Enteral Warming and Heat Transfer

 

Neonatal enteral fluid is often warmed for a variety of clinical reasons, such as reducing residuals and encouraging better digestion. Yet a question needs to be asked: what if it’s a drain on valuable clinician time, or worse yet, an exercise in futility?

In many cases, enteral nutrition is warmed prior to delivering a feed to patients. This is an attempt to bring the fluid temperature close to the temperature of 98˚F (body temperature), which is a fine practice… until physics enters the picture.

The Physics Roadblock

Let’s consider the concept of heat transfer, or thermal equilibrium. In a nutshell, heat is a transfer of energy. When heat encounters something cool, it transfers itself in an effort to equalize energy, or temperature. When placing a syringe of cold milk in heated water (as is common practice for enteral warming), the heat in the hot water transfers to the syringe and milk in an attempt to create a thermal equilibrium. This warms the syringe of milk while the water loses energy and cools. Now you have a warm syringe with warm milk inside. You’re ready to feed.

This is where the “waste of valuable clinician time” and “exercise in futility” steps in.

What will happen when a heated syringe of milk encounters cool air after the pre-warming process? Think back to the concept of thermal equilibrium. The heat (energy) in the syringe and milk will seek to transfer to the cooler temperatures (less energy) it encounters. In other words, the heat will transfer to the very air surrounding the syringe.

(But really… how quickly does that happen?)

Most pump feeds have pre-warmed syringes of milk exposed to room temperature air from thirty minutes up to four hours. How long would it take before thermal equilibrium is accomplished between the syringe with milk and room temperature air? The problem is the small amount of heat in the syringe and milk cannot possibly regulate the entire room. In fact, heat loss is quite dramatic when a syringe of milk sits exposed to room temperature for this very reason.

Based on our own repeated experiments to explore this occurrence, heat transfer is so immediate that the 98˚F milk literally drops to room temperature (72˚F average) by the time it exits the extension set. Part of the reason for this dramatic heat loss is the smaller volume of fluid traveling in the extension set path (smaller volume = faster heat loss). The 60” extension set large surface area also plays a factor, as it allows energy (heat) to transfer across its length. And while physics will show the same outcome to this test every time, we have been asked, “How many times did you perform this experiment?” The answer is over 100 times, with the same outcome each and every time.

In Conclusion

All health care professionals seek to offer the best patient experience for the best patient outcome. Warming an enteral feed is a way to meet these goals in the NICU. But physics demonstrates that the 20-minute time investment of cup and water warming does not deliver the near-or-at body temperature feed most clinicians wish to provide. It instead delivers a room temperature feed, at roughly 72˚F. While nurses agree a 72˚F feed is much better than a cold feed, the fact remains that the pre-warming process utilizes valuable time and resources only to deliver an end result which falls short of the original goal.

Whether you employ a cup and water warming method or some other chosen pre-warming method in your NICU, take into consideration the properties of heat transfer and what physics means to the process of delivering a small volume warm feed. Then evaluate and take steps to improve the process where possible.

 

Learn more about the physics of heat energy:

http://www.physicsclassroom.com/class/thermalP/u18l1d.cfm

http://www.answers.com/topic/temperature-and-heat