifying this in terms of their own medicolegal safety, but also sometimes believing that allowing that transgression will somehow encourage more flagrant disrespect of “fasting rules.” We would advocate for more liberal fasting guidelines in neonates and infants. The complications of excessive fasting are greater with restrictive fasting guidelines. In the infant and neonate group being considered, apart from avoidance of hypovolemia and hypoglycemia, more liberal proffering of appropriate fluids will settle babies at induction of anesthesia. A stormy distressed induction heightens CNS activity, increases anesthesia requirements, and has greater risk of airway issues. High-quality practice and good management of these airway issues remain a prime concern for any practitioner involved in anesthesia for neonates and infants. The climate where many anesthetists feel a greater responsibility and concern about the risk of being blamed for an aspiration event than being credited with changing the more common incidence of the complications of excessive fasting should change. The authors have nothing to report. This work was funded from institutional resources. The authors have nothing to report. Thomas Engelhardt sits on the Editorial Board of the journal Pediatric Anesthesia. The authors have no other conflicts of interest to declare. Data sharing not applicable to this article as no datasets were generated or analysed during the current study.