The practice of warming breast milk for feedings has long been a standard in the NICU. In no small part, this is related to the fact that much of the expressed breast milk has been previously frozen, requiring at least thawing.
One of the earliest publications concerning warming of infant feedings looked at infant’s acceptance of cool or cold feedings.1 Most infants (67%) accepted cold feedings and no difference in growth patterns was found between the groups fed cold, cool or warm milk. These infants were likely bigger, healthier infants than what we commonly see in the NICU today. In spite of the finding that infants “accepted” the milk, the practice of warming milk for feedings following freezing or refrigeration has been a long-standing practice for both in-hospital and home feeding. There are several theoretical reasons to warm the milk prior to feeding. In the very low birth weight infant, feeding of cold milk may lead to changes in body temperature, although this has never been well-researched. Studies have attempted to show effects of feeding temperature variations and effect on metabolic rate, residuals, effect on body temperature and growth patterns. Gonzales did show an increase in residuals related to cold feedings but this has not been replicated since and was a fairly small sample size.2 Many other practices have changed as well since that time. So, the benefit of warming feedings and to what ideal temperature is still without good evidence. There is, however, no evidence that this is harmful. So, the accepted practice is to warm to some as yet, undefined temperature. The Human Milk Banking Association advocated warming feeds to body temperature for premature infants, particularly those at risk for necrotizing enterocolitis.3 For term infants, feedings may be given at body temperature, room temperature or straight from the refrigerator. In attempts to determine at what temperature feeds are actually delivered, several studies have been undertaken (Dumm et al, 2013, Lawlor-Klean, Lefaivor, Weisbrock, 2013). Dumm et al demonstrated a range of milk temperature from 21.8°C to 36.2°C.4 Lawlor-Klean, Lefaiver, Weisbrock demonstrated a range of milk temperature from 22°C to 46.4°C.5 Over-warming may be as detrimental as under warming. Over warming may lead to deterioration of some benefit from human milk and/or effects on infant temperature. Under warming may lead to effects on digestion and/or body temperature of the infant. Is it possible that milk warmed to a higher temperature should not be left as long in a continuous feed, due to concerns about bacterial growth?
Warming may occur in a variety of methods. Clear recommendations have been adopted regarding avoidance of microwave warming.3 Past recommendations have included a preference for warming under “running water”. This is not usually practical in the real world, though. The length of time required to thaw or warm a feeding to an adequate temperature is an unrealistic time for the nurse to spend at the sink holding a bottle under running water! Thawing milk may also occur in a variety of ways. Thawing at room temperature, in the refrigerator or in warm water are all used. If milk is thawed in warm water, it often reaches room temperature or warmer before taken from the water bath. Once the milk has been warmed to room temperature or beyond, it should not be returned to the refrigerator or used for more than a few hours, usually a maximum of 4 hours.6 If thawed milk is not going to be used within this 4 hour window, it should be thawed but not warmed. Thawing in the refrigerator or with a controlled mechanism such as is now available on the market should be done to avoid warming. All of these available methods require time to thaw. Therefore, feeding our infants requires advance planning for the day or the shift, including the next shift!
Back to warming of feedings…. The usual is a warm water bath or one of the mechanical methods now available. Concerns about the warm water bath besides an unreliable end temperature includes possible contamination of the feeding. The issue of contamination is worth discussion. Tap water often contains an “acceptable” level of bacteria that in the normal, healthy population is not of concern. However, contamination of a feeding to be given to a high risk neonate could certainly cause a problem. Tap water often contains an “acceptable” level of pseudomonas and other bacteria we recognize as lethal in the NICU. Even a small amount of these bacteria can be life-threatening to our high-risk infants. In some recent performance improvement work done in California to decrease blood stream infections, it was found that one suspected cause of blood stream infection in at least 15% of cases not related to central lines was thought to be related to bacterial translocation from the gut, even in the absence of diagnosed necrotizing enterocolitis (unpublished data). Therefore, careful management of feedings is a must in this vulnerable population. In addition, if the temperature of the milk is indeed important in preventing feeding intolerance and necrotizing enterocolitis and possibly enhancing growth, what effect does the extended hang time of a pre-warmed continuous feed or feed over time have on the ultimate temperature of the milk when it reaches the infant? If milk is infusing over 30 minutes to 4 hours with tubing extending from the pump to the feeding tube both inside and outside the incubator, temperature will change over this time period, more or less, depending on room temperature, length of infusion time and incubator temperature.
As with many other neonatal issues, milk warming is a practice that cries out for additional research. Several questions that need to be answered include:
What is the ideal milk feeding temperature?
Does milk temperature have an effect on growth and if so, for whom?
Does milk temperature have an effect on feeding tolerance and if so, for whom?
Does milk temperature have an effect on overall body temperature? One study found no matter what the milk temperature was, body temperature increased. This is likely related to the increase in metabolic rate following feeding. Is this also true in premature infants and does it extend to very premature infants? Likely, the small feedings we give to the very premature infant does not cause this increase in metabolic rate and therefore, body temperature.
We often think of feeding babies as a simple, almost mindless task in the NICU. As we find out more and more about their nutritional needs and preparation for feeding, we know this, as much as the delicacy of gas exchange during lung disease, requires finesse and attention. The proper nutrition at the proper time is critical for tiny babies whose brains are requiring the majority of the nutrition.
1. Gibson JP. Reaction of 150 infants to cold formulas. J Pediatr. 1958; 52: 404–406
2. Gonzales, I, DurveaEJ, Vasquez E, Garahty N Effect of enteral feeding temperature on feeding tolerance in preterm infants. Neonatal Network. 1995;14(3):39-43.
3. The Human Milk Banking Association of North America, Inc. (2011). Best Practice for Expressing, Storing and Handling Human Milk in Hospitals, Homes and Child Care Settings. HMBANA: West Hartford, CT.
4. Dumm M, Hamms M, Sutton J, Ryan-Wenger N. NICU breast milk warming practices and the physiological effects of breast milk feeding temperatures on preterm infants. Advances in Neonatal Care. 2013;13(4):279-287.
5. Lawlor-Klean P, Lefaiver CA, Wiesbrock J. Nurses’ perception of milk temperature at delivery compared to actual practice in the neonatal intensive care unit. Advances in Neonatal Care. 2013;13(5):E1-E10.
6. Robbins S, Meyers R. Pediatric Nutrition Practice Group. (2011). Infant feedings: Guidelines for preparation of human milk and formula in health care facilities 2nd ed. American Dietetic Association.
Looking for additional reading from Sandy Beauman’s professional perspective?
View her blog entry Breast Milk Use.